Toxic effects in juvenile sea cucumber Apostichopus japonicas (Selenka) exposure to benzo[a]pyrene

Sea cucumber physiological response to abiotic stress: Emergent contaminants and climate change

The ocean is facing a multitude of abiotic stresses due to factors such as climate change and pollution. Understanding how organisms in the ocean respond to these global changes is vital to better predicting consequences. Sea cucumbers are popular echinoderms with multiple ecological, nutritional, and pharmaceutical benefits. Here, we reviewed the effects of environmental change on an ecologically important echinoderm of the ocean, aiming to understand their response better, which could facilitate healthy culture programs under environmental changes and draw attention to knowledge gaps. After screening articles from the databases, 142 studies were included on the influence of emergent contaminants and climate variation on the early developmental stages and adults of sea cucumbers. We outlined the potential mechanism underlying the physiological response of sea cucumbers to emerging contaminants and climate change. It can be concluded that the physiological response of sea cucumbers to emergent contaminants differs from their response to climate change. Sea cucumbers could accumulate pollutants in their organs but are aestivated when exposed to extreme climate change. Research showed that the physiological response of sea cucumbers to pollutants indicates that these pollutants impair critical physiological processes, particularly during the more susceptible early phases of development compared to adults, and the accumulation of these pollutants in adults is often observed. For climate change, sea cucumbers showed gradual adaptation to the slight variation. However, sea cucumbers undergo aestivation under extreme conditions. Based on this review, critical suggestions for future research are presented, and we call for more efforts focusing on the co-occurrence of different stressors to extend the knowledge regarding the effects of environmental changes on these economically and ecologically important species.

Bisphenol A (BPA) Directly Activates the G Protein-Coupled Estrogen Receptor 1 and Triggers the Metabolic Disruption in the Gonadal Tissue of Apostichopus japonicus

The sea cucumber, Apostichopus japonicus, is a marine benthic organism that feeds on small benthic particulate matter and is easily affected by pollutants. Bisphenol A (BPA, 4,4'-isopropylidenediphenol) has been identified as an endocrine disruptor. It is ubiquitously detectable in oceans and affects a variety of marine animals. It functions as an estrogen analog and typically causes reproductive toxicity by interfering with the endocrine system. To comparatively analyze the reproductive effects of estradiol (E₂) and BPA on sea cucumbers, we identified a G protein-coupled estrogen receptor 1 (GPER1) in A. japonicus and investigated its effects on reproduction. The results showed that BPA and E₂ exposure activated A. japonicus AjGPER1, thereby mediating the mitogen-activated protein kinase signaling pathways. High-level expression of AjGPER1 in the ovarian tissue was confirmed by qPCR. Furthermore, metabolic changes were induced by 100 nM (22.83 μg/L) BPA exposure in the ovarian tissue, leading to a notable increase in the activities of trehalase and phosphofructokinase. Overall, our findings suggest that AjGPER1 is directly activated by BPA and affects sea cucumber reproduction by disrupting ovarian tissue metabolism, suggesting that marine pollutants pose a threat to the conservation of sea cucumber resources.

Immunotoxicity pathway and mechanism of benzo[a]pyrene on hemocytes of Chlamys farreri in vitro

Benzo[a]pyrene (B[a]P), a typical PAHs widely existing in the marine environment, has been extensively studied for its immunotoxicity due to its persistence and high toxicity. Nevertheless, the immunotoxicity mechanism remain incompletely understood. In this study, isolated hemocytes of Chlamys farreri were exposed at three concentrations of B[a]P (5, 10 and 15 μg/mL), and the effects of B[a]P on detoxification metabolism, signal transduction, humoral immune factors, exocytosis and phagocytosis relevant proteins and immune function at 0, 6, 12, 24 h were studied. Results illustrated the AhR, ARNT and CYP1A1 were significantly induced by B[a]P at 12 h. Additionally, the content of B[a]P metabolite BPDE increased in a dose-dependent manner with pollutants. Under B[a]P stimulation, the expressions of PTK (Src, Fyn) and PLC-Ca²⁺-PKC pathway gene increased significantly, while the transcription level of AC-cAMP-PKA pathway gene decreased remarkably. Additionally, the expressions of nuclear transcription factors (CREB, NF-κB), complement system genes and C-type lectin genes up-regulated obviously. The gene expressions of phagocytosis and exocytosis related proteins were also notably affected. 5 μg/mL B[a]P could promote phagocytosis in a transitory time, but with the increase of exposure time and concentration of B[a]P, the phagocytosis, antibacterial and bacteriolytic activities gradually decreased. These results indicated that similar to vertebrates, BPDE, the metabolite of B[a]P, mediated downstream signal transduction via PTK in bivalves. The declined of the immune defense ability of hemocytes might be closely related to the inhibition of AC-cAMP-PKA pathway and the imbalance of intracellular Ca²⁺ pathway. In addition, the results manifested that complement and lectin systems play a significant role in regulating immune response. In this study, the direct relationship between detoxification metabolism and immune signal transduction in bivalves under B[a]P stress was demonstrated for the first time, which provided important information for the potential molecular mechanism of B[a]P-induced immune system disorder in bivalves.

Synergistic Effects of Dietary Selenomethionine and Vitamin C on the Immunity, Antioxidant Status, and Intestinal Microbiota in Sea Cucumber (Apostichopus japonicus)

A 30-day feeding trial was carried out to investigate the interactive effects of dietary selenium (selenomethionine) and vitamin C (Vc) in Apostichopus japonicus. Two selenium (0 and 5 mg/kg) and three vitamin C (0, 5000, and 10,000 mg/kg) combined groups of feed were formulated (Designated as LSeLVc, LSeMVc, LSeHVc, HSeLVc, HSeMVc and HSeHVc, respectively) and fed the sea cucumbers. Our results showed no significant effects on the growth-related parameters in sea cucumber (P > 0.05). Furthermore, the reciprocal action between Se and Vc had significant (P < 0.05) effects on Se accumulation in the respiratory tree and intestines. Also, the lysozyme, glutathione peroxidase activity, and the relative expression levels such as LZM, GPX, Hsp70, and Hsp90 in different tissues were significantly increased in the group of sea cucumber fed diet with 5 mg Se in combination with 5000 mg Vc compared with the control group (P < 0.05). However, MDA and H2O2 contents in the body wall were significantly reduced in the HSeHVc group (P < 0.05). In addition, analysis of intestinal flora revealed that Haloferula abundance was highest in the LSeMVc group than other treatment groups, and Vibrio abundance was decreased with combined Se and Vc supplement. Finally, the species diversity of the gut microbial community of sea cucumber in HSeMVc group was lower than those in other treatment groups. The results showed that the interaction of selenium and vitamin C had positive effects on improving the immune status, antioxidant capacity, and digestive ability of A. japonicus.

The mechanism of Mitogen-Activated Protein Kinases to mediate apoptosis and immunotoxicity induced by Benzo[a]pyrene on hemocytes of scallop Chlamys farreri in vitro

Benzo [a]pyrene (B [a]P) has received widespread attention for serious pollution in the sea, which may reduce immunity and lead to the outbreak of disease in bivalves. However, the mechanism of immunotoxicity induced by B [a]P in bivalves was still unclear. Previous studies have found that Mitogen-Activated Protein Kinases (MAPKs) including three classic pathways (ERK, p38 and JNK) play an important role in mediating this process. Thus, in order to explore the mechanism of immunotoxicity induced by B [a]P in scallop Chlamys farreri, hemocytes were treated with PD98059 (ERK inhibitor), SB203580 (p38 inhibitor) and SP600125 (JNK inhibitor) for 1 h and then incubation with B [a]P for 24 h at 1 μg/mL. Indexes including oxidative damage, apoptotic rate, and immune indicators were detected in the present study. The results showed that the increase of Reactive Oxygen Species (ROS) and DNA damage induced by B [a]P was inhibited with PD98059 and SB203580. Besides, lysosomal membrane stability (LMS) damage was promoted by PD98059, while it was opposite when treated with SB203580. Moreover, the ascended apoptosis rate induced by B [a]P was increased significantly after treatment with PD98059, but it was remarkably attenuated by SB203580 and SP600125. However, the opposite pattern was showed in phagocytosis compared with apoptosis rate in all of three inhibitors. In addition, antibacterial activity and bacteriolytic activity were enhanced by SB203580 while inhibited by PD98059. Therefore, these results showed that MAPKs directly or indirectly mediate the decrease of oxidative damage, apoptosis and immune defense ability of C. farreri hemocytes, which suggesting ERK/p38/JNK pathways have different functions in the apoptosis and immunity of C. farreri hemocytes after B [a]P exposure. In conclusion, this study intended to enrich the theoretical basis for immunotoxicology of bivalves exposed to pollutants.

Effects of aerial exposure on oxidative stress, antioxidant and non-specific immune responses of juvenile sea cucumber Apostichopus japonicus under low temperature

Desiccation is a commonly stressful situation experienced by sea cucumber during transportation without/less water. The present study was conducted to investigate the effects of aerial exposure on the survival, oxidative damage, antioxidant capacity, immune-related response and gene expression of Apostichopus japonicus at different low temperatures. After acclimation, sea cucumbers were randomly divided into 3 groups, which were exposed to 5 °C, 10 °C and 15 °C in the closed laboratory condition, respectively. Each group has three parallel replicates. During the experiment, coelomic fluid and respiratory tree of A. japonicus were sampled at the time points of 0, 3, 6, 12, 24 and 48 h post-desiccation for further analysis. The results showed that the survival rates of sea cucumber significantly decreased as time prolonged, and those of 5 °C at 6-48 h of desiccation were significantly higher than 15 °C. Most oxidative damage parameters (e.g., O₂^- production, MDA, LPO and PC contents) significant increased after 6-12 h of desiccation. Antioxidant enzyme activities and T-AOC in coelomic fluid firstly increased and then decreased during aerial exposure, indicating that sea cucumber could adjust antioxidant defense to reduce the concentrations of ROS and MDA as a strategy for protecting organisms from oxidative damage in the early stage (0-6 h) of desiccation. The relative expression levels of Hsp90 and Hsp70 mRNA in respiratory tree of sea cucumber exhibited similar rise-fall trends with antioxidant parameters, while immune enzyme activities of ACP, AKP, LSZ and T-NOS, and gene expression of TLR, Rel and p105 all significantly decreased as time prolonged. Overall, low temperature postponed the process of ROS formation and the depression of antioxidant and non-specific immune responses of sea cucumber within a certain extent, which implied that it might play a positive role in improvement of desiccation tolerance. This study not only contribute to better understand the adaption mechanisms of A. japonicus to desiccation stress, but also provide valuable information for sea cucumber culture and transportation.

Transcriptomic changes in western mosquitofish (Gambusia affinis) liver following benzo[a]pyrene exposure

Widely distributed western mosquitofish (Gambusia affinis) has been used as a new model species for hazard assessment of environmental stressors such as polycyclic aromatic hydrocarbons (PAHs). However, most of the PAH studies using G. affinis rely on targeted biomarker-based analysis, and thus may not adequately address the complexity of the toxic mechanisms of the stressors. In the present study, the whole transcriptional sequencing of G. affinis liver after exposure to a PAH model, benzo[a]pyrene (BaP) (100 μg/L), for 20 days was performed by using the HiSeq XTen sequencers. In total, 58,156,233 and 51,825,467 clean nucleotide reads were obtained in the control and BaP-exposed libraries, respectively, with average N50 lengths of 1419 bp. In addition, after G. affinis was exposed for 20 days, 169 genes were upregulated, and 176 genes were downregulated in liver. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses were applied to all the genes to determine the genes' biological functions and processes. The results clearly showed that the differentially expressed genes were mainly related to immune pathways and metabolic correlation pathways. Interestingly, almost all the pathways related with the immunity were upregulated, while the metabolism pathways were downregulated. Lastly, quantitative real-time PCR (qRT-PCR) was performed to measure expressional levels of twelve genes confirmed through the DGE analysis. These results demonstrate that BaP damages immunity and enhances the consumption of all available energy storage to activate mechanisms of the detoxification in G. affinis. Up until now, the present study is the first time that a whole transcriptome sequencing analysis in the liver of G. affinis exposed to BaP has been reported.

The effects of benzo[a]pyrene on the composition of gut microbiota and the gut health of the juvenile sea cucumber Apostichopus japonicus Selenka

The gut microbiota is essential for health and physiological functions in the host organism. However, the toxicological evaluation of environmental pollutants on the gut microbiota is still insufficient. In the present study, the juvenile sea cucumber Apostichopus japonicus was exposed for 14 days to Benzo[a]pyrene (BaP), which is a model polycyclic aromatic hydrocarbon (PAH), at four different concentrations (0, 0.5, 5, and 25 μg/L). We analyzed the intestinal microbial community of A. japonicas using 16S rRNA gene amplicon sequencing. Our results demonstrate that BaP exposure caused alterations to the microbiome community composition in sea cucumbers. At the phylum level, Planctomycetes were significantly more abundant in BaP exposure groups at 14 d compared with the control group, and the abundance of Proteobacteria and Bacteroidetes increased while the abundance of Firmicutes decreased following BaP exposure. At the genus level, multiple beneficial and autochthonous genera declined in the BaP treatment groups compared to the control, including Lactococcus, Bacillus, Lactobacillus, Enterococcus, Leuconostoc and Weissella; however, a bloom of alkane-degrading bacteria was found in BaP-exposed guts and included Lutibacter, Pseudoalteromonas, Polaribacter, Rhodopirellula and Blastopirellula. Furthermore, histological morphology, enzymatic activity and gene expression analysis revealed that BaP exposure also negatively impacted gut structure and function and presented as inflammation or atrophy, oxidative stress and immune suppression in sea cucumber intestines. Collectively, these findings provide insights into the toxic effects of BaP exposure on A. japonicas associated with intestinal microbiota and health.

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.

Benzo[a]pyrene (BaP) exposure generates persistent reactive oxygen species (ROS) to inhibit the NF-κB pathway in medaka (Oryzias melastigma)

Benzo[a]pyrene (BaP), a common environmental pollutant, can modulate the immune-associated signal pathway NF-κB, which is one of the critical signal pathways involved in various immune responses. BaP exposure usually generates reactive oxygen species (ROS), but whether ROS are predominantly involved in the modulation mechanism of the NF-κB pathway has not been clearly understood. In this study, an in vivo examination of Oryzias melastigma demonstrated that BaP exposure led to a down-regulation of the NF-κB pathway and increased levels of ROS. Conversely, in vitro results using the medaka liver cell line DIT-29 and a widely applied H₂O₂ method showed the opposite: up-regulation of the NF-κB pathway. However, the down-regulation of NF-κB upon BaP exposure in vitro was inhibited by the addition of a ROS inhibitor, indicating ROS are involved in the modulation of NF-κB. The discrepancy between in vivo and in vitro results of ROS impacts on NF-κB activation might be related to the concentration and persistence of ROS. Using a modified luminol detection system, BaP was found to generate sustained physiological concentrations of ROS for 24 h, while an H₂O₂ bolus generated ROS for less than 30 min. Furthermore, a steady-state sub-micromolar H₂O₂ system (H₂O₂ss) was developed in parallel as a positive control of ROS, by which H₂O₂ could be maintained for 24 h. Comparative evaluation using H₂O₂, H₂O₂ss and BaP exposures on the medaka cell line with pGL4.32 demonstrated that the persistent physiological concentrations of ROS generated upon BaP exposure or treatment with H₂O₂ss inhibited the NF-κB pathway, but direct H₂O₂ exposure had the opposite effect. Moreover, a western-blot assay and EMSA detection further confirmed the modulation of the NF-κB pathway in DIT-29. Taken together, this study shows that BaP exposure inhibits the NF-κB pathway by generating sustained physiological concentrations of ROS.

Transcriptomic analysis revealing hepcidin expression in Oryzias melastigma regulated through the JAK-STAT signaling pathway upon exposure to BaP

Our previous study revealed that an antimicrobial peptide hepcidin, can be significantly up-regulated either with LPS challenge or upon exposure to Benzo[a]pyrene (BaP) in red sea bream, but the molecular mechanism involved in whether the transcriptional expression of hepcidin induced by LPS or BaP is regulated through a similar signaling pathway is not yet known. To elucidate the underlying molecular mechanism, the marine model fish Oryzias melastigma was exposed to 1 μg/L BaP as well as challenged with 5 μg of LPS per fish. Samples at 3 h post-LPS challenge, and 2 d and 3 d post-BaP exposure were separately collected for transcriptome analysis. General analysis of the predicted immune-associated unigenes based on the transcriptomic data showed that the percentages of modulated immune-associated genes were 7% with LPS challenge, and 3% and 7% with BaP exposure at 2 and 3 days, respectively. Genes involved in functions like antimicrobial activity, neutrophil activation, and leukocyte chemotaxis were up-regulated with LPS challenge, whereas more than half of the immune associated genes including the KLF family were down-regulated upon BaP exposure, indicating a difference in the modulated immune genes between LPS challenge and BaP exposure. Specific comparative analyses of the immune-associated signal pathways NOD, TOLL, NF-κB and JAK-STAT with LPS challenge or upon exposure to BaP, indicated that most of the modulated genes in association with the NOD, TOLL and NF-κB pathways were induced with LPS challenge but only a few after exposure to BaP, suggesting that BaP exposure was generally not associated with any of the three signal pathways. Interestingly, further transcriptomic analysis revealed that 5 of the 8 modulated genes associated with the JAK-STAT pathway were down-regulated, while 2 inhibiting genes were up-regulated after BaP exposure for 2 days whereas LPS challenge resulted in only less than half modulated, suggesting the possibility of down-regulation caused by BaP exposure through JAK-STAT pathway. Further testing using an EPC cell culture demonstrated that expression of the hepcidin1 gene was less involved in the known signal pathways, such as c/EBP, BMP, and NF-κB, but instead mostly in association with the JAK-STAT pathway upon BaP exposure.

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

Bioflocs are not only a source of supplemental nutrition but also provide substantial probiotic bacteria and bioactive compounds, which play an important role in improving physiological health of aquatic organisms. A 60-day experiment was conducted to investigate the growth, intestinal microbiota, non-specific immune response and disease resistance of sea cucumber in biofloc systems with different carbon sources (glucose, sucrose and starch). Control (no biofloc) and three biofloc systems were set up, and each group has three replicates. The results showed that biofloc volume (BFV) and total suspended solids (TSS) increased in the sequences of glucose > sucrose > starch and green sea cucumber > white sea cucumber during the experiment. The highest specific growth rates (SGRs) were observed in biofloc system with glucose as carbon source, which also had relatively lower glucose, lactate and cortisol levels in coelomic fluid and higher glycogen content in muscle compared to other groups. There were significant increased Bacillus and Lactobacillus counts of sea cucumber intestine in biofloc systems, and the activities of superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPx) also showed obvious ascending trends. Significant increases in total coelomocytes counts (TCC), phagocytosis, respiratory burst, complement C3 content and lysozyme (LSZ) and acid phosphatase (ACP) activities of sea cucumber were all found in biofloc system (glucose). The expression patterns of most immune-related genes (i.e. Hsp90, Hsp70, c-type lectin (CL), toll-like receptor (TLR)) were up-regulated, suggesting the promotion of pathogen recognition ability and immune signaling pathways activation by biofloc. Furthermore, green and white sea cucumber had significantly higher survival rates in biofloc systems during the 14-day challenge test. In conclusion, biofloc technology could improve growth and physiological health of A. japonicus, by optimizing intestinal microbiota, strengthening antioxidant ability, enhancing non-specific immune response and disease resistance against pathogens, meanwhile glucose was recommended as optimal carbon source in biofloc system of sea cucumber culturing.

Effects of dietary supplementation of four strains of lactic acid bacteria on growth, immune-related response and genes expression of the juvenile sea cucumber Apostichopus japonicus Selenka

A feeding experiment was conducted to evaluate the effects of four strains of lactic acid bacteria (LAB) [i.e. Lactobacillus plantarum LL11 (LP), Weissella confuse LS13 (WC), Lactococcus lactis LH8 (LL) and Enterococcus faecalis LC3 (ES)] isolated from marine fish on growth, immune response and expression levels of immune-related gens in body wall of juvenile sea cucumber Apostichopus japonicus. As a result, sea cucumber had better growth performance fed supplementation of LP and ES than the control group (P < .05). Survival rate in each LAB supplementation group was significantly higher than that in control group after Vibrio splendidus challenge (P < .05). In regards to the enzyme activities, LP supplementation significantly imporved the enzyme activities of alkaline phosphatase (AKP) (P < .05), acid phosphatase (ACP) and superoxide dismutase (SOD), ES supplementation significantly imporved AKP activity (P < .05), and WC supplementation significantly imporved ACP activity (P < .05). However, lysozyme (LSZ) activity was not significantly changed in the four LAB supplementation treatments (P > .05). For the gene expression levels, different expression patterns were observed among four groups, heat shock proteins (HSP60, HSP70 and HSP90) and caspase-2 showed dramatic up-regulation at 30 d while NF-kappa-B transcription factor p65 was down-regulated at 15 d and up-regulated at 30 d, and nitric oxide synthase was down-regulated at both timepoints in almost all the four groups. In conclusion, the four LAB strains screened from marine fish supplemented in diets indicated positive effects on immune response for A. japonicus, especially, the L. plantarum LL11 and E. faecalis LC3 indicated better growth performance.

Dietary supplementation of biofloc influences growth performance, physiological stress, antioxidant status and immune response of juvenile sea cucumber Apostichopus japonicus (Selenka)

Bioflocs are rich in various probiotics and bioactive compounds, which play an important role in improving growth and health status of aquatic organisms. A 60-day experiment was conducted to investigate the effects of dietary supplementation of biofloc on growth performance, digestive enzyme activity, physiological stress, antioxidant status, expression of immune-related genes and disease resistance of sea cucumber Apostichopus japonicus. Juvenile sea cucumbers were fed five experimental diets containing graded levels of biofloc from 0% to 20% (referred as B0, B5, B10, B15 and B20, respectively). The results showed that the sea cucumbers at dietary supplementation levels of 10%-15% biofloc had significantly higher specific growth rate (SGR) compared to control group (diet B0). Digestive enzyme activity increased with the increasing of dietary biofloc level, while no significant difference was found between diets B15 and B20. Dietary supplementation of biofloc also had significant influences on physiological stress parameters except for lactate. There was no significant discrepancy in total coelomocytes counts (TCC) in coelomic fluid of sea cucumber between the treatments. Phagocytosis and respiratory burst of cellular immune at 15% and 20% biofloc levels were significantly higher than those of control group. Significant increases in superoxide dismutase (SOD), total nitric oxide synthase (T-NOS), lysozyme (LSZ), acid phosphatase (ACP) and alkaline phosphatase (AKP) activities of sea cucumber were found at highest dietary supplementation level of 20% biofloc. The expression patterns of immune-related genes (i.e., Hsp90, Hsp70, p105, Rel, NOS and LSZ) in tissues of sea cucumber were analyzed between the experimental diets, and a general trend of up-regulation was observed at higher biofloc levels. Furthermore, dietary 10%-20% biofloc significantly reduced cumulative mortality of sea cucumber after being challenged with Vibrio splendidus. In conclusion, dietary supplementation of biofloc could improve growth performance of A. japonicus, by increasing digestive enzyme activity, releasing physiological stress, enhancing immune response and disease resistance of sea cucumber. The suitable supplemental level of approximately 15% biofloc was recommended in the present study.

Benzo[a]pyrene exposure under future ocean acidification scenarios weakens the immune responses of blood clam, Tegillarca granosa

Persistent organic pollutants (POPs) are known to converge into the ocean and accumulate in the sediment, posing great threats to marine organisms such as the sessile bottom burrowing bivalves. However, the immune toxicity of POPs, such as B[a]P, under future ocean acidification scenarios remains poorly understood to date. Therefore, in the present study, the impacts of B[a]P exposure on the immune responses of a bivalve species, Tegillarca granosa, under present and future ocean acidification scenarios were investigated. Results obtained revealed an increased immune toxicity of B[a]P under future ocean acidification scenarios in terms of reduced THC, altered haemocyte composition, and hampered phagocytosis, which may attribute to the synergetic effects of B[a]P and ocean acidification. In addition, the gene expressions of pathogen pattern recognition receptors (TLR1, TLR2, TLR4, TLR6), pathway mediators (TRAF6, TAK1, TAB2, IKKα and Myd88), and effectors (NF-ĸB) of the important immune related pathways were significantly down-regulated upon exposure to B[a]P under future ocean acidification scenarios. Results of the present study suggested an increased immune toxicity of B[a]P under future ocean acidification scenarios, which will significantly hamper the immune responses of T. granosa and subsequently render individuals more susceptible to pathogens challenges.