Crowding of white shrimp Litopenaeus vananmei depresses their immunity to and resistance against Vibrio alginolyticus and white spot syndrome virus

Effects of high stocking density on growth performance and expression of MyD88, and its temporal expression under the challenge of Vibrio parahaemolyticus in the noble scallop Chlamys nobilis

High stocking density has been regarded as an adverse factor in bivalve aquaculture. However, its subsequent molecular response to pathogenic bacteria has been little studied. In order to study the question, a novel MyD88 was first cloned using adult noble scallops Chlamys nobilis (CnMyD88), and its tissue distribution was investigated. Then, 1860 juvenile scallops were divided into two groups with two initial densities of high density (200 individuals/layer, HD) and normal density (110 individuals/layer, ND) and in-situ cultured for three months, in which their growth, survival, and the differential expression of CnMyD88 were examined, respectively. Finally, scallops were injected with the Vibrio parahaemolyticus to assess the temporal expression of CnMyD88. As the results show, CnMyD88 cDNA has a full length of 2241 bp and contains an 1107 bp ORF that encodes a 368-derived protein. It was widely expressed in examined tissues with a significantly higher level in hemolymph, intestine, mantle, and gonad than others. Besides, the HD group showed lower growth (0.39 ± 0.05 mm/day) and survival (37.00 ± 8.49%) than the ND group (0.55 ± 0.02 mm/day and 76.82 ± 5.78%). More importantly, the HD group exhibited significantly lower expression levels of CnMyD88 in their examined tissues than the ND group. After V. parahaemolyticus challenging, CnMyD88 had significantly lower expression levels in the scallops from the HD group than that of the scallops from the ND group at 6th, 24th, and 36th. The present results indicated that high stocking density not only made adverse impacts on growth and survival but also may induce immunosuppression in the noble scallop. Therefore, appropriate low stocking density may be worth considering to adopt in scallop aquaculture.

Growth, immunity and transcriptome response to different stocking densities in Litopenaeus vannamei

The effects of different stocking densities on Litopenaeus vannamei were investigated from the aspects of growth performance, immune response and transcriptome in this experiment. L. vannamei (initial body weight: 0.30 ± 0.02 g) were reared for 8 weeks at three stocking densities of 100 (LSD), 200 (MSD) and 300 (HSD) shrimp/m³, respectively. The results showed that the survival rate (SR), final body weight (FBW), weight gain rate (WGR), specific growth ratio (SGR) and protein efficiency ratio (PER) of L. vannamei significantly decreased, while the feed factor (FCR) significantly increased with the increase of stocking density. After Vibrio parahemolyticus infection, the SR of L. vannamei in the HSD group was significantly lower than that in the LSD and MSD groups. Increasing stocking density significantly increased the activities of aspartate aminotransferase (AST), alanine aminotransferase (ALT) and lysozyme (LYS) while significantly decreased the activities of catalase (CAT) and phenol oxidase (PO) in the serum of L. vannamei. Similar changes of the gene expression as the activities of immune enzymes were found in the hemocytes. Pairwise comparison between the LSD, MSD and HSD group in the transcriptome analysis identified that there were 304, 1376 and 2083 differentially expressed genes (DEGs) in LSD vs MSD, MSD vs HSD and LSD vs HSD, respectively. Among them, most of the immune-related DEGs were down-regulated and metabolism-related DEGs were up-regulated with the increasing stocking density. In addition, KEGG enrichment pathway analysis revealed that several immune and metabolic related pathways including PI3K-Akt signaling pathway and AMPK signaling pathway were significantly enriched. Of these, the PI3K-Akt signaling pathway had the most DEGs and was also the most significantly enriched pathway. Furthermore, 16 DEGs (such as FOXO, PCK2 and CTSC, etc.) and partial immune enzyme activity (such as AST, CAT and PO, etc.) changes were closely correlated with the increase of stocking density when partial immune-related DEGs and immune-related enzymes were analyzed jointly. All these results indicated that changes in stocking density had a significant effect on the growth performance, immunity and transcriptome of L. vannamei.

Effects of stocking density on the growth performance, mitophagy, endocytosis and metabolism of Cherax quadricarinatus in integrated rice-crayfish farming systems

Red claw crayfish (Cherax quadricarinatus) is an economic freshwater shrimp with great commercial potential. However, the suitable stocking density of C. quadricarinatus is still unclear in integrated rice-crayfish farming system. Thus, this study aimed to investigate the effects of stocking density on growth performance, mitophagy, endocytosis and metabolism of C. quadricarinatus. The C. quadricarinatus was reared at low density (LD, 35.73 g/m²), middle density (MD, 71.46 g/m²) and high density (HD, 107.19 g/m²) in an integrated rice-crayfish farming system. After 90 days of farming, the growth performance of C. quadricarinatus significantly decreased in the MD and HD groups relative to that in the LD group. The HD treatment caused oxidative stress and lipid peroxidation at the end of the experiment in hepatopancreas. Transcriptome analysis showed that there were 1,531 DEGs (differently expressed genes) between the LD group and HD group, including 1,028 upregulated genes and 503 downregulated genes. KEGG (Kyoto Encyclopedia of Genes and Genomes) enrichment analysis indicated that the DEGs were significantly enriched in endocytosis and mitophagy pathways. Meanwhile, four lipid metabolism pathways, including biosynthesis of unsaturated fatty acids, fatty acid biosynthesis, glycerolipid metabolism and glycerophospholipid metabolism, exhibited an upregulated tendency in the HD group. In conclusion, our data showed that when the stocking density reached up to 207.15 g/m² in HD group, the growth performance of C. quadricarinatus was significantly inhibited in this system. Meanwhile, the data indicated that C. quadricarinatus may respond to the stressful condition via activating antioxidant defense system, endocytosis, mitophagy and metabolism-related pathways in hepatopancreas.

Microplastic-Contaminated Feed Interferes with Antioxidant Enzyme and Lysozyme Gene Expression of Pacific White Shrimp (Litopenaeus vannamei) Leading to Hepatopancreas Damage and Increased Mortality

Microplastic pollution can interfere with aquatic animal health and nonspecific immunity, increasing the potential for pathogen infection in crustaceans. However, the long-term effects of microplastics on crustacean immunity are less understood, especially regarding their toxicity in Pacific white shrimp (Litopenaeus vannamei). Effects of high-density polyethylene microplastics (HDPE-MPs) in feed on the mortality rate, hepatopancreas, and nonspecific immune system gene expression of Pacific white shrimp are presented. The LC₅₀ at day 28 of HDPE-MP exposure was determined as 3.074% HDPE-MP in feed. A significant upregulation of the superoxide dismutase (SOD) and glutathione peroxidase (GPx) genes was observed in shrimp that were fed with 0.1 and 0.5% of HDPE-MP; then, they were downregulated significantly, except for the SOD gene expression of shrimp fed with 0.1% of HDPE-MP. The lysozyme (LYZ) gene was upregulated significantly in shrimp that were fed with 0.5, 1, and 3% HDPE-MP for 7 days and downregulated significantly in HDPE-receiving groups for at least 14 days. Significant histopathological changes in the hepatopancreas were observed in the treatment groups. The histopathological score of each lesion was correlated with the increase in HDPE-MP concentration. This study shows that the ingestion of HDPE microplastics can alter the expression of nonspecific immune system genes and damage the hepatopancreas in Pacific white shrimp.

Assessment of Performance, Microbial Community, Bacterial Food Quality, and Gene Expression of Whiteleg Shrimp (Litopenaeus vannamei) Reared under Different Density Biofloc Systems

Biofloc shrimp culture, as a way of improving shrimp production, gains worldwide consideration. However, the effects of the biofloc system on shrimp culture at high densities could be a challenge. Here, this study is aimed at identifying a better stocking density of whiteleg shrimp (Litopenaeus vannamei) between two intensive biofloc systems of 100 and 300 org./m². Achieving that was done by comparing growth performance, water quality, feed utilization, microbial loads from water and shrimps, and gene expression of growth, stress, and immune-related genes. Shrimp postlarvae with a mean weight of 35.4 ± 3.7 mg were reared in six indoor cement tanks (36 m³ total capacity each) at two stocking densities (3 replicates each) for a rearing period of 135 days. Better final weight, weight gain, average daily weight gain, specific growth rate, biomass increase percentage, and survival rate were associated with lower density (100/m²), whereas high-density showed significantly higher total biomass. Better feed utilization was found in the lower density treatment. Lower density treatment enhanced water quality parameters, including higher dissolved oxygen and lower nitrogenous wastes. Heterotrophic bacterial count in water samples was recorded as 5.28 ± 0.15 and 5.11 ± 0.28 log CFU/ml from the high- and low-density systems, respectively, with no significant difference. Beneficial bacteria such as Bacillus spp. were identified in water samples from both systems, still, the Vibrio-like count was developed in the higher density system. Regarding shrimp food bacterial quality, the total bacterial count in the shrimp was recorded as 5.09 ± 0.1 log CFU/g in the 300 org./m² treatment compared to 4.75 ± 0.24 log CFU/g in the lower density. Escherichia coli was isolated from the shrimps in a lower density group while Aeromonas hydrophila and Citrobacter freundii were associated with shrimps from a higher density system. Immune-related genes including prophenoloxidase, superoxide dismutase (SOD), and lysozyme (LYZ) expressions were all significantly higher expressed in the shrimp from the lower density treatment. Toll receptor (LvToll), penaiedin4 (PEN4), and stress-related gene (HSP 70) showed a decreased gene expression in the shrimp raised in the lower density. Significant upregulation of growth-related gene (Ras-related protein-RAP) expression was associated with the lower stocking density system. In conclusion, the current study found that applying high stocking density (300 org./m²) contributes negatively to performance, water quality, microbial community, bacterial food quality, and gene expression of immune, stress, and growth-related genes when compared with the lower stocking density system (100 org./m²) under biofloc system.

Effects of high stocking density on the galectin gene expression in noble scallop Chalmys nobilis under bacterial infection

High stocking densities have been shown to have adverse effects on the physiology of bivalves. The noble scallop Chlamys nobilis is one of the most important cultured shellfish in Southern China. However, the effects of scallop stocking density on its immunity is not well understood. In this context, this study was conducted to assess the effect of high stocking density on the galectin (an important protein in innate immunity) gene expression of C. nobilis during bacterial infection. A full-length galectin (CnGal) gene was cloned. The ORF of the CnGal cDNA encodes a predicted protein containing 549 aa with four CRDs and no signal peptide. Our results reveal that high stocking density in the scallop not only led to high mortality and slow growth, but also changed tissue distribution of the CnGal expression. The individuals from the high stocking density group exhibited more differences among tissues than those from the control group, but the highest expression were both recorded in hemolymph. After the Vibrio parahaeomlyticus challenge, the gene's expression levels were all significantly up-regulated in the hemolymph and gill, but the time up to peak was different between the two tissues. The findings of this study could fill a gap in knowledge about how high stocking density affect scallop immunity at the molecular level.

Differentially proteomic analysis of the hemocytes against Aeromonas hydrophila infection in oriental river prawn Macrobrachium nipponense by iTRAQ approach

As the direct executors of biological function, the expression level of proteins in host will reveal the molecular mechanisms regulating bacteria infection more directly. In the present study, the differential proteomes of Macrobrachium nipponense hemocytes response to Aeromonas hydrophila infection were identified with isobaric tags for relative and absolute quantitation (iTRAQ) labeling followed by liquid chromatography electrospray ionization tandem mass spectrometry. The hemocyte proteins from the unchallenged and A. hydrophila challenged prawn, M. nipponense, at 12, 24 and 36 h post infection were compared. From this, a total of 3372 proteins were identified and 1014 proteins were considered differentially expressed, of which 117 common differentially expressed proteins were indicated between the time points. Hierarchical clustering, Gene Ontology, Kyoto Encyclopedia of Genes and Genomes enrichment and protein-protein interaction network analyses were performed for the general characterization of overall enriched proteins. Cytoskeletal proteins including myosin heavy chain, myosin regulatory light chain, actin, tubulin alpha/beta chain, troponin I and troponin T as well as antioxidant enzymes such as catalase and cytosolic MnSOD were found significantly up-regulated in hemocytes, indicating that the phagocytosis process and ROS system were induced after challenge with A. hydrophila. And other proteins such as integrin β, innexin inx2-like and heat shock protein 60 also participate in prawn immune response against bacteria. Parallel reaction monitoring analyses were carried out for validation of the expression levels of differentially expressed proteins, which indicated high reliability of the proteomic results. This is the first report on proteome of M. nipponense hemocytes against A. hydrophila infection, which contributes to better understanding on the molecular mechanisms of prawns.

Metabolite profiling of whiteleg shrimp Litopenaeus vannamei from super-intensive culture in closed aquaculture systems: a recirculating aquaculture system and a hybrid zero water discharge-recirculating aquaculture system

INTRODUCTION

The production of the whiteleg shrimp Litopenaeus vannamei now accounts for approximately 75% of the total shrimp production in Indonesia. The techniques used to produce whiteleg shrimp in Indonesia are still dominated by conventional rearing strategies using open-pond systems, which often contribute to unpredictable culture performance and weak sustainability. Alternative production strategies of closed aquaculture systems, including the recirculating aquaculture system (RAS) and hybrid zero water discharge-recirculating aquaculture system (hybrid system), have been developed and implemented for higher productivity, stability and sustainability of whiteleg shrimp grow-out production in Indonesia. Despite the positive aspects of the application of closed aquaculture systems in shrimp aquaculture, the differences in the characteristics of shrimp grown in closed RAS and hybrid systems compared to open-pond systems remain unclear.

OBJECTIVE

This study aims to investigate the differences in the metabolite profiles of shrimp grown in intensive closed aquaculture systems, including an RAS and hybrid system, compared to those of shrimp grown in a semi-intensive, open, earthen pond system by means of non-targeted GC-MS metabolite profiling.

METHODS

Shrimp cultured in the closed systems (RAS and hybrid system) and an open system (pond) were harvested and subjected to GC-MS non-targeted metabolomics analysis. A total of 112 metabolites were annotated from shrimp samples and subjected to principal component analysis (PCA).

RESULTS

The metabolites annotated from GC-MS mainly included organic compounds, proteinogenic and non-proteinogenic amino acids, sugars, nucleosides and fatty acids. The results of principal component analysis showed several metabolites with high variable importance in projection (VIP) scores, including shikimic acid, β-alanine, uric acid, hypoxanthine, inosine, homocysteine, methionine, phenylalanine, tryptophan and lysine, as the main metabolites differentiating the shrimp grown in the three production systems.

CONCLUSION

Our findings showed that shrimp cultured in different aquaculture systems exhibited distinct metabolite profiles, and the metabolites showing high VIP scores, including shikimic acid, β-alanine, uric acid, hypoxanthine, inosine, homocysteine, methionine, phenylalanine, tryptophan and lysine, may serve as candidate markers to indicate the differences in shrimp from different production systems.

Mitochondrial manganese superoxide dismutase from the shrimp Litopenaeus vannamei: Molecular characterization and effect of high temperature, hypoxia and reoxygenation on expression and enzyme activity

Climate warming has been increasing ocean water temperature and decreasing oxygen concentrations, exposing aquatic organisms to environmental stress conditions. The shrimp Litopenaeus vannamei manages to survive these harsh environmental conditions by enhancing their antioxidant defenses, among other strategies. In this study, we report the mitochondrial manganese superoxide dismutase (mMnSOD) nucleotide and deduced amino acid sequences and its gene expression in L. vannamei tissues. The deduced protein has 220 amino acids with a signal peptide of 20 amino acids. Expression of mMnSOD was analyzed in hepatopancreas, gills and muscle, where gills had highest expression in normoxic conditions. In addition, shrimp were subjected to high temperature, hypoxia and reoxygenation to analyze the effect on the expression of mMnSOD and SOD activity in mitochondria. High temperature and hypoxia showed a synergistic effect in the up-regulation on expression of mMnSOD in gills and hepatopancreas. Moreover, induction in SOD activity was found in the mitochondrial fraction from gills of normoxia at high temperature, probably due to an overproduction of reactive oxygen species caused by an elevated metabolic rate due to the stress temperature. These results suggest that the combined stress conditions of hypoxia and high temperature trigger molecularly the antioxidant response in L. vannamei in a higher degree than only one stressor.

Evaluation of immunological response in shrimp Penaeus vannamei submitted to low temperature and air exposure

The objective of this study is to explore the immune response of the shrimp Penaeus vannamei to low temperature and air exposure during the mimic waterless transportation. Shrimp were cold shocked at 13 °C for 3 min, then exposed to air at 15 °C for 12 h, and finally revived in water at 25 °C. Hemocyte structure remained intact with only slight distortions of some organelles and nuclear membrane under the stress. Phenoloxidase (PO), lysozyme (Lys) and gamma-glutamyl transferase (GGT) activities, glutamine (Gln) content and relative mRNA expressions of prophenoloxidase (proPO), β-1,3-glucan binding protein (LGBP), ferrin (Fer) and glucose regulated protein 78 (GRP 78) increased and reached peak levels after 3 h-9 h of air exposure, and then decreased to relatively stable levels in the prolonged period of air exposure. The total hemocyte count (THC) and gene expressions of proPO, Fer and LGBP at the end of waterless storage were significantly lower (p < 0.05) than those in fresh shrimp while no significant difference (p ≥ 0.05) was found between revived and fresh shrimp in PO, Lys, GGT activities, Gln content and gene expression level of GRP 78. Of all the hemocytes, the percentage of semi granular cells (SGC) and granular cells (GC) significantly decreased after 6-9 h of storage, while hyaline cells (HC) dramatically increased after 9 h of storage to compensate the loss of SGC and GC. Results suggested that the low temperature and air exposure caused significant immunological response to the shrimp, but the damages to the immune system were partly reparable.

The effect of rearing density on immune responses of hepatopancreas and intestine in Litopenaeus vananmei against Vibrio paraheamolyticus E1 challenge

Rearing density and disease management are considered as pivotal factors determining shrimp farm productivity and profitability. To systematically investigate the potential mechanisms for density-related differences between disease susceptibility and rearing densities, we conducted comparative transcriptome analysis of the molecular differences between hepatopancreas and intestine of Litopenaeus vannamei under two different rearing densities (800- and 400- shrimp/m³) for 15 d and further analyzed the differences in immune response to Vibrio parahaemolyticus E1 (VPE1) raised under two density conditions. Totally 45 different expression genes (DEGs) were identified in the hepatopancreas under two different rearing densities, the DEGs were grouped into four processes or pathways related to animal immune system. Then, exposure to the VPE1 resulted in 639 DEGs, involved into fourteen immune related processes or pathways. In the intestine, seventeen processes or pathways related to the immune system were identified among the 5470 DEGs under two different rearing densities. 279 DEGs were identified post VPE1 challenge, classified into five processes or pathways associated with the immune system. Meanwhile, the results of growth performance, histopathology and the activities of antioxidant enzymes in the hepatopancreas and intestines of shrimp showed that high density decreased weight gain rate (63.20 ± 1.67% and 18.73 ± 3.35% in the high and low rearing density groups, respectively), severely destroyed the histopathology and inhibited the antioxidant enzymes activities. This study demonstrated that rearing density in L. vannamei significantly impacts susceptibility to the VPE1, via altered transcriptional challenge responses, and thus higher mortality due to disease.

Molecular characterization of a novel white spot syndrome virus response protein (dubbed LvWRP) from Litopenaeus vannamei

White spot syndrome, which is caused by white spot syndrome virus (WSSV), is a highly contagious disease of penaeid shrimp. However, there is currently incomplete understanding of the infection mechanism and pathogenesis of WSSV. In this study, a novel gene of a previously uncharacterized WSSV response protein (LvWRP) in Litopenaeus vannamei was identified and characterized. The LvWRP gene has an open reading frame (ORF) of 879 bp encoding a putative protein of 292 amino acids. Sequence analysis revealed that LvWRP shared 24.9% identity with an uncharacterized protein of Penaeus monodon nudivirus. Real-time qPCR analysis showed that LvWRP was ubiquitously expressed in shrimp tissues, with transcript levels induced in hemocytes upon immune challenge with Vibrio parahaemolyticus, Streptoccocus iniae, lipopolysaccharide (LPS), and WSSV. In addition, RNA interference-mediated knockdown of LvWRP followed by WSSV challenge revealed significant decrease in the transcript levels of WSSV IE1 and VP28 genes coupled with a reduction in WSSV copies in shrimp hemocytes. Moreover, depletion of LvWRP followed by WSSV challenge significantly increased the transcript levels of Vago4 and Vago5 as well as increased the phosphorylation of STAT, while hemocytes apoptosis in terms of caspase 3/7 activity was decreased. These results suggest that LvWRP is important for WSSV replication in shrimp, and therefore one of the vital host factors in WSSV infection.

Effects of stocking density on the growth performance, bacterial load and antioxidant response systems of noble scallop Chlamys nobilis

Stocking density is a crucial factor in shellfish aquaculture that affects overall growth performance and health status. Present study analyzes the effects of stocking densities on growth, survival and hemolymph immune status of noble scallop Chlamys nobilis. The scallops with the same size were separately placed in the lantern cages (10 layers per cage) using high stocking density (500 scallops per cage) and low stocking density (100 scallops per cage) and cultivated in the same location for 60 days. The results indicated that the scallops cultivated at high stocking density had significantly higher mortality and slower growth than those cultivated at low stocking density. Moreover, the hemolymph of scallops cultivated at high density showed significantly higher bacterial load, higher reactive oxygen species (ROS), higher expression level of Nrf2 and lower expression level of Keap1, as well as lower antibacterial ability of Vibrio parahemolyticus than that of scallops cultivated at low density. The present results demonstrated that long-term overcrowding is detrimental for the scallops, which can not only lead to high mortality and slow growth, but also cause more vulnerable to pathogenic bacteria. Therefore, we speculated that high stocking density culture practice of scallops in China might be the root of infectious bacteria outbreaks.

Influence of stocking density on growth, digestive enzyme activities, immune responses, antioxidant of Oreochromis niloticus fingerlings in biofloc systems

A 120-day feeding trial was conducted to evaluate the effect of different stocking densities on growth, the non-specific immunities, antioxidant status and digestive enzyme activities of Oreochromis niloticus fingerlings under a zero-water exchange biofloc system. Tilapias (0.51 ± 0.05 g) were randomly distributed in twelve tanks, each with 300 L water. The experimental design was completely randomized using three replications with four treatments 166 orgs m^-3 (LD, low density), 333 orgs m^-3 (MD, middle density) and 600 orgs m^-3 (HD, high density) with glucose added as biofloc groups, and a clear water group without glucose added as a control 333 orgs m^-3. The fish cultured in LD and MD group showed higher final body weight. For the digestive enzymes, the lipase, trypsin, and amylase activities were all depressed in HD group and control group. Regarding the immune and antioxidant abilities, significantly lower values (P < 0.05) of the lysozyme, complement 3, and glutathione were observed for the fish that reared in the control group and HD group. The stress indicator, the cortisol, 5-hydroxytryptamine, and glucose concentrations were also depressed in HD group and control group, meanwhile the alanine aminotransferase, aspartate transaminase and alkaline phosphatase were all higher in HD group and control group. The significant higher survival was observed in the LD and MD group after Vibrio harveyi challenge test. The results of the experiment indicated that the biofloc in situ had the effects of anti-crowding stress.

Sponge chemical defenses are a possible mechanism for increasing sponge abundance on reefs in Zanzibar

Coral reefs are experiencing increasing anthropogenic impacts that result in substantial declines of reef-building corals and a change of community structure towards other benthic invertebrates or macroalgae. Reefs around Zanzibar are exposed to untreated sewage and runoff from the main city Stonetown. At many of these sites, sponge cover has increased over the last years. Sponges are one of the top spatial competitors on reefs worldwide. Their success is, in part, dependent on their strong chemical defenses against predators, microbial attacks and other sessile benthic competitors. This is the first study that investigates the bioactive properties of sponge species in the Western Indian Ocean region. Crude extracts of the ten most dominant sponge species were assessed for their chemical defenses against 35 bacterial strains (nine known as marine pathogens) using disc diffusion assays and general cytotoxic activities were assessed with brine shrimp lethality assays. The three chemically most active sponge species were additionally tested for their allelopathic properties against the scleractinian coral competitor Porites sp.. The antimicrobial assays revealed that all tested sponge extracts had strong antimicrobial properties and that the majority (80%) of the tested sponges were equally defended against pathogenic and environmental bacterial strains. Additionally, seven out of ten sponge species exhibited cytotoxic activities in the brine shrimp assay. Moreover, we could also show that the three most bioactive sponge species were able to decrease the photosynthetic performance of the coral symbionts and thus were likely to impair the coral physiology.

Effects of stocking density of the white shrimp Litopenaeus vannamei (Boone) on immunities, antioxidant status, and resistance against Vibrio harveyi in a biofloc system

Determining optimum stocking density of the white shrimp Litopenaeus vannamei (Boone) is a big concern for shrimp farmers. However, few studies have assessed the influence of stocking density on the antioxidant status, immunology, digestive enzyme activities, and growth performance of white shrimp in biofloc systems. In this study, these parameters of white shrimp in a biofloc system were compared at three stocking densities: 300 orgs m^-3 as low stocking density (LD), 400 orgs m^-3 as medium stocking density (MD), and 500 orgs m^-3 as high stocking density (HD). The feed conversion ratio in the LD group was significantly lower than that in the MD and HD groups (P < 0.05), and the ultimate individual weight in the LD group was significantly higher than that in the other two groups (P < 0.05). The antioxidant status and immunology parameters, including complement 3, complement 4, lysozyme, superoxide dismutase, glutathione peroxidase and malondialdehyde were all depressed in the HD groups. Furthermore, activities of the digestive enzymes, amylase, trypsin, and lipase were lower in the MD and HD groups than that in the LD group. The highest relative percentage survival was observed in the LD group 10 days after challenge with the pathogen Vibrio harveyi. Results of this study indicated that the immune status and welfare of white shrimp can be seriously impaired in the HD condition (i.e., ≥500 m^-3) in biofloc systems. These findings can be used to determine suitable stocking densities in the white shrimp farming industry using the biofloc system.

Proteomics Analysis Reveals Previously Uncharacterized Virulence Factors in Vibrio proteolyticus

Members of the genus Vibrio include many pathogens of humans and marine animals that share genetic information via horizontal gene transfer. Hence, the Vibrio pan-genome carries the potential to establish new pathogenic strains by sharing virulence determinants, many of which have yet to be characterized. Here, we investigated the virulence properties of Vibrio proteolyticus, a Gram-negative marine bacterium previously identified as part of the Vibrio consortium isolated from diseased corals. We found that V. proteolyticus causes actin cytoskeleton rearrangements followed by cell lysis in HeLa cells in a contact-independent manner. In search of the responsible virulence factor involved, we determined the V. proteolyticus secretome. This proteomics approach revealed various putative virulence factors, including active type VI secretion systems and effectors with virulence toxin domains; however, these type VI secretion systems were not responsible for the observed cytotoxic effects. Further examination of the V. proteolyticus secretome led us to hypothesize and subsequently demonstrate that a secreted hemolysin, belonging to a previously uncharacterized clan of the leukocidin superfamily, was the toxin responsible for the V. proteolyticus-mediated cytotoxicity in both HeLa cells and macrophages. Clearly, there remains an armory of yet-to-be-discovered virulence factors in the Vibrio pan-genome that will undoubtedly provide a wealth of knowledge on how a pathogen can manipulate host cells.

The pan-genome of the genus Vibrio is a potential reservoir of unidentified toxins that can provide insight into how members of this genus have successfully risen as emerging pathogens worldwide. We focused on Vibrio proteolyticus, a marine bacterium that was previously implicated in virulence toward marine animals, and characterized its interaction with eukaryotic cells. We found that this bacterium causes actin cytoskeleton rearrangements and leads to cell death. Using a proteomics approach, we identified a previously unstudied member of the leukocidin family of pore-forming toxins as the virulence factor responsible for the observed cytotoxicity in eukaryotic cells, as well as a plethora of additional putative virulence factors secreted by this bacterium. Our findings reveal a functional new clan of the leukocidin toxin superfamily and establish this pathogen as a reservoir of potential toxins that can be used for biomedical applications.

Differential expression of microRNAs in shrimp Marsupenaeus japonicus in response to Vibrio alginolyticus infection

Till date numerous microRNAs (miRNAs) have been discovered from various organisms, including mammals, plants, insects, nematodes and viruses. They are known to have antiviral functions in crustaceans such as shrimp Marsupenaeus japonicas. However, little is known about the role of miRNAs against bacterial infection in this shrimp caused by Vibrio alginolyticus. We performed small RNA sequencing to characterize the differentially expressed microRNAs in V. alginolyticus challenged shrimp, in comparison to that in control uninfected shrimp, at 24 h and 48 h. In total, 55 host miRNAs were differentially expressed in response to the infection and most of these were downregulated at both the time-points. TargetScan and miRanda algorithms showed that the target genes of these down-regulated miRNAs were related to innate immune functions such as production of phenoloxidase enzyme, apoptosis and phagocytosis. Further, gene ontology analysis revealed that many immune signaling pathways were mediated by these miRNAs. This study is one of the earliest attempts at characterizing shrimp miRNAs that respond to V. alginolyticus infection, and will help unravel the miRNA pathways involved in antibacterial action in shrimp.