Sea water acclimation of rainbow trout (Oncorhynchus mykiss) modulates the mucosal transcript immune response induced by Vibrio anguillarum and Aeromonas salmonicida vaccine, and prevents further transcription of stress-immune genes in response to acute stress

Mucosal tissues appear to be more important in fish than in mammals due to living in a microbial-rich aquatic milieu, yet the complex interaction between the immune and the neuroendocrine system in these tissues remains elusive. The aim of this work was to investigate the mucosal immune response in immunized rainbow trout vaccinated with Alpha ject vaccine (bivalent), kept in fresh water (FW) or transferred to seawater (SW), and to evaluate their response to acute stress (chasing). Acute stress resulted in higher levels of plasma cortisol (Sham+Stress and Vaccine+Stress). A similar response was observed in skin mucus, but it was lower in Vaccine+Stress compared with stressed fish. With a few exceptions, minimal alterations were detected in the transcriptomic profile of stress-immune gene in the skin of vaccinated and stressed fish in both FW and SW. In the gills, the stress elicited activation of key stress-immune components (gr1, mr, β-ar, hsp70, c3, lysozyme, α-enolase, nadph oxidase, il1β, il6, tnfα, il10 and tgfβ1) in FW, but fewer immune changes were induced by the vaccine (nadph oxidase, il6, tnfα, il10 and igt) in both SW and FW. In the intestine, an array of immune genes was activated by the vaccine particularly those related with B cells (igm, igt) and T cells (cd8α) in FW with no stimulation observed in SW. Therefore, our survey on the transcriptomic mucosal response demonstrates that the immune protection conferred by the vaccine to the intestine is modulated in SW. Overall, our results showed: i) plasma and skin mucus cortisol showed no additional stress effect induced by prolonged SW acclimation, ii) the stress and immune response were different among mucosal tissues which indicates a tissue-specific response to specific antigens/stressor. Further, the results suggest that the systemic immune organs may be more implicated in infectious events in SW (as few changes were observed in the mucosal barriers of immunized fish in SW) than in FW.

Serum proteinogram of gilthead seabream (Sparus aurata) and European seabass (Dicentrarchus labrax) as a new useful approach for detecting loss of haemostasis

Proteinograms, a semiquantitative analytical method that separates proteins into multiple bands, have not been explored in teleosts for diagnostic or prognostic purposes. This study aimed to establish reference values for proteinograms in the serum of gilthead seabream (Sparus aurata) and European seabass (Dicentrarchus labrax), two important farmed fish species in the Mediterranean region. Serum proteins were studied using SDS-PAGE, electropherogram, and HPLC-mass spectrometry. SDS-PAGE analysis revealed four major bands of proteins around 11, 25, 70, and 100 kDa in the serum of gilthead seabream and European seabass. Electropherogram results showed that a protein with a molecular weight of 76.8 kDa was the most abundant protein in the serum of gilthead seabream, while a peak of 75.5 kDa was the most abundant in European seabass. HPLC-mass spectrometry detected 87 proteins and 119 proteins in the serum of gilthead seabream and European seabass, respectively, including α1-globulins, α2-globulins, β-globulins, and γ-globulins. Notably, the albumin sequence was not detected in either of the two species. These results help to characterise the serum protein profile and to establish reference proteinograms for these two fish species. They also provide a basis for the development of novel approaches for the rapid detection of loss of haemostasis due to stress, health disorders or disease in farmed fish.

Participation of Hepcidins in the Inflammatory Response Triggered by λ-Carrageenin in Gilthead Seabream (Sparus aurata)

The role of hepcidins, antimicrobial peptides involved in iron metabolism, immunity, and inflammation, is studied. First, gilthead seabream (Sparus aurata L.) head-kidney leucocytes (HKLs) were incubated with λ-carrageenin to study the expression of hepcidin and iron metabolism-related genes. While the expression of most of the genes studied was upregulated, the expression of ferroportin gene (slc40a) was downregulated. In the second part of the study, seabream specimens were injected intramuscularly with λ-carrageenin or buffer (control). The expression of the same genes was evaluated in the head kidney, liver, and skin at different time points after injection. The expression of Hamp1m, ferritin b, and ferroportin genes (hamp1, fthb, and slc40a) was upregulated in the head kidney of fish from the λ-carrageenin-injected group, while the expression of Hamp2C and Hamp2E genes (hamp2.3 and hamp2.7) was downregulated. In the liver, the expression of hamp1, ferritin a (ftha), slc40a, Hamp2J, and Hamp2D (hamp2.5/6) genes was downregulated in the λ-carrageenin-injected group. In the skin, the expression of hamp1 and (Hamp2A Hamp2C) hamp2.1/3/4 genes was upregulated in the λ-carrageenin-injected group. A bioinformatic analysis was performed to predict the presence of transcription factor binding sites in the promoter region of hepcidins. The primary sequence of hepcidin was conserved among the different mature peptides, although changes in specific amino acid residues were identified. These changes affected the charge, hydrophobicity, and probability of hepcidins being antimicrobial peptides. This study sheds light on the poorly understood roles of hepcidins in fish. The results provide insight into the regulatory mechanisms of inflammation in fish and could contribute to the development of new strategies for treat inflammation in farm animals.

Interwoven processes in fish development: microbial community succession and immune maturation

Fishes are hosts for many microorganisms that provide them with beneficial effects on growth, immune system development, nutrition and protection against pathogens. In order to avoid spreading of infectious diseases in aquaculture, prevention includes vaccinations and routine disinfection of eggs and equipment, while curative treatments consist in the administration of antibiotics. Vaccination processes can stress the fish and require substantial farmer's investment. Additionally, disinfection and antibiotics are not specific, and while they may be effective in the short term, they have major drawbacks in the long term. Indeed, they eliminate beneficial bacteria which are useful for the host and promote the raising of antibiotic resistance in beneficial, commensal but also in pathogenic bacterial strains. Numerous publications highlight the importance that plays the diversified microbial community colonizing fish (i.e., microbiota) in the development, health and ultimately survival of their host. This review targets the current knowledge on the bidirectional communication between the microbiota and the fish immune system during fish development. It explores the extent of this mutualistic relationship: on one hand, the effect that microbes exert on the immune system ontogeny of fishes, and on the other hand, the impact of critical steps in immune system development on the microbial recruitment and succession throughout their life. We will first describe the immune system and its ontogeny and gene expression steps in the immune system development of fishes. Secondly, the plurality of the microbiotas (depending on host organism, organ, and development stage) will be reviewed. Then, a description of the constant interactions between microbiota and immune system throughout the fish's life stages will be discussed. Healthy microbiotas allow immune system maturation and modulation of inflammation, both of which contribute to immune homeostasis. Thus, immune equilibrium is closely linked to microbiota stability and to the stages of microbial community succession during the host development. We will provide examples from several fish species and describe more extensively the mechanisms occurring in zebrafish model because immune system ontogeny is much more finely described for this species, thanks to the many existing zebrafish mutants which allow more precise investigations. We will conclude on how the conceptual framework associated to the research on the immune system will benefit from considering the relations between microbiota and immune system maturation. More precisely, the development of active tolerance of the microbiota from the earliest stages of life enables the sustainable establishment of a complex healthy microbial community in the adult host. Establishing a balanced host-microbiota interaction avoids triggering deleterious inflammation, and maintains immunological and microbiological homeostasis.

Effect of transporting Oreochromis niloticus in water with and without sodium chloride on skin morphology and some immunity-related genes expression

The aim of this study was to examine the effects of salt addition on the skin gene expression of Mucin, Antimicrobial peptides, cortisol, and glucose in Oreochromis niloticus after 5-hour transportation in water. Three groups were compared: Control, post-transport without salt (PT-S), and post-transport with 5 g salt-1(PT + S), with a stocking density of 28.6 gL-1, 20 fish for each experimental group. The results showed that the PT-S group had more significant changes in gene expression than the PT + S group, suggesting that salt alleviated the stress and immune responses of O. niloticus. The PT-S group had higher expression of mucin- 2(MUC + 2) (7.58 folds) and mucin-5AC (MUC5-AC) (6.29 folds) than the PT + S group (3.30 folds and 4.16 folds, respectively). The PT-S group also had lower expression of β-defensin-1 (Dβ1) (0.42 folds), β-defensin-2 (Dβ2) (0.29 folds), and Cath1 (0.16 folds) than the PT + S group (0.82 folds, 0.69 folds, and 0.75 folds, respectively). The skin morphology of the PT-S group revealed some white patches with no goblet cell openings, while the PT + S group had better preservation of skin features with some goblet cell openings and slight white patches. This study indicates that O. niloticus can benefit from sodium chloride during transportation, as it helps to reduce stress and inflammation, balance mineral levels, enhance health and immunity, and regulate mucous secretion.

Skin histopathological responses of Oreochromis niloticus (Nile tilapia) to transportation in water with and without salt

Oreochromis niloticus (Nile tilapia) is a well-known economic fish species that can thrive under the right environmental circumstances. The transport of live fish, either for food or as companion animals, presents a big issue for animal welfare at the same time it is considered one of stressful conditions. Hence, the present study investigated the skin histopathological responses of O. niloticus that were attributed to stress and salt addition during transportation. Three experimental groups of O. niloticus the 1st is the control non-transported group (CG), the 2nd is transport in water without salt (PT-S) and the 3rd is transport in water containing 5gL- 1salt (PT + S), the last 2 groups were transported in 5 h transport model. Results indicate that the skin of PT-S fish showed a marked decrease in epidermal thickness, decreased number of goblet cells, and an increase in the sub-epidermal and dermal pigments with the presence of large edematous vacuoles. Fish skin from PT + S demonstrated mild hydropic swelling in epidermal cells with normal goblet (mucous) cells density, and more or less normal melanin pigment distribution in sub epidermis and on the dermis layers, however, dermis showed mild edematous spaces. Scanning microscopy of PT-S skin tissue showed few scratched white patches among normal regions that may represent a thickened surface with the decreased number of goblets cell opening, while the PT + S group showed moderate preservation of surface skin architectures with the presence of goblet (mucous) cells opening in spite of presence of slight thickened white patches. The estimated total lesion changes present in PT-S group showed a significant increase (P < 0.001) compared with the control (CG) group. On the other hand, PT + S showed significant (P < 0.001) improvement in the overall previously recorded changes compared with the PT-S group, and a non- significant change in the histological architectures compared with the control group. Our findings underlined the importance of skin and its mucous cover health during transportation. The use 5 gL- 1salt during O. niloticus transportation appears to preserve the surface skin features, and keep the goblet (mucous) cells open to the external surface, and may act as a deterrent for the release of mucus from goblet (mucous) cells in response to stress and lessen the stress of transportation.

A comprehensive review on the dynamic role of toll-like receptors (TLRs) in frontier aquaculture research and as a promising avenue for fish disease management

Toll-like receptors (TLRs) represent a conserved group of germline-encoded pattern recognition receptors (PRRs) that recognize pathogen-associated molecular patterns (PAMPs) and play a crucial role in inducing the broadly acting innate immune response against pathogens. In recent years, the detection of 21 different TLR types in various fish species has sparked interest in exploring the potential of TLRs as targets for boosting immunity and disease resistance in fish. This comprehensive review offers the latest insights into the diverse facets of fish TLRs, highlighting their history, classification, architectural insights through 3D modelling, ligands recognition, signalling pathways, crosstalk, and expression patterns at various developmental stages. It provides an exhaustive account of the distinct TLRs induced during the invasion of specific pathogens in various fish species and delves into the disparities between fish TLRs and their mammalian counterparts, highlighting the specific contribution of TLRs to the immune response in fish. Although various facets of TLRs in some fish, shellfish, and molluscs have been described, the role of TLRs in several other aquatic organisms still remained as potential gaps. Overall, this article outlines frontier aquaculture research in advancing the knowledge of fish immune systems for the proper management of piscine maladies.

Effects of environmental translocation and host characteristics on skin microbiomes of sun-basking fish

Variation in the composition of skin-associated microbiomes has been attributed to host species, geographical location and habitat, but the role of intraspecific phenotypic variation among host individuals remains elusive. We explored if and how host environment and different phenotypic traits were associated with microbiome composition. We conducted repeated sampling of dorsal and ventral skin microbiomes of carp individuals (Cyprinus carpio) before and after translocation from laboratory conditions to a semi-natural environment. Both alpha and beta diversity of skin-associated microbiomes increased substantially within and among individuals following translocation, particularly on dorsal body sites. The variation in microbiome composition among hosts was significantly associated with body site, sun-basking, habitat switch and growth, but not temperature gain while basking, sex, personality nor colour morph. We suggest that the overall increase in the alpha and beta diversity estimates among hosts were induced by individuals expressing greater variation in behaviours and thus exposure to potential colonizers in the pond environment compared with the laboratory. Our results exemplify how biological diversity at one level of organization (phenotypic variation among and within fish host individuals) together with the external environment impacts biological diversity at a higher hierarchical level of organization (richness and composition of fish-associated microbial communities).

Riding the wave of innovation: immunoinformatics in fish disease control

The spread of infectious illnesses has been a significant factor restricting aquaculture production. To maximise aquatic animal health, vaccination tactics are very successful and cost-efficient for protecting fish and aquaculture animals against many disease pathogens. However, due to the increasing number of immunological cases and their complexity, it is impossible to manage, analyse, visualise, and interpret such data without the assistance of advanced computational techniques. Hence, the use of immunoinformatics tools is crucial, as they not only facilitate the management of massive amounts of data but also greatly contribute to the creation of fresh hypotheses regarding immune responses. In recent years, advances in biotechnology and immunoinformatics have opened up new research avenues for generating novel vaccines and enhancing existing vaccinations against outbreaks of infectious illnesses, thereby reducing aquaculture losses. This review focuses on understanding in silico epitope-based vaccine design, the creation of multi-epitope vaccines, the molecular interaction of immunogenic vaccines, and the application of immunoinformatics in fish disease based on the frequency of their application and reliable results. It is believed that it can bridge the gap between experimental and computational approaches and reduce the need for experimental research, so that only wet laboratory testing integrated with in silico techniques may yield highly promising results and be useful for the development of vaccines for fish.

Alterations in certain immunological parameters in the skin mucus of the carp, Cirrhinus mrigala, infected with the bacteria, Edwardsiella tarda

The bacterial fish pathogen Edwardsiella tarda causes heavy stock mortality, severely hampering fish production, resulting in great economic loss to the farming industry. The first biological barriers that confer immune protection against pathogen entry are the fish mucosal surfaces. The present study was undertaken to investigate the influence of E. tarda on certain enzymatic and non-enzymatic parameters in the skin mucous secretions of the fish Cirrhinus mrigala using spectrophotometry and zymography. Fish were randomly divided into three groups: control, vehicle control, and infected. A sublethal dose of E. tarda (2.2 × 106 CFU/fish) suspended in 50 μL of PBS was injected intra-peritoneally at 0 day (d). Subsequently, mucus samples were collected at 2 d, 4 d, 6 d and 8 d post-infection. The activities of lysozyme (LYZ), protease (PROT), alkaline phosphatase (ALP), acid phosphatase (ACP), catalase (CAT), peroxidase (PER), superoxide dismutase (SOD), and glutathione S-transferase (GST) decreased significantly in the skin mucus of the challenged fish, indicating the suppressed immune system and decreased antioxidant capacity of C. mrigala to E. tarda infection. Lipid peroxidation (LPO) and total nitrate-nitrite were significantly higher at several time points post-infection, suggesting that physiological functions have been impaired following pathogen challenge. The present findings could be relevant for fish aquaculture and underline the importance of skin mucus not only for assessing fish immune status but also for identifying early warning signals of disease caused by pathogens.

Welfare and performance of ballan wrasse (Labrus bergylta) reared at two different temperatures after a preparatory feeding trial with enhanced dietary eicosapentaenoic acid

Concerns have long been raised about the welfare of ballan wrasse (Labrus bergylta) used for the biological control of sea lice in Atlantic salmon (Salmo salar) aquaculture. This study assessed the effect of increased dietary eicosapentaenoic acid (EPA) levels and initial condition factor (CF) on the subsequent performance and welfare of ballan wrasse farmed in high and low water temperatures. Fish were fed a diet with either commercial or high EPA levels for 3 months at 15°C. Subsequently, fish were tagged with a passive integrated transponder, measured for their CF and divided into two groups consisting of fish from both treatments and reared for 4.5 months at either 15 or 6°C fed a commercial diet. Each fish was categorized as high (≥2.7) or low CF (<2.7) fish based on the calculated average CF of the population. Dietary composition influenced the fatty acid (FA) profile of the stored lipids without affecting the growth or welfare of ballan wrasse. Fish reared at 15°C showed higher growth, more fat and energy reserves and less ash content. Fish reared at 6°C lost weight, using up their body lipids at the end of the temperature trial. Gene expression analyses showed upregulation of the positive growth marker (GHrα) and two genes involved in the synthesis and oxidation of FAs (elovl5, cpt1) and downregulation of the negative growth marker (mstn) in fish reared at 15°C compared to those reared at 6°C. Fish reared at 6°C showed upregulated levels of il-6 compared to those reared at 15°C, suggesting an enhanced immune reaction in response to low temperature. Fish with high CF showed better survival, growth and performance compared to those with low CF. External welfare scoring showed higher prevalence and severity in emaciation, scale loss and the sum index score (of all measured welfare parameters) in fish reared at 6°C compared to those reared at 15°C and better welfare in fish with high CF compared to those with low CF. Histological examination of the skin showed that fish reared at 6°C had decreased epidermal thickness, a lower overall number of mucous cells in the inner and outer epidermis and a different organization of mucous cells compared to fish reared at 15°C, indicating stress in fish reared at 6°C. Overall, low water temperatures had profound effects on the performance and external and internal welfare parameters of ballan wrasse and can be considered a stressor likely affecting the delousing efficacy. These findings support the seasonal use of different cleaner fish species. High CF, but not increased dietary EPA levels, appeared to help fish cope better with low water temperatures and should thus be assessed and considered before deploying them in salmon cages.

Transcriptomic landscape of Atlantic salmon (Salmo salar L.) skin

In this study, we present the first spatial transcriptomic atlas of Atlantic salmon skin using the Visium Spatial Gene Expression protocol. We utilized frozen skin tissue from 4 distinct sites, namely the operculum, pectoral and caudal fins, and scaly skin at the flank of the fish close to the lateral line, obtained from 2 Atlantic salmon (150 g). High-quality frozen tissue sections were obtained by embedding tissue in optimal cutting temperature media prior to freezing and sectioning. Further, we generated libraries and spatial transcriptomic maps, achieving a minimum of 80 million reads per sample with mapping efficiencies ranging from 79.3 to 89.4%. Our analysis revealed the detection of over 80,000 transcripts and nearly 30,000 genes in each sample. Among the tissue types observed in the skin, the epithelial tissues exhibited the highest number of transcripts (unique molecular identifier counts), followed by muscle tissue, loose and fibrous connective tissue, and bone. Notably, the widest nodes in the transcriptome network were shared among the epithelial clusters, while dermal tissues showed less consistency, which is likely attributable to the presence of multiple cell types at different body locations. Additionally, we identified collagen type 1 as the most prominent gene family in the skin, while keratins were found to be abundant in the epithelial tissue. Furthermore, we successfully identified gene markers specific to epithelial tissue, bone, and mesenchyme. To validate their expression patterns, we conducted a meta-analysis of the microarray database, which confirmed high expression levels of these markers in mucosal organs, skin, gills, and the olfactory rosette.

Histological and ultrastructural characterization of the dorso-ventral skin of the juvenile and the adult starry puffer fish (Arothron stellatus, Anonymous 1798)

BACKGROUND

The starry puffer fish (Arothron stellatus, Anonymous, 1798) is a poisonous tetradontidae fish inhabiting the Red sea. The skin constitutes an important defense against any external effects. The study aims to characterize the dorso-ventral skin of the juvenile and the adult starry puffer fish using light and scanning electron microscopies. Twenty specimens of juvenile and adult fresh fishes were used.

RESULTS

The scanning electron microarchitecture of the skin of the juvenile and adult fish showed delicate irregular-shaped protrusions, and well-defined bricks-like elevations on the dorsal side and interrupted folds as well as irregular-shaped protrusions on the ventral side. In adult fish, the patterned microridges of the superficial and deep epithelial cells (keratinocytes) were larger and well-defined in the dorsal skin than in the ventral side, the contrary was seen in the juvenile fish. The microridges were arranged in a fingerprint or honeycomb patterns. The openings of the mucous cells were more numerous in the dorsal skin in both age stages but more noticeable in adult. Furthermore, the sensory cells were more dominant in the juveniles than the adults. The odontic spines were only seen in adult. Histologically, few taste buds were observed in the epidermis of the dorsal skin surface of the adult fish. Both mucous and club cells were embedded in the epidermis of the juvenile and adult fish with different shapes and sizes. Melanophores were observed at the dorsal skin of both juvenile and adult fishes while fewer numbers were noticed at the ventral surfaces. Several dermal bony plates with different shapes and sizes were demonstrated in the skin of both adult and juvenile fishes.

CONCLUSION

The structural variations of skin of the juvenile and adult fishes may reflect the various environmental difficulties that they confront.

Quality of DNA extracted from freshwater fish scales and mucus and its application in genetic diversity studies of Perca fluviatilis and Rutilus rutilus

Studies on genetic diversity require biological material containing a reliable source of DNA that can be extracted and analyzed. Recently, non-invasive sampling has become a preferred sampling method of biological material. The suitability of a less invasive approach that involves obtaining samples by swabbing the fish skin (including live, non-anesthetized fish) should be considered. In this study, we compared the efficiency of DNA extraction, amplification, and sequencing of mtDNA fragments of two fish species Perca fluviatilis and Rutilus rutilus based on DNA collected from the scales and mucus using the modified Aljanabi and Martinez method. The results revealed a higher quality of DNA extracted from the mucus; however, the mean DNA concentration obtained from the scales of both fish species was higher. We verified the method suitable for amplification and sequencing of mtDNA fragments of both fish species using newly designed markers (D-loop, ATP6) and examined the potential risk of intraspecific cross-contamination. The DNA sequence alignment analysis revealed identical sequences attributed to the same individual when DNA, extracted from two different sources (scales and mucus), was used. We demonstrated that the quantity and quality of DNA extracted from the scales and mucus using the proposed method were high enough to carry out genetic diversity studies based on sampling of live fish with the possibility to release it after collecting samples.

Dietary Debaryomyces hansenii promotes skin and skin mucus defensive capacities in a marine fish model

The present study explores the effects of two supplementation levels of Debaryomyces hansenii (1.1% and 2.2%) as a probiotic in a reference low fish meal-based diet on the skin mucosal tissue in Sparus aurata. This study includes the evaluation of fish performance coupled with a holistic study of the skin mucosa: i) a transcriptomic study of the skin tissue, and ii) the evaluation of its secreted mucus both in terms of skin mucosal-associated biomarkers and its defensive capacity by means of co-culture analysis with two pathogenic bacteria. Results showed that after 70 days of diet administration, fish fed the diet supplemented with D. hansenii at 1.1% presented increased somatic growth and a better feed conversion ratio, compared to fish fed the control diet. In contrast, fish fed the diet including 2.2% of the probiotic presented intermediate values. Regarding gene regulation, the probiotic administration at 1.1% resulted in 712 differentially expressed genes (DEGs), among which 53.4% and 46.6% were up- and down-regulated, respectively. In particular, D. hansenii modulated some skin biological processes related to immunity and metabolism. Specifically, D. hansenii administration induced a strong modulation of some immune biological-related processes (61 DEGs), mainly involved in B- and T-cell regulatory pathways. Furthermore, dietary D. hansenii promoted the skin barrier function by the upregulation of anchoring junction genes (23 DEGs), which reinforces the physical defense against potential skin damage. In contrast, the skin showed modulated genes related to extracellular exosome and membrane organization (50 DEGs). This modulated functioning is of great interest, particularly in relation to the increased skin mucus defensive capacity observed in the bacterial co-culture in vitro trials, which could be related to the increased modulation and exudation of the innate immune components from the skin cells into the mucus. In summary, the modulation of innate immune parameters coupled with increased skin barrier function and cell trafficking potentiates the skin's physical barrier and mucus defensive capacity, while maintaining the skin mucosa's homeostatic immune and metabolic status. These findings confirmed the advantages of D. hansenii supplementation in low fish meal-based diets, demonstrating the probiotic benefits on cultured marine species.

Immune responses in carp strains with different susceptibility to carp edema virus disease

Carp edema virus disease (CEVD), also known as koi sleepy disease (KSD), represents a serious threat to the carp industry. The expression of immune-related genes to CEV infections could lead to the selection of crucial biomarkers of the development of the disease. The expression of a total of eleven immune-related genes encoding cytokines (IL-1β, IL-10, IL-6a, and TNF-α2), antiviral response (Mx2), cellular receptors (CD4, CD8b1, and GzmA), immunoglobulin (IgM), and genes encoding-mucins was monitored in gills of four differently KSD-susceptible strains of carp (Amur wild carp, Amur Sasan, AS; Ropsha scaly carp, Rop; Prerov scaly carp, PS; and koi) on days 6 and 11 post-infection. Carp strains were infected through two cohabitation infection trials with CEV genogroups I or IIa. The results showed that during the infection with both CEV genogroups, KSD-susceptible koi induced an innate immune response with significant up-regulation (p < 0.05) of IL-1β, IL-10, IL-6a, and TNF-α2 genes on both 6 and 11 days post-infection (dpi) compared to the fish sampled on day 0. Compared to koi, AS and Rop strains showed up-regulation of IL-6a and TNF-α2 but no other cytokine genes. During the infection with CEV genogroup IIa, Mx2 was significantly up-regulated in all strains and peaked on 6 dpi in AS, PS, and Rop. In koi, it remained high until 11 dpi. With genogroup I infection, Mx2 was up-expressed in koi on 6 dpi and in PS on both 6 and 11 dpi. No significant differences were noticed in selected mucin genes expression measured in gills of any carp strains exposed to both CEV genogroups. During both CEV genogroups infections, the expression levels of most of the genes for T cell response, including CD4, CD8b1, and GzmA were down-regulated in AS and koi at all time points compared to day 0 control. The expression data for the above experimental trials suggest that both CEV genogroups infections in common carp strains lead to activation of the same expression pattern regardless of the fish's susceptibility towards the virus. The expression of the same genes in AS and koi responding to CEV genogroup IIa infection in mucosal tissues such as gill, gut, and skin showed the significant up-regulation of all the cytokine genes in gill and gut tissues from koi carp at 5 dpi. Significant down-regulation of CD4 and GzmA levels were only detected in koi gill on 5 dpi but not in other tissues. AS carp displayed significant up-expression of Mx2 gene in all mucosal tissues on 5 dpi, whereas in koi, it was up-regulated in gill and gut only. In both carp strains, gill harbored a higher virus load on 5 dpi compared to the other tissues. The results showed that resistance to CEV could not be linked with the selected immune responses measured. The up-regulation of mRNA expression of most of the selected immune-related genes in koi gill and gut suggests that CEV induces a more systemic mucosal immune response not restricted to the target tissue of gills.

Novel insights on toxicology of ochratoxin A contaminated diets to skin: Residues, mucus disruption and barrier impairment in teleost model grass carp (Ctenopharyngodon idella)

Ochratoxin A (OTA), a notorious pollutant widely present worldwide, seriously pollutes aquafeeds. This paper aims to explore the toxicity effects of OTA by the way of diet on the skin barrier in grass carp (Ctenopharyngodon idella). Results were shown as follows in the skin: (1) OTA increased the mRNA abundances of uptake transporter proteins (e.g., OAT3) and decreased efflux transporter proteins (e.g., ABCG2), which caused the accumulation of OTA in the skin of grass carp. (2) OTA upregulated the gene expression related to ROS production by enhancing the NOX (1, 2, 4) signaling pathway and decreased the ability to ROS elimination with downregulation of GPx1 (a,b), Trx by inhibiting the PGC1-α/Nrf2 signaling pathway, which caused oxidative damage to the skin. (3) OTA exacerbated apoptosis in the skin by upregulating the expression of apoptosis-related proteins mediated by ways of endoplasmic reticulum stress and mitochondrial apoptosis. Moreover, OTA down-regulated the mRNA and protein abundances of tight junction-related proteins by inhibiting the MLCK signaling pathway, which in turn disrupted the tight junctions. (4) OTA reduced the number of mucous cup cells and decreased f LZ activities and IgM contents, and finally down-regulated the mRNA abundances of mucin (2, 3), LEAP-2 (A, B), and β-defensin (1, 2, 3), which in turn resulted in impairing skin chemical barrier. Moreover, based on the antimicrobial-related indexes (LZ activities and IgM contents), the OTA-safe upper doses were 814.827 and 813.601 μg/kg.

Long-Term Influence of PCB- and PBDE-Spiked Microplastic Spheres Fed through Rotifers to Atlantic Cod (Gadus morhua) Larvae

Omnipresent microplastics (MPs) in marine ecosystems are ingested at all trophic levels and may be a vector for the transfer of persistent organic pollutants (POPs) through the food web. We fed rotifers polyethylene MPs (1-4 µm) spiked with seven congeners of polychlorinated biphenyls (PCBs) and two congeners of polybrominated diphenyl ethers (PBDEs). In turn, these rotifers were fed to cod larvae from 2-30 days post-hatching (dph), while the control groups were fed rotifers without MPs. After 30 dph, all the groups were fed the same feed without MPs. Whole-body larvae were sampled at 30 and 60 dph, and four months later the skin of 10 g juveniles was sampled. The PCBs and PBDEs concentrations were significantly higher in MP larvae compared to the control larvae at 30 dph, but the significance dissipated at 60 dph. Expression of stress-related genes in cod larvae at 30 and 60 dph showed inconclusive minor random effects. The skin of MP juveniles showed disrupted epithelial integrity, fewer club cells and downregulation of a suite of genes involved in immunity, metabolism and the development of skin. Our study showed that POPs were transferred through the food web and accumulated in the larvae, but that the level of pollutants decreased once the exposure was ceased, possibly related to growth dilution. Considering the transcriptomic and histological findings, POPs spiked to MPs and/or MPs themselves may have long-term effects in the skin barrier defense system, immune response and epithelium integrity, which may potentially reduce the robustness and overall fitness of the fish.

Transfer of CeO2 nanoparticles between freshwater omnivorous organisms: Effect of feces and necrophagy

Transfer of CeO₂ engineered nanoparticles (NPs) through feces was investigated between two omnivorous organisms, red crucian carp (Carassius auratus red var.) and crayfish (Procambarus clarkii). Upon water exposure (5 mg/L, 7 days), the highest bioaccumulation was observed in carp gills (5.95 μg Ce/g D.W.) and crayfish hepatopancreas (648 μg Ce/g D.W.), with the bioconcentration factors (BCFs) at 0.45 and 3.61, respectively. In addition, 97.4% and 73.0% of ingested Ce were excreted by carp and crayfish, respectively. The feces of carp and crayfish were collected and fed to crayfish and carp, respectively. After feces exposure, bioconcentration was observed in both carp (BCF, 3.00) and crayfish (BCF, 4.56). After feeding crayfish with carp bodies (1.85 μg Ce/g D.W.), CeO₂ NPs were not biomagnified (biomagnification factor, 0.28). Upon water exposure, CeO₂ NPs were transformed into Ce(III) in the feces of both carp (24.6%) and crayfish (13.6%), and the transformation was stronger after subsequent feces exposure (100% and 73.7%, respectively). Feces exposure lowered histopathological damage, oxidative stress, and nutritional quality (e.g., crude proteins, microelements, amino acids) to carp and crayfish in comparison with water exposure. This research highlights the importance of feces exposure on the transfer and fate of NPs in aquatic ecosystems.

Using immune cell-based bioactivity assays to compare the inflammatory activities of oil sands process-affected waters from a pilot scale demonstration pit lake

In this study, we provide evidence that oil sands process-affected waters (OSPW) contain factors that activate the antimicrobial and proinflammatory responses of immune cells. Specifically, using the murine macrophage RAW 264.7 cell line, we establish the bioactivity of two different OSPW samples and their isolated fractions. Here, we directly compared the bioactivity of two pilot scale demonstration pit lake (DPL) water samples, which included expressed water from treated tailings (termed the before water capping sample; BWC) as well as an after water capping (AWC) sample consisting of a mixture of expressed water, precipitation, upland runoff, coagulated OSPW and added freshwater. Significant inflammatory (i.e. macrophage activating) bioactivity was associated with the AWC sample and its organic fraction (OF), whereas the BWC sample had reduced bioactivity that was primarily associated with its inorganic fraction (IF). Overall, these results indicate that at non-toxic exposure doses, the RAW 264.7 cell line serves as an acute, sensitive and reliable biosensor for the screening of inflammatory constituents within and among discrete OSPW samples.

Mediterranean Sea heatwaves jeopardize greater amberjack's (Seriola dumerili) aquaculture productivity through impacts on the fish microbiota

Climate change is dramatically increasing the frequency and severity of marine heatwaves (MHWs) in the Mediterranean basin, strongly affecting marine food production systems. However, how it will shape the ecology of aquaculture systems, and the cascading effects on productivity, is still a major knowledge gap. The present work aims to increase our understanding of future impacts, caused by raising water temperatures, on the interaction between water and fish microbiotas, and consequential effects upon fish growth. Thus, the bacterial communities present in the water tanks, and mucosal tissues (skin, gills and gut), of greater amberjack farmed in recirculatory aquaculture systems (RAS), at three different temperatures (24, 29 and 33 °C), were characterized in a longitudinal study. The greater amberjack (Seriola dumerili) is a teleost species with high potential for EU aquaculture diversification due to its fast growth, excellent flesh quality and global market. We show that higher water temperatures disrupt the greater amberjack's microbiota. Our results demonstrate the causal mediation exerted by this bacterial community shifts on the reduction of fish growth. The abundance of members of the Pseudoalteromonas is positively correlated with fish performance, whereas members of the Psychrobacter, Chryseomicrobium, Paracoccus and Enterovibrio are suggested as biomarkers for dysbiosis, at higher water temperatures. Hence, opening new evidence-based avenues for the development of targeted microbiota-based biotechnological tools, designed to increase the resilience and adaptation to climate change of the Mediterranean aquaculture industry.

Leukolectin-proteins are synthesized and secreted by lectocytes, a distinct category of fish embryonic mucus cells

Fish perivitelline fluid (PVF) is a vital extra-embryonic compartment. At hatching, PVF-contents dissolve into the hatching fluid (HF). Analysis of Atlantic salmon HF reveals nearly hundred distinct proteins, most of which were identified by advanced mass-spectometry. However, one entity with apparent molecular weight 26 kDa, necessitated identification from its tryptic peptides. Subsequent cloning and sequencing revealed novel leukolectin-proteins. From bioinformatic analysis, leukolectins (LL) belong in the tectonin protein-family, with recognized functions in innate immunity. This study aims to identify LL-expressing cells in diverse fish species, and to characterize the ll-gene in order to predict bio-functions of leukolectins. LL-proteins were detected in HF from several fish species and one invertebrate, using polyclonal LL-specific IgGs. Embryonic LL-immunoreactive cells were numerous in Atlantic salmon, rainbow trout, fewer in Atlantic cod, and rare in Oikopleura dioica, Atlantic halibut. LL-immunoreactive cells were termed lectocytes, which corresponded to peridermal mucus-cells stained by PAS, but unstained by eosin. Hence, lectocytes and hatching-gland cells were clearly distinguished. Northern blots revealed two salmon LL-transcripts at mid-embryogenesis. Such transcripts were detected in epithelial cells of the periderm, gills and oral cavity. LL-transcripts predominated in the periderm, while choriolysin-transcripts were dominant in the gills. No co-expression of choriolysins and LL-transcripts was detected. BAC-library screening yielded salmon LL's gene-structure with 4 introns, 5 exons, TATA-box, multiple upstream putative transcription-factor binding-sites and polyadenylation site. ll-gene location on chromosome ssa17 was identified in Ssal_v3.1, the 2021-version of the salmon genome. In conclusion, larvae from several fish species are outfitted with mucus enriched by LL-proteins. Mucus cells are present in embryos of all fishes, but embryonic lectocyte-numbers are far higher in species with near total larval survival. When (maternal) chorionic first-line immuno-defence is lost at hatching, leukolectin-enriched mucus may provide vital protection for larvae.

Comparative transcriptome analysis reveals sex bias in expression patterns of genes related to sex steroids and immunity in the skin of spinyhead croaker Collichthys lucidus

Fish skin is the first barrier against external invasion, and also an important interface for communication between males and females during reproduction. Nonetheless, sexual dimorphism in the physiology of fish skins is still poorly understood. Herein, transcriptomes of skin were comparatively analysed between males and females in spinyhead croaker, Collichthys lucidus. Totally, 170 differentially expressed genes (DEG) were detected, including 79 female-biased genes and 91 male-biased genes. Gene ontology (GO) annotation items of the DEGs were mainly enriched in biological process items (86.2%), including regulation of biological processes, responses to chemical and biological stimuli, transport and secretion, movement, immune response, tissue development, etc. In KEGG (Kyoto Encyclopedia of Genes and Genomes) pathway enrichment analysis, the male-biased genes were enriched in pathways including those related to immunity such as the TNF signalling pathway and IL-17 signalling pathway, whereas the female-biased genes were enriched in pathways including those related to female steroids such as ovarian steroidogenesis and oestrogen signalling pathway. In addition, odf3 was found to be a male-specific expression gene, being a candidate marker for phenotypic sex. Thus, the sexual difference in gene expression in fish skin in spawning season was uncovered by transcriptome analysis for the first time, providing new insights into sexual dimorphism in the physiology and functions of fish skin.

Isolation and characterization of autochthonous probiotics from skin mucus and their in vivo validation with dietary probiotic bacteria on growth performance and immunity of Labeo calbasu (Hamilton, 1822)

The present study was performed to isolate and identify antimicrobial bacteria from the skin mucus of Labeo calbasu and assess their effects as water additives alone and in synergism, with dietary probiotic bacteria Aneurinibacillus aneurinilyticus LC1 isolated from intestinal tracts of L. calbasu on physiology and survival of same fish. Eight treatments (T1-T8) were conducted in triplicate, containing 10 fishes (2.02 ± 0.01 g) in each treatment: T1, control group (diet without probiotics); T2-T4, a diet with water additive probiotics; Bacillus cereus LC1, B. albus LC7, and B. cereus LC10, respectively, at 1000 CFU ml^-1; T5, a diet with dietary probiotic A. aneurinilyticus at 3000 CFU g^-1, T6-T8, a diet with water additives Bacillus cereus LC1, B. albus LC7, and B. cereus LC10 at 1000 CFU ml^-1 along with dietary probiotic A. aneurinilyticus at 3000 CFU g^-1. Results revealed improved growth, nutritive physiology, immune response, water quality, and survival in fish of group T8 (fingerlings fed on a probiotic diet at 3000 CFU g^-1 and reared in holding water treated with skin mucus bacteria B. cereus LC10 at 1000 CFU g^-1) as compared to other treatments, suggesting autochthonous intestinal and cutaneous mucosal bacteria as robust candidates for their collective application in aquaculture.

Fish Epidermal Mucus as a Source of Diverse Therapeutical Compounds

Microbes are helpful and destructive to human health and other living organisms. Microbes can be eliminated by using antibiotics against them, but their capability to resist regularly encountering antibiotics makes them more injurious. Microbes can adjust and adapt according to the chemicals used against them and become antibiotic resistant. Thus, the requirement for novel antimicrobial compounds increases with time to treat antibiotic-resistant microbes. Fish epidermal mucus encounters various pathogens present in their surrounding environment. It has become a rich source of novel antimicrobial compounds mainly antimicrobial peptides that can be used against various antibiotic-resistant pathogenic microbes. Compounds extracted from epidermal mucus can be used synergistically with other antibiotics or resistance modifying agents to inhibit the growth of resistant microbes. Fishes are consumed as a protein-rich food source worldwide and contribute to the world economy. Diseases in fish cause significant losses in the economic benefits exploited by fishermen and industries based on fisheries products. This paper will review compounds from fish epidermal mucus and their use to control the growth of antibiotic-resistant or non-resistant pathogenic microbes of humans and fishes. So, to increase fisheries' economic benefits and decrease infections involving resistant microbes.

Immunoprotective role of aloin and disease resistance in Labeo rohita, infected with bacterial fish pathogen, Aeromonas hydrophila

The effect of aloin on mucosal immune response and disease resistance was elucidated in Labeo rohita infected with the bacterial fish pathogen, Aeromonas hydrophila. Fishes were divided into four groups: (i) control, (ii) vehicle control, (iii) bacteria infected and (iv) bacteria infected and aloin treated. Fish were intraperitoneally injected with A. hydrophila suspension at the dose of 2 × 10⁶ CFU/fish at 0 day (d). Following bacterial injection at 0 d, fish were treated with aloin at a dose of 1 mg/kg body weight intraperitoneally at an interval of 24 h for 4 consecutive days. Mucus collected from fish of each group was analyzed at 2 d, 4 d, 6 d, 8 d and 10 d. In bacteria-infected fish, a significant decrease (P < 0.05) in the activity of certain enzymatic and non-enzymatic immune parameters was observed. The activity of these immune parameters showed a gradual recovery on administration of aloin in bacteria-infected fish. Cumulative mortality was also found to be low in the aloin-treated group as compared to that in the infected group. Thus, aloin could act as an immunostimulant and play a protective role against disease caused by bacteria.

Divergence in metabolomic profile in clownfish and damselfish skin mucus

Introduction

The clownfish - sea anemone mutualism was suggested to have triggered the adaptive radiation of clownfishes, but the origin of clownfish resistance to stinging tentacles of host anemones remains unclear. The presence of specific compounds in the mucus of clownfishes conferring them the unique ability to prevent nematocyst discharge from their hosts has been the most supported hypothesis. Yet the mystery regarding the types of compounds found in clownfish skin mucus remains unsolved.

Methods

We analyzed the chemical composition of clownfish and damselfish mucus using an untargeted metabolomics (HILIC-HRMS) and lipidomics (RPLC-HRMS) approach.

Results and Discussion

The polar and lipid metabolome signatures were highly specific and allowed to discriminate between the clownfish and damselfish clades. The most discriminative part of the signature was the sphingolipid profile, displaying a broader diversity of ceramides present in significantly higher levels in clownfish mucus. Importantly, the inter-specific variability of metabolic signature was significantly higher in clownfishes, although their diversification is evolutionarily more recent, thus implying the impact of symbiosis on metabolic variability and adaptation. Furthermore, specialists and generalists clownfish species displayed distinctive metabolite signature. Two strict clownfish specialists, which are phylogenetically distant but share the same host species, clustered together based on their molecular signature, suggesting a link with their mutualistic nature. Overall, comparative analyses of metabolic signatures highlight differences in chemical composition of clownfish mucus and provide insight into biochemical pathways potentially implicated in clownfish adaptation to inhabit sea anemones and consequently diversify.

Dietary Effects of Probiotic Lactobacillus casei on Some Immunity Indices of Common Carp (Cyprinus carpio) Exposed to Cadmium

The present study investigated the effect of feeding common carp with Lactobacillus probiotic supplement on growth, blood indices, and immunity of serum and mucus exposed to cadmium. A total of 250 common carp with an average weight of 15 ± 0.62 were divided into three groups: fish without probiotics, and fish with two levels of probiotic. At the end of a 42-day feeding period, 50% of the lethal concentrations of cadmium were exposed for 14 days and blood and immune indices of serum and mucus were measured. The results showed that growth indices, blood indices, and serum and mucus immunity were significantly different from the control group (P < 0.05). Finally, this study showed probiotics could improve the decreasing effect of cadmium in BWI and PBWI indices and the additive effect of cadmium in FCR and ALT. Probiotic was also able to completely improve the decreasing effect of cadmium on lymphocyte indices and the additive effect of cadmium on ALP and AST and monocyte.

Reef Location and Client Diversity Influence the Skin Microbiome of the Caribbean Cleaner Goby Elacatinus evelynae

Fish-associated microorganisms are known to be affected by the environment and other external factors, such as microbial transfer between interacting partners. One of the most iconic mutualistic interactions on coral reefs is the cleaning interactions between cleaner fishes and their clients, during which direct physical contact occurs. Here, we characterized the skin bacteria of the Caribbean cleaner sharknose goby, Elacatinus evelynae, in four coral reefs of the US Virgin Islands using sequencing of the V4 region of the 16S rRNA gene. We specifically tested the relationship between gobies' level of interaction with clients and skin microbiota diversity and composition. Our results showed differences in microbial alpha- and beta-diversity in the skin of gobies from different reef habitats and high inter-individual variation in microbiota diversity and structure. Overall, the results showed that fish-to-fish direct contact and specifically, access to a diverse clientele, influences the bacterial diversity and structure of cleaner gobies' skin. Because of their frequent contact with clients, and therefore, high potential for microbial exchange, cleaner fish may serve as models in future studies aiming to understand the role of social microbial transfer in reef fish communities.

Sex and reproductive development impact skin mucosal epithelium immunity, antimicrobial capacity, and up-regulation of immune-related gene of goldfish (Carassius auratus)

This study evaluated the epidermis mucosal capacity of goldfish (Carassius auratus) during different stages of reproductive development in both females and males. In this regard, the activity of mucolytic immune enzymes, i.e., lysozyme, complement and peroxidase, as well as the activity of alkaline phosphatase (ALP) were evaluated. There were five stages for females i.e., immature (f1), cortical alveoli (f2), early and late-vitellogenesis (vtg) (f3 and f4) and ripe (f5); as well as two stages for males spermatogenesis (m1) and spermiation (m2). Some stages were also examined for the mucosal antimicrobial activity against specific pathogens. The results showed that the mucosal lysozyme activity increased significantly during vitellogenesis (P < 0.05), but no lysozyme activity was detected in plasma. On the contrary, the complement activity was only observed in female plasma, and it was significantly higher at f3 compared to the other developmental stages. Both the plasma and mucosal ALP and peroxidase activities showed a significant increase by female reproductive development with the highest amounts at f4. Contrary to the female, no significant changes were observed in plasma and mucosal immune agents and biochemistry of the male. The f5-staged goldfish showed the highest antimicrobial activities against Gram-positive bacteria, i.e., Streptococcus faecium, Staphylococcus aureus and Micrococcus luteus (P < 0.05). Our results also represented the up-regulation of lysozyme (c-lys) gene expression by effects of female maturational development in ovary, liver and skin, while male goldfish showed no significant changes in c-lys expression. Moreover, there were positive correlations between c-lys expression, mucosal lysozyme activity and calcium levels in females (P < 0.01). Overall, our findings revealed that vtg process improves mucosal innate immunity that leads to activate antimicrobial components at spawning season.

Establishment and characterization of a new fibroblast-like cell line from the skin of a vertebrate model, zebrafish (Danio rerio)

BACKGROUND

The available fully sequenced genome and genetic similarities compared to humans make zebrafish a prominent in vitro vertebrate model for drug discovery & screening, toxicology, and radiation biology. Zebrafish also possess well developed immune systems which is ideal for studying infectious diseases. Fish skin confers immunity by serving as a physical barrier against the invading pathogens in the aquatic habitat. Therefore in vitro models from the skin tissue of zebrafish help to study the physiology, functional genes in vitro, wound healing, and pathogenicity of microbes. Hence the study aimed to develop and characterize a skin cell line from the wild-type zebrafish Danio rerio.

METHODS AND RESULTS

A novel cell line designated as DRS (D. rerio skin) was established and characterized from the skin tissue of wild-type zebrafish, D. rerio, by the explant technique. The cells thrived well in the Leibovitz's -15 medium supplemented with 15% FBS and routinely passaged at regular intervals. The DRS cells mainly feature fibroblast-like morphology. The culture conditions of the cells were determined by incubating the cells at varying concentrations of FBS and temperature; the optimum was 15% FBS and 28 °C, respectively. Cells were cryopreserved and revived with 70-75% viability at different passage levels. Two extracellular products from bacterial species Aeromonas hydrophila and Edwardsiella tarda were tested and found toxic to the DRS cells. Mitochondrial genes, namely COI and 16S rRNA PCR amplification and partial sequencing authenticated the species of origin of cells. The modal diploid (2n) chromosome number of the cells was 50. The cell line DRS was found to be free from mycoplasma. The cells were transfected with pMaxGFP plasmid and tested positive for green fluorescence at 24-48 h post-transfection.

CONCLUSION

The findings from this study thus confirm the usefulness of the developed cell line in bacterial susceptibility and transgene expression studies.

Effects of Bougainvillea glabra leaf on growth, skin mucosal immune responses, and disease resistance in common carp Cyprinus carpio

This study evaluated the effects of Bougainvillea glabra (BG) leaf as a feed supplement on growth, skin mucosal immune parameters, serum oxidative stress, expression of immune-related genes, and susceptibility to pathogen infection in carp Cyprinus carpio. Diets containing four different BG concentrations (g kg^-1), i.e., 0 g (basal diet), 20 g (BG20), 30 g (BG30), 40 g (BG40), and 50 g (BG50), were fed to the carp (average weight: 14.03 ± 0.81 g) for 8 weeks. Skin mucosal immunological and serum antioxidant parameters were examined 8 weeks post-feeding. Growth performance was significantly higher in BG40. Among the examined skin mucosal immune parameters, lysozyme (33.79 ± 0.98 U mL^-1), protein (6.88 ± 0.37 mg mL^-1), immunoglobulin (IgM; 5.34 ± 0.37 unit-mg mL^-1), and protease activity (3.18 ± 0.36%) were significantly higher in BG40 than in the control; whereas, there was no significant effect on the alkaline phosphatase level. Among serum immune activity, activities of lysozyme, the alternative complement pathway, and IgM were significantly higher in BG40. Phagocytic, and superoxide dismutase (SOD) activities were higher (P < 0.05) in BG30-BG50. Serum ALT, AST, and MDA levels were lower in BG40 than in the control (P < 0.05). Intestinal enzymatic activities were enhanced in BG40 and BG50 (P < 0.05), except for lipase in BG50. Gene expression analysis revealed that the mRNA expressions of antioxidant genes (SOD, GPx, and Nrf2), an anti-inflammatory gene (IL-10), and IκBα were significantly upregulated in BG40. Conversely, the pro-inflammatory gene IL-1β and the signaling molecule NF-κB p65 were downregulated in BG40 and BG50, respectively. BG supplementation had no significant effect on TNF-α, TLR22, or HSP70 mRNA expressions. Moreover, fish in BG40 exhibited the highest relative post-challenge survival (67.74%) against Aeromonas hydrophila infection. These results suggested that dietary supplementation with BG leaves at 40 g/kg can significantly improve the growth performance, immune responses, and disease resistance of C. carpio. BG leaves are a promising food additive for carp in aquaculture.

Alteration of hemoglobin ß gene expression in mucosal tissues of Japanese flounder, Paralichthys olivaceus, in response to heat stress, Edwardsiella piscicida infection, and immunostimulants administration

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Hbβ gene expression in the gills is decreased upon heat stress.

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Epidermal Hbß gene expression was increased upon AsA and LF feeding or infection.

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Mucosal Hbβ expression may be a useful indicator for monitoring fish health status.

Hemoglobin beta (Hbß) is a heme-binding protein capable of oxygen delivery. The oligopeptides derived from Hbβ in fish mucus are active against a variety of gram-negative bacteria and protozoa. To gain information on the physiological and immunological roles of Hbβ in the mucosal tissues of fish, we analyzed changes in Hbß gene expression levels in the epidermis, gills, and intestine of Japanese flounder, Paralichthys olivaceus, in response to heat stress, Edwardsiella piscicida infection, and trial feeding of immunostimulants, high-concentration ascorbic acid (AsA) or lactoferrin (LF). The results of quantitative real-time PCR showed that expression of the Hbß gene in the gills decreased markedly when exposed to heat stress, whereas that in the epidermis exhibited an increase 3h after infection with E. piscicida. Seven days after starting to feed either immunostimulant, epidermal Hbß gene expression in all AsA or LF dose groups was significantly higher than in the control group. The results of in situ hybridization showed that the abundance and intensity of the stained cells in the epidermis and in the gills were consistent with the expression levels of Hbß gene obtained from the infection and immunosuppressant experiments and the heat stress experiment, respectively. Our results suggest that mucosal Hbβ gene expression is closely related to physiological and immunological status and could be a useful indicator for monitoring condition of fish health.

Effect of Dietary Administration of Green Macroalgae (Ulva intestinalis) on Mucosal and Systemic Immune Parameters, Antioxidant Defence, and Related Gene Expression in Zebrafish (Danio rerio)

This study investigated the effects of adding green macroalgae gutweed (Ulva intestinalis) powder to zebrafish (Danio rerio) feed at different levels on innate immune responses, antioxidant defence, and gene expression. A total of 600 zebrafish (0.3 ± 0.08 g) were randomly allocated to 12 aquariums in four treatments with three replicates (50 fish per aquarium). Zebrafish were fed with different levels of U. intestinalis powder 0, 0.25, 0.5, and 1% for eight weeks. Whole-body extract (WBE) immune parameters including total protein level, globulin level, and lysozyme activity were evaluated and revealed statistically significant increased in all U. intestinalis supplemented groups compared to the control (P < 0.05). However, mucus immune parameters (total protein, globulin, and lysozyme) were statistically different in only 1% gutweed supplemented groups from other groups. While glutathione peroxidase (GPx) and superoxide dismutase (SOD) increased with the addition of gutweed (P < 0.05), catalase (CAT) did not change (P > 0.05). The study results showed that dietary gutweed remarkably upregulated immune-related genes such as lysozyme (Lyz) and Interleukin 1 beta (IL-1β). Antioxidant-related genes (SOD and CAT) and growth-related genes, including growth hormone (GH) and insulin-like growth factor-I (IGF-1), were remarkably upregulated with gutweed treatment (P < 0.05). In conclusion, dietary U. intestinalis showed beneficial effects on immunity, and same effects were observed in case of antioxidant and growth related genes expression in zebrafish.

Role of the melanocortin system in zebrafish skin physiology

The agouti-signaling protein (ASIP) acts as both a competitive antagonist and inverse agonist of melanocortin receptors which regulate dorsal-ventral pigmentation patterns in fish. However, the potential role of ASIP in the regulation of additional physiological pathways in the skin is unknown. The skin plays a crucial role in the immune function, acting as a physical limitation against infestation and also as a chemical barrier due to its ability to synthesize and secrete mucus and many immune effector proteins. In this study, the putative role of ASIP in regulating the immune system of skin has been explored using a transgenic zebrafish model overexpressing the asip1 gene (ASIPzf). Initially, the structural changes in skin induced by asip1 overexpression were studied, revealing that the ventral skin of ASIPzf was thinner than that of wild type (WT) animals. A moderate hypertrophy of mucous cells was also found in ASIPzf. Histochemical studies showed that transgenic animals appear to compensate for the lower number of cell layers by modifying the mucus composition and increasing lectin affinity and mucin content in order to maintain or improve protection against microorganism adhesion. ASIPzf also exhibit higher protein concentration under crowding conditions suggesting an increased mucus production under stressful conditions. Exposure to bacterial lipopolysaccharide (LPS) showed that ASIPzf exhibit a faster pro-inflammatory response and increased mucin expression yet severe skin injures and a slight increase in mortality was observed. Electrophysiological measurements show that the ASIP1 genotype exhibits reduced epithelial resistance, an indicator of reduced tissue integrity and barrier function. Overall, not only are ASIP1 animals more prone to infiltration and subsequent infections due to reduced skin epithelial integrity, but also display an increased inflammatory response that can lead to increased skin sensitivity to external infections.

Immersion challenge model for Flavobacterium psychrophilum infection of Atlantic salmon (Salmo salar L.) fry

Flavobacterium psychrophilum is the causative agent of bacterial cold-water disease (CWBD) and rainbow trout fry syndrome (RTFS), which affect salmonids. To better understand this pathogen and its interaction with the host during infection, including to support the development of resistant breeds and new vaccines and treatments, there is a pressing need for reliable and reproducible immersion challenge models that more closely mimic natural routes of infection. The aim of this present study was to evaluate a challenge model developed previously for rainbow trout for use in Atlantic salmon. First, preliminary challenges were conducted in Atlantic salmon (n = 120) and rainbow trout (n = 80) fry using two F. psychrophilum isolates collected from each fish species, respectively; fish had been pretreated with 200 mg/L hydrogen peroxide for 1 h. Thereafter, the main challenge was performed for just one F. psychrophilum isolate for each species (at 2 × 10⁷  CFU/mL) but using larger cohorts (Atlantic salmon: n = 1187; rainbow trout: n = 2701). Survival in the main challenge was 81.2% in Atlantic salmon (21 days post-challenge) and 45.3% in rainbow trout (31 days post-challenge). Mortalities progressed similarly during the preliminary and main challenges for both species, demonstrating the reproducibility of this model. This is the first immersion challenge model of F. psychrophilum to be developed successfully for Atlantic salmon.

Phytogenic Substances: A Promising Approach Towards Sustainable Aquaculture Industry

The aquaculture industry has shown rapid growth over the last three decades, especially with improving the farming systems. However, the rapid expansion and intensification practices in the aquaculture sector have been marred by increased stress levels and disease outbreaks, and subsequently, high fish mortality. Excessive use of veterinary drugs and antibiotics in aquaculture poses a great threat to human and aquatic animals' health, as well as to the biosystem. Furthermore, exposure to various pollutants such as industrial effluents and agricultural pesticides may cause devastating toxicological aspects of fish and adversely affect their health and growth. Besides, with a growing world population, there is a growing interest in intensifying aquaculture production to meet the global demand for nutritional security needs. Uncontrolled intensification of aquaculture production makes aquatic animals both vulnerable to, and potential sources of a wide range of hazards include pathogen transmission, disease outbreak, immunosuppression, impaired growth performance, malnutrition, foodborne illness, and high mortality. Plant-derived compounds are generally recognized as safe for fish, humans, and the environment and possess great potential as functional ingredients to be applied in aquaculture for several purposes. Phytogenic additives comprise a wide variety of medicinal plants and their bioactive compounds with multiple biological functions. The use of phytogenic compounds can open a promising approach towards enhancing the health status of aquatic animals. However, further in-vivo trials are necessary under favorable conditions with controlled amounts of identified bioactive compounds along with toxicity testing for fish safety towards a realistic evaluation of the tested substance efficacy.

Fishing for the Microbiome of Tropical Tuna

Although tunas represent a significant part of the global fish economy and a major nutritional resource worldwide, their microbiome still remains poorly documented. Here, we conducted an analysis of the taxonomic composition of the bacterial communities inhabiting the gut, skin, and liver of two most consumed tropical tuna species (skipjack and yellowfin), from individuals caught in the Atlantic and Indian oceans. We hypothesized that each organ harbors a specific microbial assemblage whose composition might vary according to different biotic (sex, species) and/or abiotic (environmental) factors. Our results revealed that the composition of the tuna microbiome was totally independent of fish sex, regardless of the species and ocean considered. Instead, the main determinants of observed diversity were (i) tuna species for the gut and (ii) sampling site for the skin mucus layer and (iii) a combination of both parameters for the liver. Interestingly, 4.5% of all amplicon sequence variants (ASV) were shared by the three organs, highlighting the presence of a core-microbiota whose most abundant representatives belonged to the genera Mycoplasma, Cutibacterium, and Photobacterium. Our study also revealed the presence of a unique and diversified bacterial assemblage within the tuna liver, comprising a substantial proportion of potential histamine-producing bacteria, well known for their pathogenicity and their contribution to fish poisoning cases. These results indicate that this organ is an unexplored microbial niche whose role in the health of both the host and consumers remains to be elucidated.

In vitro effects of λ-carrageenin in the head-kidney leucocytes of gilthead seabream (Sparus aurata)

The λ-carrageenin is a sulphated mucopolysaccharide that has been used for decades to induce experimental inflammation in mammals. However, it has been little considered in fish. We studied the in vitro effects of λ-carrageenin on gilthead seabream (Sparus aurata L.) head-kidney leucocytes (HKLs). For this purpose, HKLs were incubated with serial concentrations (from 0 to 1,000 μg mL-1) of λ-carrageenin for 3, 6, 12 and 24 h to assess its influence on cell viability and morphology, cell activity and modulation of several selected inflammation-related genes. The viability results demonstrated that λ-carrageenin has no negative effects on HKLs. The respiratory burst activity and phagocytic ability of HKLs after being incubated with λ-carrageenin (100 and 1,000 μg mL^-1) for 24 h were increased, whereas the phagocytic capacity was inhibited by the higher dose at the same experimental time compared with control samples. However, the peroxidase activity of HKLs was not changed by incubation with λ-carrageenin. According to transmission electron microscopy results, incubation of HKLs with the higher dose of λ-carrageenin appeared to activate the cells being evident different morphological changes without sign of cell death. Furthermore, up-regulation of three proinflammatory cytokines (il1b, tnfa, and il6) and down-regulation of anti-inflammatory genes (tgfb) were denoted in HKLs incubated with carrageenin. The present results provide a detailed approach to the effects of λ-carrageenin on fish leucocytes, which could have some impact on how we understand the response of these cells when inducing an inflammatory process in fish.

Skin immune response to Aeromonas hydrophila infection in crucian carp Carassius auratus revealed by multi-omics analysis

Fish skin is an essential protective barrier and functions as the first line of immune defense against pathogens. However, the molecular mechanism at the proteome-level remains unclear in the skin of fish. In this study, the comparative proteomics of skin immune responses of crucian carp Carassius auratus infected with Aeromonas hydrophila was investigated by isobaric tags for relative and absolute quantification (iTRAQ), two-dimensional gel electrophoresis combined with mass spectrometry (2-DE/MS) as well as high-throughput transcriptome (RNA-seq) techniques. A total of 241 and 178 differentially expressed proteins (DEPs) at 6 and 12 h post-infection (hpi) were respectively identified by iTRAQ, and key-DEPs were furtherly verified with 2-DE/MS analysis. GO and KEGG analysis showed that these DEPs were mostly related to metabolism, regulation of the cytoskeleton, stress and immune responses. Co-association results of proteome and transcriptome revealed the lysozyme (LYZ), complement C3, DnaJ (Hsp40) homolog subfamily C member 8 (DNAJC8) and allograft inflammatory factor 1-like (AIF1L) play important roles in skin immune responses of crucian carp. The significantly up-regulated expression of detected immune-related genes (c3, mapk3, f5, nlr, hsp90, itgb2, fnl, flnca, p47, mhc and pros1) were validated by qRT-PCR analysis. To our knowledge, this is first report on multi-omics analysis of the differential proteomics for the skin immune response of C. auratus against A.hydrophila infection, which contribute to the understanding the mechanisms of skin mucosal immunity in cyprinid fish.

Temperature affects settlement success of ectoparasitic salmon lice (Lepeophtheirus salmonis) and impacts the immune and stress response of Atlantic salmon (Salmo salar)

In this study, the effect of temperature on Atlantic salmon (Salmo salar) stress and immune response to the ectoparasitic salmon lice (Lepeophtheirus salmonis) was investigated. We found that infestation affected the expression of several immune and wound healing transcripts in the skin especially at the site of lice attachment compared to un-infested control fish. Moreover, expression patterns in the skin of infested fish suggest that host immune responses towards salmon lice are impaired at low temperatures. However, reduced lice infestation success and survival at the lowest investigated temperatures suggest that cold water temperatures are more detrimental to the lice than their fish hosts. Finally, temperature affected the stress response of the fish and infected fish had a higher increase in cortisol levels in response to handling (a stressor) than un-infested controls.

Aeromonas hydrophila Induces Skin Disturbance through Mucosal Microbiota Dysbiosis in Striped Catfish (Pangasianodon hypophthalmus)

Bacterial pathogens are well equipped to adhere to and initiate infection in teleost fish. Fish skin mucus serves as the first barrier against environmental pathogens. The mucus harbors commensal microbes that impact host physiological and immunological responses. However, how the skin mucosal microbiota responds to the presence of pathogens remains largely unexplored. Thus, little is known about the status of skin mucus prior to infection with noticeable symptoms. In this study, we investigated the interactions between pathogens and the skin mucosal microbiota as well as the fish skin immune responses in the presence of pathogens. Striped catfish (Pangasianodon hypophthalmus) were challenged with different concentrations of the bacterial pathogen Aeromonas hydrophila (AH), and the skin immune response and the mucosal microbiota were examined by quantitative PCR (qPCR) and 16S rRNA gene sequence analysis. We determined that the pathogen concentration needed to stimulate the skin immune response was associated with significant mucosal microbiota changes, and we reconfirmed these observations using an ex vivo fish skin model. Further analysis indicated that changes in the microbiota were attributed to a significant increase in opportunistic pathogens over AH. We concluded that the presence and increase of AH result in dysbiosis of the mucosal microbiota that can stimulate skin immune responses. We believe that our work sheds light on host-pathogen-commensal microbiota interactions and therefore contributes to aquaculture fish health.

IMPORTANCE The fish skin mucosal microbiota is essential in modulating the host response to the presence of pathogens. Our study provides a platform to study both the correlation and causation of the interactions among the pathogen, fish skin, and the skin mucosal microbiota. Based on these findings, we provide the first mechanistic information on how mucosal microbiota changes induced by the pathogen AH result in skin disturbance with immune stimulation in striped catfish in the natural state and a potential direction for early-infection screening. Thus, this study is highly significant in the prevention of fish disease.

The Zebrafish model in dermatology: an update for clinicians

Recently, the zebrafish has been established as one of the most important model organisms for medical research. Several studies have proved that there is a high level of similarity between human and zebrafish genomes, which encourages the use of zebrafish as a model for understanding human genetic disorders, including cancer. Interestingly, zebrafish skin shows several similarities to human skin, suggesting that this model organism is particularly suitable for the study of neoplastic and inflammatory skin disorders. This paper appraises the specific characteristics of zebrafish skin and describes the major applications of the zebrafish model in dermatological research.

Immune responses of Cyprinus carpio induced by protein extracts of Lernaea cyprinacea Linnaeus, 1758

Lernaea cyprinacea Linnaeus, 1758 is an ectoparasite showing widespread infections in tropical aquaculture, and the present study aimed to determine the specific immune responses against this parasite. For the experiment, whole parasite extracts were injected intraperitoneally into Cyprinus carpio Linnaeus, 1758, and samples of epidermal mucus and blood were drawn at 0, 1, 7 and 14-days post-injection (DPI). The results revealed high levels of protein, protease and lysozyme activities in the experimental fish which were injected with L. cyprinacea protein extract. In the epidermal mucus, the total protein concentration of the control fish was 460 μg/mL, and the level raised significantly to 800 μg/mL in the experimental fish. The lysozyme activity increased from 741.5 u/mL to a peak level of 1448.5 u/mL at 7DPI. The protease activity was also found elated gradually from 2.91 u/μL to 4.49 u/μL at 1 to 14 DPI. In the serum samples, the protein concentration remained steady throughout the experiment period. However, all the experimental fish displayed statistically high levels of lysozyme and protease activity, from 890 u/mL to 1220 u/mL, and 6.10 u/μL to 11.88 u/μL, respectively. In the whole blood samples, the haemoglobin content and the red blood cells (RBC) count did not show any significant change in any of the experimental groups. But, the percentage of lymphocytes showed a marginal increase from 0.47 to 0.6 in the experimental groups. Overall, the immune responses induced by L. cyprinacea protein extracts depicts a pattern of specific responses, in which the local humoral responses dominate the systemic humoral/cellular response. The results further revealed the possibility of futuristic approaches to control freshwater ectoparasites.

Integrated Analysis of lncRNA and circRNA Mediated ceRNA Regulatory Networks in Skin Reveals Innate Immunity Differences Between Wild-Type and Yellow Mutant Rainbow Trout (Oncorhynchus mykiss)

Fish skin is a vital immune organ that forms the first protective barrier preventing entry of external pathogens. Rainbow trout is an important aquaculture fish species that is farmed worldwide. However, our knowledge of innate immunity differences between wild-type (WR_S) and yellow mutant rainbow trout (YR_S) remains limited. In this study, we performed whole transcriptome analysis of skin from WR_S and YR_S cultured in a natural flowing water pond. A total of 2448 mRNAs, 1630 lncRNAs, 22 circRNAs and 50 miRNAs were found to be differentially expressed (DE). Among these DEmRNAs, numerous key immune-related genes, including ifih1, dhx58, trim25, atp6v1e1, tap1, tap2, cd209, hsp90a.1, nlrp3, nlrc3, and several other genes associated with metabolism (gstp1, nampt, naprt and cd38) were identified. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses of DEmRNAs revealed that many were significantly enriched in innate immune-related GO terms and pathways, including NAD+ADP-ribosyltransferase activity, complement binding, immune response and response to bacterium GO terms, and RIG-I-like receptor signaling, NOD-like receptor signaling and phagosome KEGG pathways. Furthermore, the immune-related competing endogenous RNA networks were constructed, from which we found that lncRNAs MSTRG.11484.2, MSTRG.32014.1 and MSTRG.29012.1 regulated at least three immune-related genes (ifih1, dhx58 and irf3) through PC-5p-43254_34, PC-3p-28352_70 and bta-miR-11987_L-1R-1_1ss8TA, and tap2 was regulated by two circRNAs (circRNA5279 and circRNA5277) by oni-mir-124a-2-p5_1ss13GA. The findings expand our understanding of the innate immune system of rainbow trout, and lay the foundation for further study of immune mechanisms and disease resistance breeding.

Structural and Functional Aspects of the Spleen in Molly Fish Poecilia sphenops (Valenciennes, 1846): Synergistic Interactions of Stem Cells, Neurons, and Immune Cells

In fish, the spleen is the prime secondary lymphoid organ. It has a role in the induction of adaptive immune responses, in addition to its significance in the elimination of immune complexes. This study was conducted on 18 randomly obtained adult molly fish (Poecilia sphenops) of both sexes using histological, immunohistochemical, and ultrastructural studies to highlight the cellular components of the spleen and their potential role in the immune system. The spleen of molly fish was characterized by the presence of well-distinct melanomacrophage centers, and other basic structures present in higher vertebrates including red and white pulps, blood vessels, and ellipsoids. Some mitotic cells could also be identified in the red pulp. Mast cells with characteristic metachromatic granules could be seen among the splenic cells. Rodlet cells were randomly distributed in the spleen and were also observed around the ellipsoids. The white pulp of the spleen expressed APG5. The expressions were well distinct in the melanomacrophages, leukocytes, and macrophages. Myostatin was expressed in leukocytes and epithelial reticular cells. IL-1β showed immunoreactivity in monocytes and macrophages around the ellipsoids. NF-κB and TGF-β were expressed in macrophages and epithelial reticular cells. Nrf2 expression was detected in stem cells and rodlet cells. Sox-9 had a higher expression in epithelial reticular cells and stem cells. The high frequency of immune cells in the spleen confirmed its role in the regulation of both innate and adaptive immunity, cell proliferation, and apoptosis.

Chronic Inflammation Modulates Opioid Receptor Gene Expression and Triggers Respiratory Burst in a Teleost Model

Stress-inducing husbandry and rearing conditions, bacterial infections or parasitic diseases may all lead to chronic inflammation. The immune response will then channel energy away from growth, reproduction and other important physiological processes, to fuel immune-related metabolic responses. The present study aims to unravel the mechanisms and contribute with new information on the molecular, cellular and humoral parameters of European seabass (Dicentrarchus labrax) undergoing chronic inflammation that can be used as health indicators for application in fish health management. European seabass individuals were intra-peritoneally injected with either Freund’s Incomplete Adjuvant (FIA) to induce inflammation or Hanks Balanced Salt Solution (HBSS) to serve as sham. Fish were sampled at 24 h, 7, 14 and 21 days post-injection and blood, plasma and head-kidney were collected. The results found were clear indicators of an inflamed peritoneal cavity and an ongoing systemic immune response that persisted for at least 21 days. Locally, inflammation was characterized by an intense recruitment of immune cells that was still evident 21 days after injection, thus illustrating the chronic character of the immune response. Cellular response was also noticed peripherally with leukocyte numbers rising in the blood of FIA-injected fish. Furthermore, the cellular-mediated respiratory burst peaked at 21 days post-FIA injection, suggesting that phagocytes were still actively fighting the phlogistic agent. Regarding the head-kidney molecular analysis, cxcr4 and il34 appear to be good markers of a chronic inflammation response due to their importance for pathways with high relevance in chronic inflammation settings. In addition, opioid receptor nopr seems to be a good marker of a chronic inflammation response due to its role in detecting noxious stimuli. The present study can serve as a baseline to assess long-term immune-related responses in future studies. For that, more research is nonetheless required to select more responsive and specific molecular markers.

Diversity and structure of sparids external microbiota (Teleostei) and its link with monogenean ectoparasites

Background

Animal-associated microbial communities appear to be key factors in host physiology, ecology, evolution and its interactions with the surrounding environment. Teleost fish have received relatively little attention in the study of surface-associated microbiota. Besides the important role of microbiota in homeostasis and infection prevention, a few recent studies have shown that fish mucus microbiota may interact with and attract some specific parasitic species. However, our understanding of external microbial assemblages, in particular regarding the factors that determine their composition and potential interactions with parasites, is still limited. This is the objective of the present study that focuses on a well-known fish-parasite interaction, involving the Sparidae (Teleostei), and their specific monogenean ectoparasites of the Lamellodiscus genus. We characterized the skin and gill mucus bacterial communities using a 16S rRNA amplicon sequencing, tested how fish ecological traits and host evolutionary history are related to external microbiota, and assessed if some microbial taxa are related to some Lamellodiscus species.

Results

Our results revealed significant differences between skin and gill microbiota in terms of diversity and structure, and that sparids establish and maintain tissue and species-specific bacterial communities despite continuous exposure to water. No phylosymbiosis pattern was detected for either gill or skin microbiota, suggesting that other host-related and environmental factors are a better regulator of host-microbiota interactions. Diversity and structure of external microbiota were explained by host traits: host species, diet and body part. Numerous correlations between the abundance of given bacterial genera and the abundance of given Lamellodiscus species have been found in gill mucus, including species-specific associations. We also found that the external microbiota of the only unparasitized sparid species in this study, Boops boops, harbored significantly more Fusobacteria and three genera, Shewenella, Cetobacterium and Vibrio, compared to the other sparid species, suggesting their potential involvement in preventing monogenean infection.

Conclusions

This study is the first to explore the diversity and structure of skin and gill microbiota from a wild fish family and present novel evidence on the links between gill microbiota and monogenean species in diversity and abundance, paving the way for further studies on understanding host-microbiota-parasite interactions.

Supplementary Information

The online version contains supplementary material available at 10.1186/s42523-022-00180-1.

Ultrasonography and X-ray micro-computed tomography characterization of the effects caused by carrageenin in the muscle of gilthead seabream (Sparus aurata)

The current work aimed to carry out an in vivo study of the λ-carrageenin-induced inflammation in the skin of gilthead seabream (Sparus aurata). The fish were injected intramuscularly with phosphate-buffered saline (PBS, as control) or λ-carrageenin (1% in PBS), and the injection zone was evaluated by real-time ultrasonography (Vevo Lab, VisualSonics) at 1.5, 3, 6, 12, and 24 h post-injection (p.i.). Results demonstrated that the skin thickness was increased in fish injected with λ-carrageenin and sampled at 1.5, 3, and 6 h p.i. However, the skin thickness of the injected area decreased to the normal values in those fish sampled at 12 and 24 h p.i. In addition, fish injected with λ-carrageenin and analysed at 1.5, 3, and 6 h p.i. showed, in the underlying muscle at the injection place, several hyperechoic small foci surrounded by an anechoic area which were not observed in control fish. Furthermore, the fish were analysed by X-ray micro-computed tomography (micro-CT). The analysis of the micro-CT acquisitions revealed also a dark area in the place of the injection with λ-carrageenin at 1.5, 3, and 6 h. These areas were smaller in fish analysed at longer times (12 h p.i.) and were almost disappeared in fish sampled at 24 h p.i. These areas had an average density of -850 to -115 HU, which did not correspond with any tissue density of the rest of the body. Furthermore, similar dark areas at the injection zones were never observed in control fish. Present results support the use of both non-invasive techniques to study the inflammatory process in fish of commercial interest such as gilthead seabream.

In vitro effects of cantharidin on gilthead seabream (Sparus aurata) head-kidney leucocytes

Cantharidin is a toxic vesicant terpene used in folk and traditional medicine due to its various therapeutic effects. Since there are no previous data on the effect of cantharidin in fish, this study aimed to investigate the in vitro related-inflammatory effects of cantharidin in gilthead seabream (Sparus aurata L.) head-kidney leucocytes (HKLs). In the first experiment, the HKLs were incubated with 0, 5 and 10 μg mL^-1 of cantharidin for 24 h to delimit its possible toxic effects. In a second experiment, leucocytes were incubated with ranging concentrations from 0 to 10 μg mL^-1 for 3, 6, or 12 h. Cell viability was higher in acidophilic granulocytes than in monocytes/macrophages and lymphocytes. Cantharidin caused apoptosis as was evidenced by transmission electron microscopy. In addition, cantharidin produced a time- and dose-dependent decrease of respiratory burst and phagocytic activities in HKLs, while their peroxidase activity was increased at 24 h of incubation with 5 and 10 μg mL^-1 of cantharidin. Different changes in the gene expression were observed after incubation with cantharidin. While the gene expression of tnfa, il1b and crel was up-regulated in HKLs, the nfkb1 and igmh genes were down-regulated in comparison to the expression found in control HKLs. Present results offer a first view of the possible effects and action mechanisms of cantharidin in HKLs, as well as its implication in the inflammatory process, which could be of interest not only for basic research but also in the aquaculture sector.