Srebp-1 bridges gonad development and lipid accumulation by regulating lipogenesis in noble scallop Chlamys nobilis.

In bivalve, development of female gonad is accompanied with accumulating lipids which provided energy resource for non-feeding larvae development. As the major transcriptional regulators of lipid metabolism, Srebps play pivotal role in lipid homeostasis during oogenesis. However, little work was conducted on Srebps function in bivalves. The noble scallop Chlamys nobilis accumulated large amount of lipids in its gonad during oogenesis. Here, we identified a single Srebp gene (named Srebp-1) with a high similarity to human Srebp-1c. Disrupting Srebp-1 with Betulin (inhibiting the maturation of Srebp protein) repressed expression of lipogenic genes and de novo lipogenesis, and resulted in reduction of gonad index and lipid deposition, suggesting a crucial role of Srebp-1 for gonad development and lipid synthesis in female gonad. Additionally, scallops with Srebp-1 disruption released fewer eggs with a reduction in their lipid content and D-larvae formation, revealing an impair of fecundity caused by Srebp-1 disruption. Cold exposure stimulated lipid accumulation which required Srebp-1 to regulate de novo lipogenesis and lipid uptake, providing a crosstalk of Srebp-1 activity and environmental variation on lipid accumulation in noble scallop. Thus, our study identified Srebp-1 as a central regulator coordinating the lipid synthesis and accumulation with gonad development in noble scallop.

Characterization of MKK family genes and their responses to temperature stress and Vibrio parahaemolyticus infection in noble scallop Chlamys nobilis.

Mitogen-activated protein kinase kinase (MKK), the key element of the Mitogen-activated protein kinase (MAPK) signaling pathway, is crucial for the immune response to adverse environments in aquatic animals. Nevertheless, there is limited information regarding the role of the MKK gene family in mollusks. In our study, genome data and transcriptome were used to identify four MKK genes (CnMKK4, CnMKK5, CnMKK6, and CnMKK7) in the noble scallop. The result of the gene structure, motif analysis, and phylogenetic tree revealed that MKK genes are relatively conserved in bivalves. Moreover, four CnMKK genes were significantly highly expressed in immune-related tissues, suggesting that CnMKKs may related to bivalve immunity. Furthermore, CnMKK6 and CgMKK4 were significantly differentially expressed (P < 0.05) under 24 h of temperature stress, and all CnMKKs were significantly differentially expressed (P < 0.05) under 24 h of Vibrio parahaemolyticus infection. These results showed that the CnMKKs may have a significant impact under biotic and abiotic stresses. In conclusion, the result of the CnMKKs provides valuable insights into comprehending the function of MKK genes in mollusks.

Genome-wide identification of gap junction gene family and their expression profiles under low temperature stress in noble scallop Chlamys nobilis.

Gap junctions, formed by gap junction proteins (GJ), play crucial roles in cell signaling and immune responses. The structure and function of the GJ from vertebrates (called connexins) have been extensively studied. However, little is known about the proteins forming gap junctions in invertebrates (called innexins). In this study, 14 GJ genes of Chlamys nobilis were identified. GJ proteins are mainly distributed on the plasma membrane, and all proteins are hydrophilic Phylogenetic tree analysis showed that the GJ proteins in C. nobilis were distantly related to those in vertebrates but closely related to those in invertebrates. Conserved motifs analysis of these GJ proteins in C. nobilis identified to have 10 conserved motifs, similar to gap junction proteins in other bivalves. Moreover, expression profiles of CnGJ genes under chronic and acute low temperature stress were also investigated. Results showed that chronic low temperature stress had a significant effect on the expression levels of CnGJ genes, and the expression profiles of CnGJ genes showed significantly variation under acute low temperature stress. All these results indicated that CnGJ genes play important roles in environmental adaptation in scallops. The present study initially elucidated the function of gap junction genes in noble scallop C. nobilis, which provides new insights into the GJ genes in mollusks and will help us better understand their roles in environmental stress in scallops.

Genome-wide analysis of the cytochrome P450 gene family in Pacific oyster Crassostrea gigas and their expression profiles during gonad development.

The cytochrome P450 (CYP) gene superfamily plays a significant role in various physiological processes, producing different compounds such as hormones, fatty acids, and biomolecules. However, little information is known their roles during gonad development in Pacific oyster (Crassostrea gigas). In this study, total of 116 CgCYP (Crassostrea gigas cytochrome P450) genes were identified and their expression pattern was analyzed for the first time. The relative molecular weights of these CgCYP genes ranged from 63.52 to 113.41 kDa, and the length of encoded amino acids ranged from 103 to 993. And total 26 cis-acting elements of these CgCYP genes were identified. GO and KEGG enrichment analysis showed some CgCYP genes are essential for the metabolism of male and female sex hormones. Additionally, expression anslysis showed 69 CgCYP genes were over-expressed in early gonad development and triploid infertile individuals. More importantly, expression levels of CgCYP1, CgCYP15, CgCYP34, CgCYP46, CgCYP69, CgCYP87, CgCYP88, and CgCYP103, were found to be significantly higher in female gonad, suggesting their important roles in female gonad development. The results of this study will provide a better understanding of the CgCYP genes in the gonad development of Pacific oyster.

Effects of chronic cold stress on tissue structure, antioxidant response, and key gene expression in the warm-water bivalve Chlamys nobilis.

As ectothermic invertebrates, mollusks are regarded as good environmental indicator species for determining the adverse effects of climate change on marine organisms. In the present study, the effects of cold stress on the tissue structure, antioxidant activity, and expression levels of genes were evaluated in the warm-water noble scallop Chlamys nobilis by simulating natural seawater cooled down during winter from 17 °C to 14 °C, 12 °C, 10 °C, and 9 °C. Firstly, the gill was severely damaged at 10 °C and 9 °C, indicating that it could be used as a visually indicative organ for monitoring cold stress. The methylenedioxyamphetamine (MDA) content significantly increased with the temperatures decreasing, meanwhile, the antioxidant enzyme activities superoxide dismutase (SOD) and catalase (CAT) showed a similar pattern, suggesting that the scallop made a positive response. More importantly, 6179 genes related to low temperatures were constructed in a module-gene clustering heat map including 10 modules. Furthermore, three gene modules about membrane lipid metabolism, amino acid metabolism, and molecular defense were identified. Finally, six key genes were verified, and HEATR1, HSP70B2, PI3K, and ATP6V1B were significantly upregulated, while WNT6 and SHMT were significantly downregulated under cold stress. This study provides a dynamic demonstration of the major gene pathways' response to various low-temperature stresses from a transcriptomic perspective. The findings shed light on how warm-water bivalves can tolerate cold stress and can help in breeding new strains of aquatic organisms with low-temperature resistance.

Genome-wide identification of STATs and analysis of their role in sex determination in Pacific oysters (Crassostrea gigas).

STAT (signal transducer and activator of the transcription) proteins, are a group of highly conserved transcription factors and fundamental components of the JAK-STAT signaling pathway. They play crucial roles in a variety of biological processes, such as immunity, proliferation, differentiation, and growth. However, little information is known regarding their role in gonad development and sex determination in mollusks. In this study, we identified 3 STAT genes in Pacific Oyster Crassostrea gigas. Phylogenetic analysis showed that STATs from mollusks were highly conserved, and most of them had four identical motif regions, except for the STAT1 and STAT3 predicted sequences from Crassostrea hongkongensis. Tissue expression analysis indicated CgSTAT1 had a high expression level in most tissues, while CgSTAT3 had a low expression level in most tissues. Expression analysis of early developmental stages showed CgSTAT1 had a higher expression level from egg to D shaped larva and a lower expression level in subsequent stages. In contrast CgSTAT1, CgSTAT2 had a reverse expression pattern. Expression analysis of different developmental stages of diploid gonads indicated that CgSTAT1 had a higher expression level at the S1 and S3 stages relative to the S2 stage in females, while in males the S3 stage had a higher expression than than the S2 stage. The expression level of CgSTAT1 between diploids and triploids in females differed significantly, but there were no significant differences in males. Expression of CgSTAT2 differed significantly between diploid and triploid males. These data suggest an important role for STATs in sex differentiation in diploid and triploid oysters. Our study is the first to explore the role of STATs in sex differentiation and gonadal development in oysters, and will help us better understand the molecular mechanisms of sex differentiation in shellfish.

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.

Transcriptome analysis provides insights into the high ability to synthesize fatty acids in "yellow oil" mud crab (Scylla paramamosain).

Yellow oil mud crab (YOC) is a new variant of mud crab (Scylla paramamosain), which was attracted much attention in recent years due to its high level of nutrition. However, the nutritive values and the physiological changes in YOC have not been clearly understood. In this study, we aimed to identify the nutrient compositions (including total carotenoid content (TCC), total lipid content (TLC), total antioxidant capacity (TAC), and fatty acids) and differences in genes related to the biosynthesis of fatty acids using transcriptome analysis in YOC in comparison with those of normal mud crabs. As a result, observations on the morphological characteristics showed that the YOC exhibits a difference in the color of the muscle, gills (orange-yellow), and hemolymph (yellow) compared with the normal female crabs (NFC) (blue or nattier blue). The TCC and TLC (84.96 ± 9.65 µg/g in muscle and 1.39 ± 0.10 µg/mL in hemolymph) or TAC (1.52 ± 0.17 mM in hemolymph) of YOC were higher than that of NFC and normal male crab (NMC). YOC had lower saturated fatty acids, but higher unsaturated fatty acids, as well as the ratio of n-3/n-6 of fatty acids in muscle and hemolymph, compared with those of NFC and NMC. Furthermore, the transcriptome profile revealed that the unigenes in YOC were enriched in the synthesis of n-3 fatty acids. Furthermore, more unigenes related to 'Biosynthesis of unsaturated fatty acids' were identified in muscle and hemocytes, while fewer were in the gonads of YOC. Additionally, the positive (in muscle and hemocytes) and a negative correlation (in gonads) between expressions of unigenes and contents of TLC, TCC, and UFA were found, indicating a better synthesis ability of fatty acids in the muscle and hemocytes of YOC. Overall, compared to NFC and NMC, YOC has higher nutrients and is a better food nutrient source for humans.

In vitro fermentation of Gracilaria lemaneiformis and its sulfated polysaccharides by rabbitfish gut microbes.

This study intended to characterize the Gracilaria lemaneiformis (SW)-derived polysaccharide (GLP) and explore the fermentation aspects of SW and GLP by rabbitfish (Siganus canaliculatus) intestinal microbes. The GLP was mainly composed of galactose and anhydrogalactose (at 2.0:0.75 molar ratio) with the linear mainstay of α-(1 â†’ 4) linked 3,6-anhydro-α-l-galactopyranose and ß-(1 â†’ 3)-linked galactopyranose units. The in vitro fermentation results showed that the SW and GLP could reinforce the short-chain fatty (SCFAs) production and change the diversity and composition of gut microbiota. Moreover, GLP boosted the Fusobacteria and reduced the Firmicutes abundance, while SW increased the Proteobacteria abundance. Furthermore, the adequacy of feasibly harmful bacteria (such as Vibrio) declined. Interestingly, most metabolic processes were correlated with the GLP and SW groups than the control and galactooligosaccharide (GOS)-treated groups. In addition, the intestinal microbes degrade the GLP with 88.21 % of the molecular weight reduction from 1.36 × 105 g/mol (at 0 h) to 1.6 × 104 g/mol (at 24 h). Therefore, the findings suggest that the SW and GLP have prebiotic potential and could be applied as functional feed additives in aquaculture.

Evaluation of the Feasibility of Harvest Optimisation of Soft-Shell Mud Crab (Scylla paramamosain) from the Perspective of Nutritional Values.

Soft-shell crabs have attracted consumers' attention due to their unique taste and nutritional value. To evaluate the feasibility of harvest optimisation of soft-shell mud crabs, the proximate composition, mineral composition, and total carotenoid, amino acid, and fatty acid contents of edible parts of male and female soft-shell mud crabs at different moulting stages were determined and compared from a nutritional value perspective. The results showed that the sex and moulting stages could significantly affect the nutritional values of the edible portions of soft-shell crabs. The female or male soft-shell crabs in the postmoult Ⅰ stage had a much richer mineral element content than that in other moulting stages. The total carotenoid content in female soft-shell crabs was significantly higher than that in male crabs in all moulting stages, while male soft-shell crabs had better performance in amino acid nutrition than female soft-shell crabs. Moreover, it was found that soft-shell crabs in the postmoult Ⅱ stage had significantly higher contents of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), while significantly lower contents of saturated fatty acids (SFA) than those in other stages. The present study will provide a reference basis for the diversified cultivation of soft-shell crabs, and further promote the development of the mud crab industry.

Carotenoid content and composition: A special focus on commercially important fish and shellfish.

Carotenoids are natural pigments that provide many health benefits to living organisms. Although terrestrial plants are the major dietary source of carotenoids for humans, aquatic animals (especially fish and shellfish) are equally important because they are rich in certain important carotenoids lacking in fruits and vegetables. Although extensive research has focused on exploring the carotenoid content and composition in fish and shellfish, this information is poorly organized. This paper reviews the scientific evidence for the carotenoid content and composition in fish and shellfish. It makes serious attempts to summarize the relevant data published on specific research questions in order to improve the understanding of various evidence to clarify the research status of carotenoids in fish and shellfish and defining topics for future studies. From the analysis of published data, it is obvious that most fish and shellfish are rich in complex carotenoids (e.g. astaxanthin, fucoxanthin, fucoxanthinol, lutein). These carotenoids have stronger antioxidant effect, higher efficiency in removing the singlet oxygen and the peroxyl radicals, and have a variety of health benefits. Carotenoid levels in fish and shellfish depend on genotype, climatic conditions of the production area, storage and cooking methods. However, the information of the bioavailability of fish/shellfish carotenoids to human is very limited, which hinders the actual contributions to health. The findings of this study can be used as a guide to select appropriate fish and shellfish as dietary sources of carotenoids, and provide information about potential fish and shellfish species for aquaculture to produce carotenoids to meet part of the growing demand for natural carotenoids.

Identification of a key gene StAR-like-3 responsible for carotenoids accumulation in the noble scallop Chlamys nobilis.

Carotenoids play important roles in living organisms. However, animals cannot synthesize carotenoids by themselves, and they must absorb and accumulate carotenoids from their diets in which some key genes are involved. In present study, a gene named StAR-like-3 was characterized in the noble scallop Chlamys nobilis, and its function was identified using golden scallops with higher carotenoids content and brown scallops with less carotenoids content by immunohistochemistry, carotenoid binding assay and RNAi. Results showed that the StAR-like-3 encodes a 54.7 kDa transmembrane protein (named as StAR3) of 481 amino acids containing a MENTAL domain and a START (Steroidogenic acute regulatory protein-related lipid transfer) domain, and its expression level in hemocytes and intestine of golden scallops were significantly higher than those of brown ones. Subsequently, the StAR3 protein was detected in the intestinal epithelial cells of golden scallops, and recombinant StAR3 could bind lutein conjugated to protein G and antibody to form a yellow complex, suggesting it is a carotenoid binding protein involving in carotenoids accumulation in golden scallops. Furthermore, total carotenoids content of hemolymph in golden scallops was significantly decreased when the expression of StAR-like-3 suppressed, suggesting this gene plays an important role in transport of carotenoids. Conclusion, the present results indicated that the StAR-like-3 is a key gene responsible for the carotenoids accumulation in the scallop.

Climate change and n-3 LC-PUFA availability.

Omega-3 long-chain polyunsaturated fatty acids (n-3 LC-PUFA) are essential fatty acids for the growth, development and survival of virtually all organisms. There is increasing evidence that anthropogenic climate change has a direct and indirect impact on the availability of natural n-3 LC-PUFA. However, this information is fragmented and not well organized. Therefore, this article reviewed published data from laboratory experiments, field experiments and model simulations to reveal the impact of climate change on the global supply of natural n-3 LC-PUFA and how this will limit the availability of n-3 LC-PUFA in the future food web. In general, climate change can significantly reduce the availability of natural n-3 LC-PUFA in grazing food webs in the following ways: 1) decrease the total biomass of phytoplankton and shift the plankton community structure to a smaller size, which also reduce the biomass of animals in higher trophics; 2) reduce the n-3 LC-PUFA content and/or quality (n-3: n-6 ratio) of all marine organisms; 3) reduce the transfer efficiency of n-3 LC-PUFA in grazing food web. In addition, as an anthropogenic climate adaptation measure, this review also proposed some alternative sources of n-3 LC-PUFA and determined the direction of future research. The information in this article is very useful for providing a critical analysis of the impact of climate change on the supply of natural n-3 LC-PUFA. Such information will aid to establish climate adaptation or management measures, and determine the direction of future research.

Intraspecific hybridization as a mitigation strategy of ocean acidification in marine bivalve noble scallop Chlamys nobilis.

The driving factors of climate change, especially ocean acidification (OA), have many detrimental impacts on marine bivalves. Hybridization is one of the important methods to improve environmental tolerance of animals and plants. In this study, we explored the feasibility of intraspecific hybridization as an OA mitigation strategy in noble scallop Chlamys nobilis (ecologically and economically important bivalve species). The results of this study revealed that exposure of C. nobilis to OA condition significantly reduced the hatching rate, survival rate, growth rate (shell height, shell length, shell width and shell weight), and total carotenoid content (TCC), as well as increased the deformity rate of C. nobilis larvae. Interestingly, under both ambient water and OA condition, the intraspecific hybridization of C. nobilis exhibited heterosis in terms of hatching rate, survival rate and growth rate (excepted for growth in shell length under OA). Transcriptome sequencing of C. nobilis (inbreed and hybrid under ambient and OA conditions) identified four main differentially expressed genes involved in signal transduction, biological process maintenances, nucleic acid binding and post-translational modification. In addition, the expression of these four genes in hybrid C. nobilis was significantly higher than that in inbreed C. nobilis. In conclusion, hybrid C. nobilis showed heterosis in growth rate and survival rate under both ambient water and acidified seawater condition, which may be the result of enhanced expression of genes related to signal transduction, DNA replication and post-translational modification.

Whole-transcriptome RNA sequencing revealed the roles of chitin-related genes in the eyestalk abnormality of a novel mud crab hybrid (Scylla serrata ♀ × S. paramamosain ♂).

Chitin is a kind of insoluble structural polysaccharide and plays different roles in different species. In crustaceans, it forms the structural components in the exoskeleton. In our previous studies, novel mud crab hybrids have been produced from the interspecific hybridization of Scylla serrata ♀ × S. paramamosain ♂. Some of the hybrid crabs have been found to be morphologically (eyestalk) abnormal, but the genetic mechanism remains unknown. To address this question, we performed whole-transcriptome RNA sequencing on the control group (normal hybrids), abnormal hybrids, and S. paramamosain to uncover the genetic basis underlying this morphological abnormality. A total of 695 mRNAs, 10 miRNAs, 44 circRNAs, and 1957 lncRNAs were differentially expressed between normal and abnormal hybrids. Several differentially expressed genes (DEGs) associated with chitin and cuticle metabolism were identified, including chitin synthase, chitinase, chitin deacetylase, ß-N-acetylglucosaminidase, ß-1,4-endoglucanase, N-alpha-acetyltransferase, cuticle proprotein, early cuticle protein, and arthrodial cuticle protein. Functional analysis showed that DE miRNAs, DE circRNAs, DE lncRNAs, and lncRNA/circRNA-miRNA-mRNA network were enriched in pathways related to the amino acid, carbohydrate, and glycogen metabolism. Considering the importance of the chitin and cuticle in exoskeleton formation, it can be concluded that the changes in the chitin and cuticle biosynthesis might have caused the eyestalk abnormality in hybrid crabs. These findings can lay the solid foundation for a better understanding of the important roles of chitin and cuticle related genes and the development of hybridization techniques in crustaceans.

Endogenous LC-PUFA biosynthesis capability in commercially important mollusks.

Mollusks are excellent dietary sources for LC-PUFA. However, the main challenge limiting mollusk production is the high mortality rate of molluskan larvae in early life cycle stages. This paper reviews scientific evidences on molecular and biochemical studies of LC-PUFA biosynthesis in commercially important molluskan species. It carefully summarizes the pertinent data published on specific research questions to improve the understanding of the diverse evidences. It is helpful to clarify the current state of research and determine topics for future studies on LC-PUFA biosynthesis in mollusks. From the analysis of published data, mollusks have the ability to biosynthesis LC-PUFA to a certain extent. LC-PUFA biosynthesis information of commercially important molluskan species can be useful to determine the fatty acids essential for their diet. Therefore, specific management strategies or feeds can be developed to strengthen the industry by improving the health and survival rate of molluskan larvae.

Lipid nutritional quality of marine and freshwater bivalves and their aquaculture potential.

Omega-3 Long-chain polyunsaturated fatty acids (n-3 LC-PUFA) are beneficial to human health. Since the industrial revolution, with the tremendous increase of human population, the supply of natural n-3 LC-PUFA is far lower than the nutritional need of n-3 LC-PUFA. Therefore, a new alternative source of natural n-3 LC-PUFA is urgently needed to reduce the supply and demand gap of n-3 LC-PUFA. Mollusks, mainly bivalves, are rich in n-3 LC-PUFA, but the information of bivalves' lipid profile is not well organized. Therefore, this study aims to analyze the published fatty acid profiles of bivalves and reveal the potential of bivalve aquaculture in meeting the nutritional needs of human for n-3 LC-PUFA. There are growing evidence show that the nutritional quality of bivalve lipid is not only species-specific, but also geographical specific. To date, bivalve aquaculture has not been evenly practiced across the globe. It can be seen that aquaculture is predominant in Asia, especially China. Unlike fish aquaculture, bivalve aquaculture does not rely on fishmeal and fish oil inputs, so it has better room for expansion. In order to unleash the full potential of bivalve aquaculture, there are some challenges need to be addressed, including recurrent mass mortalities of farmed bivalves, food safety and food security issues. The information of this article is very useful to provide an overview of lipid nutritional quality of bivalves, and reveal the potential of bivalve aquaculture in meeting the growing demand of human for n-3 LC-PUFA.

Evidence of Sex Differentiation Based on Morphological Traits During the Early Development Stage of Mud Crab Scylla paramamosain.

In order to uncover the sexual difference in morphology and how early they appear during the development stage of mud crab Scylla paramamosain, we measured, observed, and biostatistically analyzed morphological traits related to sex. For unveiling the morphological differences between sexes, morphological traits involving abdomen width (AW), carapace length (CL), and carapace width (CW) were first measured during the crablet development stage of S. paramamosain in the present study. The correlation analyses and path analyses exhibited that sexual dimorphism in the third abdomen width (AW3) and fourth abdomen width (AW4) could be used for sex identification from stage C VI (stage VI of crablet). Based on the stepwise discriminant analysis and standardized traits, a sex discriminant equation was constructed, which is capable for sex identification in crablets from stage C VI. Observations for secondary sexual traits and abdomen morphology (shape and pleopods) using a dissecting microscope or scanning electron microscope indicated that sexes are easily identified at stage C VIII according to the abdomen shape; meanwhile, at stage C II based on pleopod difference, and at stage C I by the presence or absence of gonopores. The findings in this study contribute greatly to the accuracy of sex identification of S. paramamosain during the early development stage, which promotes the understanding of the morphological differentiation mechanism of sex.

Potential causative factors of noble scallop Chlamys nobilis mass mortality in Nan'ao Island, Shantou, China in 2017.

Noble scallop Chlamys nobilis is an important marine bivalve that has been extensively cultured in the south coast of China since the 1980s. Unfortunately, since the late 1990s, the farmed scallops often suffered from regional mass mortality, which results in enormous economic losses to farmers and industries. In 2017, another mass mortality event occurred in Nan'ao Island, Shantou, China. In this study, the cause of C. nobilis mass mortality in 2017 was first investigated in the field, and then validated in a laboratory experiment. In the field, three sampling sites were selected according to the scallop mortality rate: Hunter Bay (90% mortality), Baisha Bay (67% mortality) and Longhai (6% mortality). Meanwhile, environmental parameters (temperature, salinity, DO, pH and chlorophyll a) of each site were also measured in situ. Then, water and scallop samples were collected randomly for the analysis of phytoplankton diversity and algal toxin activity using 18S rDNA and PP2A inhibition assay, respectively. In laboratory, healthy scallops were challenged with Karenia mikimotoi (1 × 103 cells/mL) for 30 h. The field results showed that no significant difference in those environmental parameters existed among the three sites, but the relative abundance of K. mikimotoi in seawater and scallops' intestines in Hunter Bay and Baisha Bay was significantly higher than that in Longhai, and sick scallops contained significantly higher algal toxin activity than healthy ones. Laboratory results revealed that challenged scallops with K. mikimotoi showed significantly higher mortality rate and algal toxin activity than healthy ones, and low density of K. mikimotoi (1 × 103 cells/mL) was sufficient to cause >50% scallops' mortality within 26 h. This study provides the first evidence that low K. mikimotoi cell density can cause massive mortality in C. nobilis, and provides useful information as guide to prevent scallop mass mortality in the future.

mRNA profile provides novel insights into stress adaptation in mud crab megalopa, Scylla paramamosain after salinity stress.

BACKGROUND: Mud crab, Scylla paramamosain, a euryhaline crustacean species, mainly inhabits the Indo-Western Pacific region. Wild mud crab spawn in high-salt condition and the salinity reduced with the growth of the hatching larvae. When the larvae grow up to megalopa, they migrate back to estuaries and coasts in virtue of the flood tide, settle and recruit adult habitats and metamorphose into the crablet stage. Adult crab can even survive in a wide salinity of 0-35 ppt. To investigate the mRNA profile after salinity stress, S. paramamosain megalopa were exposed to different salinity seawater (low, 14 ppt; control, 25 ppt; high, 39 ppt). RESULTS: Firstly, from the expression profiles of Na+/K+/2Cl- cotransporter, chloride channel protein 2, and ABC transporter, it turned out that the 24 h might be the most influenced duration in the short-term stress. We collected megalopa under different salinity for 24 h and then submitted to mRNA profiling. Totally, 57.87 Gb Clean Data were obtained. The comparative genomic analysis detected 342 differentially expressed genes (DEGs). The most significantly DEGs include gamma-butyrobetaine dioxygenase-like, facilitated trehalose transporter Tret1, sodium/potassium-transporting ATPase subunit alpha, rhodanese 1-like protein, etc. And the significantly enriched pathways were lysine degradation, choline metabolism in cancer, phospholipase D signaling pathway, Fc gamma R-mediated phagocytosis, and sphingolipid signaling pathway. The results indicate that in the short-term salinity stress, the megalopa might regulate some mechanism such as metabolism, immunity responses, osmoregulation to adapt to the alteration of the environment. CONCLUSIONS: This study represents the first genome-wide transcriptome analysis of S. paramamosain megalopa for studying its stress adaption mechanisms under different salinity. The results reveal numbers of genes modified by salinity stress and some important pathways, which will provide valuable resources for discovering the molecular basis of salinity stress adaptation of S. paramamosain larvae and further boost the understanding of the potential molecular mechanisms of salinity stress adaptation for crustacean species.