Regulation of growth, intestinal microbiota, non-specific immune response and disease resistance of sea cucumber Apostichopus japonicus (Selenka) in biofloc systems

Evidence of size-dependent toxicity of polystyrene nano- and microplastics in sea cucumber Apostichopus japonicus (Selenka, 1867) during the intestinal regeneration

Microplastics are ubiquitous pollutants in the global marine environment. However, few studies have adequately explored the different toxic mechanisms of microplastics (MPs) and nanoplastics (NPs) in aquatic organisms. The sea cucumber, Apostichopus japonicus, is a key organism in the marine benthic ecosystem due to its crucial roles in biogeochemical cycles and food web. This study investigated the bioaccumulation and adverse effects of polystyrene micro- and nanoplastics (PS-M/NPs) of different sizes (20 μm, 1 μm and 80 nm) in the regenerated intestine of A. japonicus using multi-omics analysis. The results showed that after 30-day exposure at the concentration of 0.1 mg L-1, PS-MPs and PS-NPs accumulated to 155.41-175.04 μg g-1 and 337.95 μg g-1, respectively. This excessive accumulation led to increased levels of antioxidases (SOD, CAT, GPx and T-AOC) and reduced activities of immune enzymes (AKP, ACP and T-NOS), indicating oxidative damage and compromised immunity in the regenerated intestine. PS-NPs had more profound negative impacts on cell proliferation and differentiation compared to PS-MPs. Transcriptomic analysis revealed that PS-NPs primarily affected pathways related to cellular components, e.g., ribosome, and oxidative phosphorylation. In comparison, PS-MPs had greater influences on actin-related organization and organic compound metabolism. In the PS-M/NPs-treated groups, differentially expressed metabolites were mainly amino acids, fatty acids, glycerol phospholipid, and purine nucleosides. Additionally, microbial community reconstruction in the regenerated intestine was severely disrupted by the presence of PS-M/NPs. In the PS-NPs group, Burkholderiaceae abundance significantly increased while Rhodobacteraceae abundance decreased. Correlation analyses demonstrated that intestinal regeneration of A. japonicus was closely linked to its enteric microorganisms. These microbiota-host interactions were notably affected by different PS-M/NPs, with PS-NPs exposure causing the most remarkable disruption of mutual symbiosis. The multi-omic approaches used here provide novel insights into the size-dependent toxicity of PS-M/NPs and highlight their detrimental effects on invertebrates in M/NPs-polluted marine benthic ecosystems.

Responses of Digestive, Antioxidant, Immunological and Metabolic Enzymes in the Intestines and Liver of Largemouth Bass (Micropterus salmoides) under the Biofloc Model

To investigate the activities of intestinal digestive enzymes, liver antioxidant enzymes, immunological enzymes, and glucometabolic enzymes in largemouth bass (Micropterus salmoides) under the biofloc model, an experiment was conducted in 300-liter glass tanks. The experiment comprised a control group, which was fed a basal diet, and a biofloc group, where glucose was added to maintain a C/N ratio of 15. Each group had three parallel setups, with a stocking density of 20 fish per tank. The experiment ran for 60 days, employing a zero-water exchange aquaculture model. The results showed that at the end of the culture period, there were no significant differences between the initial weight, final weight, WGR, SGR, and SR of the biofloc group and the control group of largemouth bass (p > 0.05), whereas the lower FCR and the higher PER in the biofloc group were significant (p < 0.05); intestinal α-amylase, trypsin, and lipase activities of largemouth bass in the biofloc group were significantly increased by 37.20%, 64.11%, and 51.69%, respectively, compared with the control group (p < 0.05); liver superoxide dismutase and catalase activities, and total antioxidant capacity of largemouth bass in the biofloc group were significantly increased by 49.26%, 46.87%, and 98.94% (p < 0.05), while the malondialdehyde content was significantly reduced by 19.91% (p < 0.05); liver lysozyme, alkaline phosphatase, and acid phosphatase activities of largemouth bass in the biofloc group were significantly increased by 62.66%, 41.22%, and 29.66%, respectively (p < 0.05); liver glucokinase, pyruvate kinase, glucose-6-phosphate kinase, pyruvate kinase, glucose-6-phosphatase, and glycogen synthase activities were significantly increased by 46.29%, 99.33%, 32.54%, and 26.89%, respectively (p < 0.05). The study showed that the biofloc model of culturing largemouth bass can not only enhance digestive enzyme activities, antioxidant capacity, and immune response but can also promote the process of glucose metabolism and reduce feeding costs. This study provides data support for healthy culturing of largemouth bass in future production, provides a theoretical reference for optimizing the biofloc technology culture model, and is crucial for promoting the healthy and green development of aquaculture.

Bioaccumulation of functionalized polystyrene nanoplastics in sea cucumber Apostichopus japonicus (Selenka, 1867) and their toxic effects on oxidative stress, energy metabolism and mitochondrial pathway

Micro/nano-plastics (M/NPs) are emerging contaminants in aquatic environment, however, little knowledge regarding the adverse effects of functionalized NPs has been documented so far. This study investigated the accumulation of different polystyrene nanoplastics (PS-NPs, i.e., plain PS, carboxyl-functional PS-COOH and amino-functional PS-NH₂) at two particle sizes of 100 nm and 200 nm, and evaluated the impacts on oxidative stress, energy metabolism and mitochondrial pathway responses in intestine and respiratory tree of Apostichopus japonicus during the 20-d exposure experiment. The results showed that there were significant interactions of particle size and nanoplastic type on the accumulation of different PS-NPs. Exposure to NPs significantly increased the production of malondialdehyde, glutathione and reactive oxygen species, as well as the activities of antioxidant enzymes including glutathione reductase, superoxide dismutase and catalase, resulting in various degrees of oxidative damage in sea cucumber. The significant decrease in adenosine triphosphate content and increases in alkaline phosphatase and lactate dehydrogenase activities suggested that NPs impaired energy metabolism and modified their energy allocation. After 20-d exposure, the complex I, II and III activities in mitochondrial respiratory chain were significantly inhibited. Meanwhile, the Bax and Caspase-3 gene expression were significantly up-regulated, and Bacl-2 was down-regulated, indicating the toxicity on mitochondrial pathway of A. japonicus. The calculated IBR values elucidated the greater detriment to mitochondrial pathway than oxidative stress and energy metabolism. For 100 nm particle size, plain PS has stronger influence on all the biomarkers compared to PS-COOH/NH₂, however, the opposite trends were observed in 200 nm PS-NPs. Furthermore, 100 nm PS-NPs were recognized to be more hazardous to sea cucumber than 200 nm microbeads. These findings provide new insights for understanding the differentiated toxic effects of functionalized NPs in marine invertebrates.

Mesentery AjFGF4-AjFGFR2-ERK pathway modulates intestinal regeneration via targeting cell cycle in echinoderms

OBJECTIVES

The purpose of the study aims to understand the regeneration process and its cytology mechanism in economic echinoderms.

MATERIALS AND METHODS

The intestine regeneration process of Apostichopus japonicus was investigated by immunohistochemistry and the cell proliferation was detected by immunofluorescence and flow cytometry. Fibroblast growth factor 4 of A. japonicus (AjFGF4) was screened by RNA-seq analysis and validated to regulate cell proliferation by siAjFGF4 and recombinant-AjFGF4 treatment. The binding and co-localization of AjFGF4 and AjFGFR2 were verified by Co-IP, GST-pull down, and immunofluorescence. Then, the AjFGF4-AjFGFR2-ERK-cell cycle axis was examined by western blot, immunofluorescence, and flow cytometry techniques.

RESULTS

The mesentery was served as the epicenter of intestinal regeneration via activating cell proliferation and other cellular events. Mechanically, AjFGF4-mediated cell proliferation was dependent on the binding to its receptor AjFGFR2, and then triggered the conserved ERK-MAPK pathway but not JNK and p38 pathway. The activated ERK-MAPK subsequently mediated the expression of cell cycle regulatory proteins of CDK2, Cyclin A, and Cyclin B to promote cell proliferation.

CONCLUSIONS

We provide the first functional evidence that AjFGF4-AjFGFR2-ERK-cell cycle axis mediated cell proliferation was the engine for mesentery-derived intestine regeneration in echinoderms.

Ultra-Performance Liquid Chromatography-Mass Spectrometry-Based Untargeted Metabolomics Reveals the Key Potential Biomarkers for Castor Meal-Induced Enteritis in Juvenile Hybrid Grouper (Epinephelus fuscoguttatus♀ × E. lanceolatus♂)

The intensification of aquaculture to help kerb global food security issues has led to the quest for more economical new protein-rich ingredients for the feed-based aquaculture since fishmeal (FM, the ingredient with the finest protein and lipid profile) is losing its acceptability due to high cost and demand. Although very high in protein, castor meal (CM), a by-product after oil-extraction, is disposed-off due to the high presence of toxins. Concurrently, the agro-industrial wastes’ consistent production and disposal are of utmost concern; however, having better nutritional profiles of these wastes can lead to their adoption. This study was conducted to identify potential biomarkers of CM-induced enteritis in juvenile hybrid-grouper (Epinephelus fuscoguttatus♀ × Epinephelus lanceolatus♂) using ultra-performance liquid chromatography-mass spectrometry (UPLC-MS) alongside their growth and distal intestinal (DI) health evaluation. A total of 360 fish (initial weight = 9.13 ± 0.01g) were randomly assigned into three groups, namely, fish-meal (FM) (control), 4% CM (CM4), and 20% CM (CM20). After the 56-days feeding-trial, the DI tissues of FM, CM4, and CM20 groups were collected for metabolomics analysis. Principal components analysis and partial least-squares discriminant-analysis (PLS-DA, used to differentiate the CM20 and CM4, from the FM group with satisfactory explanation and predictive ability) were used to analyze the UPLC-MS data. The results revealed a significant improvement in the growth, DI immune responses and digestive enzyme activities, and DI histological examinations in the CM4 group than the others. Nonetheless, CM20 replacement caused DI physiological damage and enteritis in grouper as shown by AB-PAS staining and scanning electron microscopy examinations, respectively. The most influential metabolites in DI contents identified as the potential biomarkers in the positive and negative modes using the metabolomics UPLC-MS profiles were 28 which included five organoheterocyclic compounds, seven lipids, and lipid-like molecules, seven organic oxygen compounds, two benzenoids, five organic acids and derivatives, one phenylpropanoids and polyketides, and one from nucleosides, nucleotides, and analogues superclass. The present study identified a broad array of DI tissue metabolites that differed between FM and CM diets, which provides a valuable reference for further managing fish intestinal health issues. A replacement level of 4% is recommended based on the growth and immunity of fish.

Adverse effects of dietary virgin (nano)microplastics on growth performance, immune response, and resistance to ammonia stress and pathogen challenge in juvenile sea cucumber Apostichopus japonicus (Selenka)

It has been well documented that micro- and nanoplastics are emerging pollutants in aquatic environments, and their potential toxic effects has attracted widespread concerns. Here, we evaluated the adverse effects of dietary polystyrene nanoplastics and microplastics (PS-N/MPs) on growth performance, oxidative stress induction, immune response, ammonia detoxification, and bacterial pathogen resistance of sea cucumber Apostichopus japonicus. After collection and acclimation, sea cucumbers were randomized into 3 groups (i.e., control, 100 nm PS-NPs and 20 µm PS-MPs at 100 mg kg^-1 diet) for 60-day feeding experiment. Every group contained 360 sea cucumbers which were equally divided into 3 aquaria as biological triplicates. The results showed that the specific growth rate and final weight of the sea cucumbers fed with diets containing PS-N/MPs were significantly lower than those of control group. Dietary virgin PS-N/MPs significantly increased the reactive oxygen species production and malondialdehyde content in coelomic fluid, causing oxidative stress and damage to the growth and development of A. japonicus. During the experiment, 100 nm PS-NPs significantly induced the depletion in cellular and humoral immune parameters. The calculated IBR values based on multi-level biomarkers revealed the size-dependent toxic differences of PS-NPs > PS-MPs. The relative expression levels of GDH and GS mRNA showed first rise and then fall trends after exposure to ammonia, and 100 nm PS-NPs had a more profound impact on suppressing ammonia detoxification compared with 20 µm PS-MPs. Moreover, the expression of Hsp90, Hsp70, CL, TLR, and CASP2 genes were all down-regulated by ammonia exposure. Taken together of IBR results, ammonia stress test and pathogen challenge, we deduced that dietary 100 nm PS-NPs are more potentially hazardous than 20 µm PS-MPs. These findings provide valuable information for understanding the size-dependent toxic effects of PS-N/MPs and early risk warning on marine invertebrates.

Bioflocs attenuate Mn-induced bioaccumulation, immunotoxic and oxidative stress via inhibiting GR-NF-κB signalling pathway in Channa asiatica

Manganese (Mn) is a relatively common element in aquatic ecosystems and can be bio-concentration, but the mechanism of manganese poisoning on fish health is unclear. Here, this study's objective was to evaluate the potential mechanisms of bioflocs in ameliorating Mn-induced toxicity in Channa asiatica. Three hundred sixty juveniles were randomly divided into 12 tanks. Four C:N ratios in triplicate tanks were tried: C/N = 7.6:1 with a commercial diet (control), C/N 10:1, C/N 15:1 and C/N 20:1, and the bio-accumulation, immunotoxic, oxidative stress, GR-NF-κB related genes expression and intestinal histomorphology were assessed in three different periods after Mn exposure (0 h, 48 h and 96 h). The results showed that bioflocs had a significant protective effect on Mn poisoning by preventing alterations in bio-accumulation levels, LSZ, AKP, C3, C4 and IgM, of which the C/N 15:1 group had the best relief effect. Furthermore, bioflocs also assisted in the recovery of liver T-SOD, CAT, GPX and T-AOC levels while decreasing the content of MDA. Moreover, C/N 15:1 group significantly down-regulated the expression level of NF-κB, TNF-α, IL-1β and IL-8 and up-regulated significantly IκBα, GR, HSP70 and HSP90 expression levels considerably (P < 0.05). From the intestinal section, the C/N 15:1 group resistance was the best one, and there was no difference between C/N 20:1 group and control group. These results revealed that administration of bioflocs (C/N 15:1) has the potential to combat Mn toxicity in C. asiatica, and the specific pathway may be GR-NF-κB.

Molecular Aspects of Regeneration Mechanisms in Holothurians

Holothurians, or sea cucumbers, belong to the phylum Echinodermata. They show good regenerative abilities. The present review provides an analysis of available data on the molecular aspects of regeneration mechanisms in holothurians. The genes and signaling pathways activated during the asexual reproduction and the formation of the anterior and posterior parts of the body, as well as the molecular mechanisms that provide regeneration of the nervous and digestive systems, are considered here. Damage causes a strong stress response, the signs of which are recorded even at late regeneration stages. In holothurian tissues, the concentrations of reactive oxygen species and antioxidant enzymes increase. Furthermore, the cellular and humoral components of the immune system are activated. Extracellular matrix remodeling and Wnt signaling play a major role in the regeneration in holothurians. All available morphological and molecular data show that the dedifferentiation of specialized cells in the remnant of the organ and the epithelial morphogenesis constitute the basis of regeneration in holothurians. However, depending on the type of damage, the mechanisms of regeneration may differ significantly in the spatial organization of regeneration process, the involvement of different cell types, and the depth of reprogramming of their genome (dedifferentiation or transdifferentiation).

Effect of chronic exposure to microplastic fibre ingestion in the sea cucumber Apostichopus japonicus

Microplastics (MPs) in the form of microfibres (MFs) are of great concern because of their size and increasing abundance, which increase their potential to interact with or be ingested by aquatic organisms. Although MFs are the dominant shape of MPs ingested by sea cucumbers in habitats, their effect on sea cucumbers remains unclear. This study examined the effect of dietary exposure to MFs on the growth and physiological status of both juvenile and adult Apostichopus japonicus sea cucumbers. MFs were mixed into the diet of sea cucumbers for 60 d at environmentally relevant concentrations of 0.6 MFs g^-1, 1.2 MFs g^-1 and 10 MFs g^-1. Dietary exposure to MFs, with concentrations at or above those commonly found in the habitats, did not significantly affect the growth and faecal production rate of either juvenile or adult sea cucumbers. However, a disruption in immunity indices (acid phosphatase and alkaline phosphatase activity) and oxidative stress indices (total antioxidant capacity and malondialdehyde content) was observed in juvenile and adult sea cucumbers, indicating that these indices might be useful as potential biomarkers of the exposure to MF ingestion in sea cucumbers. This study provides insights into the toxicity mechanism of MF ingestion in a commercially and ecologically important species.

Composition and geographic variation of the bacterial microbiota associated with the coelomic fluid of the sea urchin Paracentrotus lividus

In the present work, culture-based and culture-independent investigations were performed to determine the microbiota structure of the coelomic fluid of Mediterranean sea urchin Paracentrotus lividus individuals collected from two distinct geographical sites neighboring a high-density population bay and a nature reserve, respectively. Next Generation Sequencing analysis of 16S rRNA gene (rDNA) showed that members of the Proteobacteria, Bacteroidetes and Fusobacteria phyla, which have been previously reported to be commonly retrieved from marine invertebrates, dominate the overall population of microorganisms colonizing this liquid tissue, with minority bacterial genera exhibiting remarkable differences among individuals. Our results showed that there is a correlation between microbiota structure and geographical location of the echinoderm collection site, highlighting over-representation of metagenomic functions related to amino acid and bioactive peptides metabolism in specimens inhabiting the nature reserve. Finally, we also described the developmental delay and aberrations exhibited by sea urchin embryos exposed to distinct bacterial isolates, and showed that these defects rely upon hydrophilic compound(s) synthesized by the bacterial strains assayed. Altogether, our findings lay the groundwork to decipher the relationships of bacteria with sea urchins in their aquatic environment, also providing an additional layer of information to understand the biological roles of the coelomic fluid.

Monitoring oxidative stress, immune response, Nrf2/NF-κB signaling molecules of Rhynchocypris lagowski living in BFT system and exposed to waterborne ammonia

The present study was implemented to evaluate oxidative stress, immune response, Nrf2 and NF-κB signaling molecules related genes expression of Rhynchocypris lagowski living in biofloc technology (BFT) system and exposed to waterborne ammonia. According to the differences of C:N ratios, the experiment was divided into four groups: C:N 10.8:1 (control group), C:N 15:1, C:N 20: 1 and C:N 25:1. The results demonstrated that BFT can effectively regulate water quality and promote growth, and the C:N 20:1 group has the most significant effect (P < 0.05). Besides, significant increases in immune enzymes (lysozyme, complement C3, C4, immunoglobulin M and nitric oxide synthase) and anti-inflammatory factor (IL-2) activity of R. lagowski were emerged in the treatment C:N 20:1 after the 56-d growth experiment and the challenging trial (P < 0.05). Comparing the antioxidant status of R. lagowski in liver and gut before and after ammonia stress: superoxide dismutase, total antioxidant capacity and catalase activity in treatments C:N 20:1 were significant increased (P < 0.05), while the level of malondialdehyde was marked lower than that in control. Meanwhile, treatment C:N 20:1 considerably upregulated Nrf2 signaling molecules related genes and significantly down-regulated the expression of pro-inflammatory factor gene in NF-κB signaling pathway compared with the control (P < 0.05). These results indicated that BFT could enhance growth, antioxidant and immune response and regulate Nrf2 and NF-κB related genes expression in R. lagowski, with most excellent effects in fish given C:N 20:1 group.

Effect of acute exposure to ammonia and BFT alterations on Rhynchocypris lagowski: Digestive enzyme, inflammation response, oxidative stress and immunological parameters

Biofloc technology (BFT) is a new green culture technology that is intended not merely to eradicate nitrogenous residues but also enhance immunity and antioxidant activity in aquatic animals. A 56-day feeding trial and a 96 h ammonia challenge test were implemented to evaluate the effect of acute exposure to ammonia and BFT alterations on Rhynchocypris lagowski: digestive enzyme, inflammation response, oxidative stress and immunological parameters in zero water exchange tanks. According to the differences of C/N ratios, the experiment was divided into four groups: C/N 10.8:1 (control group), C/N 15:1, C/N 20: 1 and C/N 25:1. The results provided evidence that weight gain rate (WGR), specific growth rate (SGR) and protein efficiency ratio (PER) were significantly elevated in C/N 20, while food conversion rate (FCR) was considerably decreased (P < 0.05) compared to the control. Significant increases in digestive enzyme amylase (AMS), lipase (LPS), protease (PRS) and cellulase (CES) activity; Immune enzyme complement C3, complement C4, Nitric Oxide Synthase (NOS) and immunoglobulin M (IgM) activity; Serum biochemical lysozyme (LSZ), glutamic pyruvic transaminase (GPT), glutamic oxaloacetic transaminase (GOT) and alkaline phosphatase (AKP) activity; Inflammation TNF-α, IL-1β, IL-2, IL-6 of R. lagowskis were found in the C/N 20 group after a 56-day feeding trial and a challenging trial (P < 0.05). Comparing the antioxidant capacity of R. lagowski in gills, brains and spleen of juveniles from the four experimental groups, the activity of superoxide dismutase (SOD), total antioxidant capacity (T-AOC), catalase (CAT) activity and glutathion reductases (GR) activity of juveniles in the C/N 20 group were significant higher (P < 0.05), and the content of malondialdehyde (MDA) was considerably lower than that in the control. Overall, these findings suggest that BFT not only can enhance R. lagowski growth performance, digestive enzyme activity, and strengthen antioxidant status but also upgrade immune response, improve the ability of resistance to ammonia stress.

Bacillus baekryungensis MS1 regulates the growth, non-specific immune parameters and gut microbiota of the sea cucumber Apostichopus japonicus

Winter is a high incidence period of skin ulceration syndrome in the sea cucumber Apostichopus japonicus. Disease control during the overwintering of sea cucumber can help increase yield and reduce losses. The purpose of this study was to study the effect of the low temperature-resistant probiotic Bacillus baekryungensis MS1 on the growth and immune parameters of sea cucumbers and preliminarily investigate the molecular mechanism of the effects. A low temperature-resistant bacterium, B. baekryungensis MS1, was isolated from a sea cucumber pond in winter and used for culture experiments. After 10 days of prefeeding, the experiment was divided into the control group (fed with commercial diet) and the MS1 group (fed with diet containing B. baekryungensis MS1 at 10⁷ cfu g^-1) for a total of 60 days. The specific growth rate was measured at the end of the culture period to evaluate the growth performance of the sea cucumber. Samples were taken on days 30 and 60 to determine the immune parameters (including superoxide dismutase activity, catalase activity, alkaline phosphatase activity, acid phosphatase activity, nitric oxide synthetase activity, phagocytosis and respiratory burst), aquaculture water microbiota and gut microbiota of the sea cucumber. Finally, transcriptome sequencing and qRT-PCR verification of the two groups of sea cucumbers were performed to study the mechanism of B. baekryungensis MS1 to improve the immunity of the sea cucumber. The results showed that after 60 days of feeding, B. baekryungensis MS1 significantly improved the growth performance and immune enzyme activity and formed a healthier structure of the gut microbiota in the sea cucumber. The challenge test showed that B. baekryungensis MS1 significantly reduced the mortality of sea cucumbers infected with Vibrio splendidus. Transcriptome and gene expression analysis indicated that B. baekryungensis MS1 activated the ubiquitin-mediated proteolysis pathway and inhibited the mTOR signaling pathway to regulate the immunity of the sea cucumber. In summary, the low temperature-resistant bacterium B. baekryungensis MS1 could be applied for the aquiculture of sea cucumber in winter to improve health status and resist pathogenic bacteria such as V. splendidus.

Poly-β-hydroxybutyrate (PHB) in bioflocs alters intestinal microbial community structure, immune-related gene expression and early Cyprinid herpesvirus 2 replication in gibel carp (Carassius auratus gibelio)

The aquaculture system based on biofloc technology (BFT) showed positive effects on prevention of Cyprinid herpesvirus 2 (CyHV-2) infection in gibel carp (Carassius auratus gibelio), which is detrimental to health and causes seriously economic losses to aquaculture. However, the enhancement mechanism of BFT regarding immunity and disease resistance of cultured species is scarce. Poly-β-hydroxybutyrate (PHB) has been proved as one of bioactive compounds in bioflocs. In this study, two groups (4% PHB supplementation diets and control with basal diets) with 30-day feeding were set to study the effect of PHB supplementation on immune-related gene expression by qRT-PCR, time-course CyHV-2 replication in vivo by qPCR and intestinal microbiota by illumine high-throughput sequencing. PHB supplementation significantly up-regulated transcriptional levels of eight immune-related genes, decreased cumulative mortality of gibel carp and early CyHV-2 replication in spleen in vivo (P < 0.05). Additionally, PHB changed the microbial structure but not diversity, and significantly increased beneficial bacteria such as Bacillus sp. KEGG pathway analysis by PICRUSt demonstrated that oral administration of PHB up-regulated abundances of genes responsible for seven pathways and down-regulated genes in eleven pathways. Histological structures of foregut, mindgut and hindgut were also affected. Our findings suggested that profitable effects of PHB on immunity and disease resistance might be gut microbiota-related, and regulated through pathways of enzymes secretion, replication and repair, and host immune system. This study will provide new insights into understanding the enhancing mechanism of BFT on immunity and disease resistance of cultured animals, and developing prebiotics/probiotics-based immunotherapies to improve animal health and disease resistance.

Dietary Approaches to Attain Fish Health with Special Reference to their Immune System

Fish despite their low collocation in the vertebrate phylum possess a complete immune system. In teleost fish both innate and adaptive immune responses have been described with melanomacrophage centers (MMCs) equivalent to mammalian germinal centers. Primary lymphoid organs are represented by the thymus and kidney, while spleen and mucosa-associated lymphoid tissues act as secondary lymphoid organs. Functions of either innate immune cells (e.g., macrophages and dendritic cells) or adaptive immune cells (T and B lymphocytes) will be described in detail, even including their products, such as cytokines and antibodies. In spite of a robust immune arsenal, fish are very much exposed to infectious agents (marine bacteria, parasites, fungi, and viruses) and, consequentially, mortality is very much enhanced especially in farmed fish. In fact, in aquaculture stressful events (overcrowding), microbial infections very frequently lead to a high rate of mortality. With the aim to reduce mortality of farmed fish through the reinforcement of their immune status the current trend is to administer natural products together with the conventional feed. Then, in the second part of the present review emphasis will be placed on a series of products, such as prebiotics, probiotics and synbiotics, β-glucans, vitamins, fatty acids and polyphenols all used to feed farmed fish. With special reference to polyphenols, results of our group using red grape extracts to feed farmed European sea bass will be illustrated. In particular, determination of cytokine production at intestinal and splenic levels, areas of MMCs and development of hepatopancreas will represent the main biomarkers considered. All together, our own data and those of current literature suggests that natural product administration to farmed fish for their beneficial effects may, in part, solve the problem of fish mortality in aquaculture, enhancing their immune responses.

Physiological and immunological responses of sea cucumber Apostichopus japonicus during desiccation and subsequent resubmersion

Desiccation is one of the extremely stressful situations experienced by aquatic animals, and sea cucumber usually suffers from desiccation stress during transportation without water. The present study was conducted to evaluate the effect of desiccation and subsequent resubmersion on physiological stress, oxidative damage, antioxidant status and non-specific immune response of Apostichopus japonicus, providing valuable information on the health management of sea cucumber culturing. Control and desiccation groups were set up, and each group has three replicates. After 1, 3 and 6 h of desiccation, individuals were resubmersed in aerated seawater for a 24 h recovery in three batches, which were represented as D1, D3 and D6, respectively. The results showed that glucose level in coelomic fluid of sea cucumber significantly decreased after desiccation, whereas lactate, cortisol and osmolality showed remarkable ascending trends. Thereafter, all stress parameters gently recovered towards normal levels as control group during 24 h resubmersion. The prolonged desiccation at D6 treatment induced the significant increases of malondialdehyde (MDA) and reactive oxygen species (ROS) contents, as well as relatively lower superoxide dismutase (SOD) and catalase (CAT) activities. During the period of desiccation and subsequent resubmersion, sea cucumber adjusted antioxidant defense to reduce the concentrations of MDA and ROS as a strategy for protecting against oxidative damage. Desiccation also had significant effects on non-specific immune parameters (total coelomocytes counts, TCC; complement C3; total nitric oxide synthase, T-NOS; lysozyme, LSZ; alkaline phosphatase, AKP) of A. japonicus, which could be recovered to some extent during resubmersion. In conclusion, less than 6 h of desiccation did not induce irreparable damage to sea cucumber, and was recommended for handling and shipping live sea cucumbers.

Effects of dietary poly-β-hydroxybutyrate supplementation on the growth, immune response and intestinal microbiota of soiny mullet (Liza haematocheila)

Soiny mullet (Liza haematocheila) is an important economic fish species in China, but stress and diseases have seriously restricted its culture. There are no effective methods including vaccines to prevent or control these diseases. Alternative methods should be employed, such as using novel immunostimulant poly-β-hydroxybutyrate (PHB). The present study aimed to evaluate effects of dietary PHB supplementation on the growth, antioxidant enzymes activity, immune-related genes expression and intestinal microbiota in soiny mullet. The fish was fed for 30 or 60 days with six diets at different PHB supplementation of 0, 0.5, 1, 2, 4 and 8%, named as groups P0, P0.5, P1, P2, P4 and P8. The results showed that the weight gain and specific growth rate of fish in P2 and P0.5 groups were significantly higher than those in control P0 group at 30 and 60 days, respectively (P < 0.05). The antioxidant enzymes activity of catalase and superoxide dismutase in serum were significantly increased in P0.5/P1/P2 groups after 30 days. The transcriptional levels of penicillin-binding protein A and interleukin-8 analyzed by qRT-PCR were significantly upregulated in P2 and P4 groups compared to those in P0/P0.5/P1/P8 groups at 30 days. The transcriptional level of major histocompatibility complex class II in P2 group was significantly upregulated, and aldehyde oxidase downregulated compared to P0 group. Intestinal microbiota analysis by Illumina high-throughput sequencing showed that the microbiota diversity was not changed significantly, but the microbiota structure shifted significantly post PHB treatment. At the phyla level, Firmicutes and Proteobacteria were predominant in both P0 and P2 groups. At the genus level, the relative abundance of Bacillus spp. in P2 group increased significantly, and abundance of Achromobacter spp. decreased significantly. KEGG pathway analysis by PICRUSt showed that oral administration PHB significantly upregulated abundances of genes responsible for 10 pathways and downregulated genes involved in 17 pathways. In conclusion, soiny mullet fed with 2% PHB supplemental diets for 30 days showed better growth performance, higher antioxidant enzymes activity and immune-related genes expression. Their regulation of growth and immunity might be related with the intestinal microbiota change post PHB supplementation. It will provide very useful basic information to study the regulation mechanism of PHB in aquatic animals, and provide good green method to prevent disease in soiny mullet.