First case of hepatopancreatic necrosis disease in pond-reared Chinese mitten crab, Eriocheir sinensis, associated with microsporidian

Overcoming research challenges: In vitro cultivation of Ameson portunus (Phylum Microsporidia)

Ameson portunus is an intracellular pathogen that infects marine crabs Portunus trituberculatus and Scylla paramamosain, causing significant economic losses. However, research into this important parasite has been limited due to the absence of an in vitro culture system. To address this challenge, we developed an in vitro cultivation model of A. portunus using RK13 cell line in this study. The fluorescent labeling assay indicated a high infection rate (∼60 %) on the first day post-infection and quantitative PCR (qPCR) detection demonstrated successful infection as early as six hours post-inoculation. Fluorescence in situ hybridization (FISH) and qPCR were used for the detection of A. portunus infected cells. The FISH probe we designed allowed detection of A. portunus in infected cells and qPCR assay provided accurate quantification of A. portunus in the samples. Transmission electron microscopy (TEM) images revealed that A. portunus could complete its entire life cycle and produce mature spores in RK13 cells. Additionally, we have identified novel life cycle characteristics during the development of A. portunus in RK 13 cells using TEM. These findings contribute to our understanding of new life cycle pathways of A. portunus. The establishment of an in vitro culture model for A. portunus is critical as it provides a valuable tool for understanding the molecular and immunological events that occur during infection. Furthermore, it will facilitate the development of effective treatment strategies for this intracellular pathogen.

Comparison of Body Characteristics, Carotenoid Composition, and Nutritional Quality of Chinese Mitten Crab (Eriocheir sinensis) with Different Hepatopancreas Redness

In this study, we investigated the body characteristics, carotenoid composition, and nutritional quality of Eriocheir sinensis with different hepatopancreas redness (a*). We distributed the crabs into two groups based on the hepatopancreas a* values and compared their body characteristics, chroma, carotenoid composition, and protein, lipid, total sugar, amino acid, and fatty acid content via paired t-test. The results revealed that the relationships between hepatopancreas a* values and crab quality are sex specific. In female crabs, the differences in nutritional characteristics were evident mainly in the hepatopancreases and ovaries. In the redder hepatopancreases, the content of zeaxanthin and β-carotene increased, and the levels of C22:6n3 and C20:5n3 decreased (p < 0.05). In the ovaries, the content of astaxanthin, canthaxanthin, β-carotene, umami, and sweet amino acids were lower in the redder hepatopancreas crabs (p < 0.05). In male crabs, there were positive relationships between hepatopancreas a* and amino acid and fatty acid content. The content of leucine, arginine, and total umami amino acids in muscles and of unsaturated fatty acids and n-6 polyunsaturated fatty acids in hepatopancreases and testicles increased with increasing hepatopancreas a* values (p < 0.05). Therefore, the redder the hepatopancreas, the higher the nutritional quality of male crabs.

Establishment of a TaqMan probe-based qPCR assay for detecting microsporidia Enterospora epinepheli in grouper

Enterospora epinepheli is an intranuclear microsporidian parasite causing serious emaciative disease in hatchery-bred juvenile groupers (Epinephelus spp.). Rapid and sensitive detection is urgently needed as its chronic infection tends to cause emaciation as well as white faeces syndrome and results in fry mortality. This study established a TaqMan probe-based real-time quantitative PCR assays targeting the small subunit rRNA (SSU) gene of E. epinepheli. The relationship between the standard curve of cycle threshold (Ct) and the logarithmic starting quantity (SQ) was determined as Ct = -3.177 lg (SQ) + 38.397. The correlation coefficient (R2 ) was 0.999, and the amplification efficiency was 106.4%. The detection limit of the TaqMan probe-based qPCR assay was 1.0 × 101 copies/μL and that is 100 times sensitive than the traditional PCR method. There is no cross-reaction with other aquatic microsporidia such as Ecytonucleospora hepatopenaei, Nucleospora hippocampi, Potaspora sp., Ameson portunus. The intra-assay and inter-assay showed great repeatability and reproducibility. In addition, the test of clinical samples showed that this assay effectively detected E. epinepheli in the grouper's intestine tissue. The established TaqMan qPCR assays will be a valuable diagnostic tool for the epidemiological investigation as well as prevention and control of E. epinepheli.

The P4' Peptide-Carrying Bacillus subtilis in Cottonseed Meal Improves the Chinese Mitten Crab Eriocheir sinensis Innate Immunity, Redox Defense, and Growth Performance

This study developed a recombinant Bacillus subtilis to carry the LGSPDVIVIR peptide (cmP4) isolated from the hydrolyzed products of cottonseed meal with excellent antioxidant and immune-enhancing properties in vitro. It was carried as a tandem of five cmP4 peptides (cmP4') to be stably expressed on a large scale. Then, its effectiveness was evaluated in Chinese mitten crab (Eriocheir sinensis) based on growth performance, redox defense, and innate immunity. A total of 280 crabs (mean body weight: 41.40 ± 0.14) were randomly assigned to seven diets including a control one (without B. subtilis) and six experimental ones with different doses (107,108, and 109 CFU/kg) of unmodified and recombinant B. subtilis, respectively, for 12 weeks. Each diet was tested in four tanks of crabs (10/tank). In terms of bacterial dosages, the final weight (FW), weight gain (WG), hemolymph and hepatopancreatic activities of superoxide dismutase (SOD), catalase (CAT), lysosome (LZM), acid phosphatase (ACP) and alkaline phosphatase (AKP), and hepatopancreatic transcriptions of cat, mitochondrial manganese superoxide dismutase (mtmnsod), thioredoxin-1 (trx1), and prophenoloxidase (propo) all increased significantly with increasing B. subtilis dosages, while hemolymph and hepatopancreatic malondialdehyde (MDA) content and the transcriptions of toll like receptors (tlrs), NF-κB-like transcription factor (relish), and lipopolysaccharide-induced TNF-α factor (litaf) all decreased remarkably. In terms of bacterial species, the recombinant B. subtilis group obtained significantly high values of FW, WG, hemolymph, and hepatopancreatic activities of SOD, CAT, LZM, ACP, and AKP, and the transcriptions of mtmnsod, peroxiredoxin 6 (prx6), and propo compared with the unmodified B. subtilis, while opposite results were noted in hemolymph and hepatopancreatic MDA content and the transcriptions of tlrs, relish, and litaf. These results indicated that dietary supplementation with 109 CFU/kg of recombinant B. subtilis can improve the growth performance, redox defense, and nonspecific immunity of E. sinensis.

The crosstalk between Toll and AMPK signaling pathways mediates growth inhibition of Eriocheir sinensis under deltamethrin stress

Hepatopancreatic necrosis disease (HPND) broke out in 2015 in the Eriocheir sinensis aquaculture region of Xinghua, Jiangsu Province; however, the specific cause of HPND remains unclear. A correlation was found between HPND outbreak and the use of deltamethrin by farmers. In this study, E. sinensis specimens developed the clinical symptoms of HPND after 93 days of deltamethrin stress. The growth of E. sinensis with HPND was inhibited. Adenosine monophosphate-activated protein kinase (AMPK) is a central regulator of energy homeostasis, and its expression was up-regulated in the intestine of E. sinensis with HPND. Growth inhibitory genes (EsCabut, Es4E-BP, and EsCG6770) were also up-regulated in the intestine of E. sinensis with HPND. The expression levels of EsCabut, Es4E-BP, and EsCG6770 decreased after EsAMPK knockdown. Therefore, AMPK mediated the growth inhibition of E. sinensis with HPND. Further analysis indicated the presence of a crosstalk between the Toll and AMPK signaling pathways in E. sinensis with HPND. Multiple genes in the Toll signaling pathway were upregulated in E. sinensis under 93 days of deltamethrin stress. EsAMPK and its regulated growth inhibition genes were down-regulated after the knockdown of genes in the Toll pathway. In summary, the crosstalk between the Toll and AMPK signaling pathways mediates the growth inhibition of E. sinensis under deltamethrin stress.

Virome Analysis Provides an Insight into the Viral Community of Chinese Mitten Crab Eriocheir sinensis

Recent advances in viromics have led to the discovery of a great diversity of RNA viruses and the identification of a large number of viral pathogens. A systematic exploration of viruses in Chinese mitten crab (Eriocheir sinensis), one of the most important aquatic commercial species, is still lacking. Here, we characterized the RNA viromes of asymptomatic, milky disease (MD)-affected, and hepatopancreatic necrosis syndrome (HPNS)-affected Chinese mitten crabs collected from 3 regions in China. In total, we identified 31 RNA viruses belonging to 11 orders, 22 of which were first reported here. By comparing viral composition between samples, we observed high variation in viral communities across regions, with most of the viral species being region-specific. We proposed to establish several novel viral families or genera based on the phylogenetic relationships and genome structures of viruses discovered in this study, expanding our knowledge of viral diversity in brachyuran crustaceans. IMPORTANCE High-throughput sequencing and meta-transcriptomic analysis provide us with an efficient tool to discover unknown viruses and explore the composition of viral communities in specific species. In this study, we investigated viromes in asymptomatic and diseased Chinese mitten crabs collected from three distant locations. We observed high regional variation in the composition of viral species, highlighting the importance of multi-location sampling. In addition, we classified several novel and ICTV-unclassified viruses based on their genome structures and phylogenetic relationships, providing a new perspective on current viral taxa.

Expression levels of serine proteases, their homologs, and prophenoloxidase in the Eriocheir sinensis with hepatopancreatic necrosis syndrome (HPNS) and their expression regulation by Runt

The occurrence of hepatopancreatic necrosis syndrome (HPNS) has seriously affected the sustainable development of Chinese mitten crab (Eriocheir sinensis) farming industry. Limited studies have focused on the immune responses in crabs with HPNS. Serine proteases (SPs) and SP homologues (SPHs) play important roles in the innate immunity of crustaceans. This study investigated the effects of HPNS on the expression levels of genes related to prophenoloxidase (proPO) activation system, and the relationship between Runt transcription factor and the transcriptions of these genes. Eight SPs and five SPHs (SPH1-4, Mas) were identified from E. sinensis. SPs contain a catalytic triad of "HDS", while SPHs lack a catalytic residue. SPs and SPHs all contain a conservative Tryp_SPc domain. Evolutionary analysis showed that EsSPs, EsSPHs, EsPO, and EsRunt were clustered with SPs, SPHs, POs, and Runts of other arthropods, respectively. In crabs with HPNS, the expression levels of six SPs (1, 3, 4, 6, 7, and 8), five SPHs, and PO were significantly upregulated in the hepatopancreas. The knockdown of EsRunt could evidently decrease the expression levels of four SPs (3, 4, 5 and 8), five SPHs (SPH1-4, Mas), and PO. Therefore, the occurrence of HPNS activates the proPO system. Furthermore, the expression levels of partial genes related to proPO system were regulated by Runt. The activation of innate immune system may be a strategy for crabs with HPNS to improve immunity and fight diseases. Our study provides a new understanding of the relationship between HPNS and innate immunity.

Current Disease Threats for Cultivated Crab Eriocheir sinensis in China

The Chinese mitten crab, Eriocheir sinensis, is a commercially important crustacean in China due to its great commercial value and compatibility in a variety of aquaculture systems. However, increases in its production have been accompanied by the emergence of various diseases affecting yield, profit, and trading potential. In this study, we review the pathogenic agents associated with E. sinensis since the start of its commercial culture. The history of crab cultivation implies that increased pathogen transfer can occur as E. sinensis aquaculture grows because polyculture of E. sinensis with other aquaculture species is a prevalent practice. With this in mind, a special focus of this review is placed on pathogens that were initially discovered in other crustacean species but have since been demonstrated to infect and cause disease in E. sinensis. We expect that this review will not only offer recommendations for disease management in the E. sinensis aquaculture sector but will also advance other crustacean cultivation.

New insight of variable lymphocyte receptor-likes in anti-bacteria activity from Eriocheir sinensis

The Chinese mitten crab, Eriocheir sinensis, is a vital freshwater aquaculture species in China, however, is also facing various crab disease threats. In the present study, we identify three novel variable lymphocyte receptor-like (VLR-like) genes-VLR-like1, VLR-like3 and VLR-like4-from E. sinensis, which play vital roles in adaptive immune system of agnathan vertebrates. The bacterial challenge, bacterial binding and antibacterial-activity experiments were applied to study immune functions of VLR-likes, and the transcriptomic data from previous E. sinensis bacterial challenge experiments were analyzed to speculate the possible signaling pathway. VLR-like1 and VLR-like4 can respond to Staphylococcus aureus challenge and inhibit S. aureus specifically. VLR-like1 and VLR-like4 possess broad-spectrum bacteria-binding ability whereas VLR-like3 do not. VLR-likes in E. sinensis could associate with the Toll-like receptor (TLR) signaling pathway. The above results suggest that VLR-likes defend against bacteria invasion though exerting anti-bacteria activity, and probably connect with the TLR signaling pathway. Furthermore, studying the immune functions of these VLR-likes will provide a new insight into the disease control strategy of crustacean culture.

Oxidative stress and immune response of hepatopancreas in Chinese mitten crab Eriocheir sinensis under lipopolysaccharide challenge

Chinese mitten crab (Eriocheir sinensis; H. Milne Edwards, 1853) is one of the important farmed crustaceans in China. Lipopolysaccharide (LPS), as a harmful factor, is prone to occur during the farming process of crabs. Aiming to test the hypothesis that damage degrees of the hepatopancreas in E. sinensis is correlated to LPS concentrations, in this study, E. sinensis were injected with LPS (50 μg/kg, and 500 μg/kg) and analyzed for the activity of antioxidant and immune-related enzymes, immune-related gene expression, and histopathological of hepatopancreas. As result, the hepatopancreas of E. sinensis immune-related genes, i.e., Dorsal, HSP90, Toll2, TLRs, Tube, and proPO, were significantly affected by LPS challenge. Among immune-related genes, Dorsal and proPO might play key roles in combating the LPS challenge. The activity of CAT gradually decreased with the increase of time, and the total antioxidant capacity was decreased after LPS challenge, indicating the inhibition of LPS on the antioxidant system. Interestingly, the decreasing trend of AKP and ACP activity suggested the immune system of crabs was affected by LPS challenge. The hepatopancreas section showed that the damage degree of hepatopancreas was different under the challenge of LPS with different concentrations, and the damage degree was proportional to the concentration. Our findings provide useful information for understanding the mechanism of hepatopancreas injury of E. sinensis induced by LPS infection.

Full-length transcriptome sequencing and comparative transcriptome analysis of Eriocheir sinensis in response to infection by the microsporidian Hepatospora eriocheir

As a new generation of high-throughput sequencing technology, PacBio Iso-Seq technology (Iso-Seq) provides a better alternative sequencing method for the acquisition of full-length unigenes. In this study, a total of 22.27 gigabyte (Gb) subread bases and 128,614 non-redundant unigenes (mean length: 2,324 bp) were obtained from six main tissues of Eriocheir sinensis including the heart, nerve, intestine, muscle, gills and hepatopancreas. In addition, 74,732 unigenes were mapped to at least one of the following databases: Non-Redundant Protein Sequence Database (NR), Gene Ontology (GO), Kyoto Encyclopaedia of Genes and Genomes (KEGG), KEGG Orthology (KO) and Protein family (Pfam). In addition, 6696 transcription factors (TFs), 28,458 long non-coding RNAs (lncRNAs) and 94,230 mRNA-miRNA pairs were identified. Hepatospora eriocheir is the primary pathogen of E. sinensis and can cause hepatopancreatic necrosis disease (HPND); the intestine is the main target tissue. Here, we attempted to identify the key genes related to H. eriocheir infection in the intestines of E. sinensis. By combining Iso-Seq and Illumina RNA-seq analysis, we identified a total of 12,708 differentially expressed unigenes (DEUs; 6,696 upregulated and 6,012 downregulated) in the crab intestine following infection with H. eriocheir. Based on the biological analysis of these DEUs, several key processes were identified, including energy metabolism-related pathways, cell apoptosis and innate immune-related pathways. Twelve selected genes from these DEUs were subsequently verified by quantitative real-time PCR (qRT-PCR) analysis. Our findings enhance our understanding of the E. sinensis transcriptome and the specific association between E. sinensis and H. eriocheir infection.

High-throughput small molecule screen identifies inhibitors of microsporidia invasion and proliferation in C. elegans

Microsporidia are a diverse group of fungal-related obligate intracellular parasites that infect most animal phyla. Despite the emerging threat that microsporidia represent to humans and agricultural animals, few reliable treatment options exist. Here, we develop a high-throughput screening method for the identification of chemical inhibitors of microsporidia infection, using liquid cultures of Caenorhabditis elegans infected with the microsporidia species Nematocida parisii. We screen a collection of 2560 FDA-approved compounds and natural products, and identify 11 candidate microsporidia inhibitors. Five compounds prevent microsporidia infection by inhibiting spore firing, whereas one compound, dexrazoxane, slows infection progression. The compounds have in vitro activity against several other microsporidia species, including those known to infect humans. Together, our results highlight the effectiveness of C. elegans as a model host for drug discovery against intracellular pathogens, and provide a scalable high-throughput system for the identification and characterization of microsporidia inhibitors.

Integrative Analysis of Transcriptome and Metabolome Reveals Molecular Responses in Eriocheir sinensis with Hepatopancreatic Necrosis Disease

Hepatopancreatic necrosis disease (HPND) is a highly lethal disease that first emerged in 2015 in Jiangsu Province, China. So far, most researchers believe that this disease is caused by abiotic factors. However, its true pathogenic mechanism remains unknown. In this study, the effects of HPND on the metabolism and other biological indicators of the Chinese mitten crab (Eriocheir sinensis) were evaluated by integrating transcriptomics and metabolomics. Our findings demonstrate that the innate immunity, antioxidant activity, detoxification ability, and nervous system of the diseased crabs were affected. Additionally, metabolic pathways such as lipid metabolism, nucleotide metabolism, and protein metabolism were dysregulated, and energy production was slightly increased. Moreover, the IL-17 signaling pathway was activated and high levels of autophagy and apoptosis occurred in diseased crabs, which may be related to hepatopancreas damage. The abnormal mitochondrial function and possible anaerobic metabolism observed in our study suggested that functional hypoxia may be involved in HPND progression. Furthermore, the activities of carboxylesterase and acetylcholinesterase were significantly inhibited, indicating that the diseased crabs were likely stressed by pesticides such as pyrethroids. Collectively, our findings provide new insights into the molecular mechanisms altered in diseased crabs, as well as the etiology and pathogenic mechanisms of HPND.

The immuneoreaction and antioxidant status of Chinese mitten crab (Eriocheir sinensis) involve protein metabolism and the response of mTOR signaling pathway to dietary methionine levels

To study the effects of dietary methionine on growth performance, immunity, antioxidant capacity, protein metabolism, inflammatory response and apoptosis factors in Chinese mitten crabs (Eriocheir sinensis). Five diets with different methionine levels (0.63%, 0.85%, 1.06%, 1.25% and 1.47%) were fed to E. sinensis for 8 weeks. Results showed that in the 1.25% Met group, both growth performance and feed utilization were significantly increased. The crude protein content of crab muscle in the 1.06% and 1.25% Met groups was significantly higher than that in the control group. The immune and antioxidant enzyme activities, as well as gene expression levels of anti-lipopolysaccharide factor 1 (ALF1), Crustin-1, prophenoloxidase (proPO), cap 'n' collar isoform C (CncC) in 1.25% Met group were significantly higher than other groups. The activities of adenosine deaminase (ADA) and glutamate transaminase (GPT) in serum decreased first and then increased with the increase of methionine content, while the changes of ADA and GPT in hepatopancreas increased first and then decreased. 1.25% Met group exhibited significantly increased levels of GOT, GPT, and ADA compared to the control group. 1.25% Met diet group significantly up-regulated protein synthesis and anti-apoptotic factors, and significantly down-regulated inflammatory and pro-apoptotic factors in hepatopancreas. At 1.25% in the diet, methionine was found to boost E. sinensis growth, muscle protein deposition and immunity, as well as its antioxidant capacity. Combined with the above results, based on the expression of factors involved in the mammalian target of rapamycin (mTOR) signaling pathway and the mitogen-activated protein kinase (MAPK) signaling pathway, it is proved that methionine can not only promote protein metabolism, improve feed utilization, but also alleviate the inflammatory response and apoptosis caused by oxidative stress in the body.

The complete mitochondrial genome sequence of the Metschnikowia bicuspidata (Metschnikoff, 1884), an emerging pathogen of 'milky disease' in Chinese mitten crab

The outbreak of milky disease of Chinese mitten crab caused by M. bicuspidata seriously restricted the development of the crab industry. In this study, the mitochondrial genome sequence of M. bicuspidata was assembled, annotated, and further analyzed. The results indicated that the complete mitochondrial genome of M. bicuspidata was 75,095 bp, which contained two rRNAs, 23 tRNAs, and 13 protein-coding genes. The phylogenetic tree of 13 yeasts based on the complete mitochondrial genome was constructed which showed that M. bicuspidata (accession number OK514652) and M. bicuspidata (accession number MW147605.1) were clustered in a clade. To sum up, our research results would further provide essential data for the systematics and evolution study of M. bicuspidata.

16S rRNA gene sequencing analysis reveals an imbalance in the intestinal flora of Eriocheir sinensis with hepatopancreatic necrosis disease

Hepatopancreas necrosis disease (HPND) is a highly fatal disease that first appeared in Jiangsu Province, China, in 2015, and later spread to many other provinces, which had a severe impact on the culture of Chinese mitten crab (Eriocheir sinensis). Here, changes in the intestinal flora of healthy and HPND-affected Chinese mitten crabs were compared via 16S rRNA sequencing. Our findings indicated that Firmicutes, Bacteroidota, and Proteobacteria were the three dominant phyla in both healthy and HPND-affected crabs and exhibited no significant differences in α-diversity (richness p = 0.0892; evenness and diversity p = 0.0630). Furthermore, there were no significant changes in the abundance of Proteobacteria between the experimental groups. However, the abundance of Bacteroidota in the HPND group was significantly higher than that of the control group (HPND: 30.12%, Control: 16.60%), whereas the abundance of Firmicutes was significantly lower (HPND: 29.90%, Control: 50.55%). At the genus level, the abundance of Candidatus Bacilloplasma, Desulfovibrio, Bacteroides, and Aeromonas also differed significantly between groups (P < 0.05). Collectively, our study confirms an imbalance in the gut microbiota of Chinese mitten crabs with HPND and we speculate that this alteration may affect the metabolism and immune function of these organisms. Furthermore, we suspect that the structural changes in the intestinal flora of sick crabs observed in our study may be related to HPND.

The forkhead box O transcription factor regulates lipase and anti-microbial peptide expressions to promote lipid catabolism and improve innate immunity in the Eriocheir sinensis with hepatopancreatic necrosis disease

Eriocheir sinensis is a crustacean with great economic value, but the occurrence of hepatopancreatic necrosis disease (HPND) severely restricts the development of crab aquaculture. Study on the survival mechanism of crabs with HPND is beneficial to provide new strategies for disease prevention and control. The Forkhead box O (FOXO) transcription factor family is involved in various key biological processes of organisms. In this study, a FOXO gene (named as EsFOXO) from E. sinensis was cloned. The full-length cDNA of EsFOXO is 2592 bp containing a 2133 bp open reading frame that encodes 710 amino acids. EsFOXO was widely distributed in multiple immune tissues. Further study found that the expression of EsFOXO in the intestine of crabs with HPND was significantly upregulated compared with that in the normal crabs. However, whether EsFOXO is involved in the immune and metabolic regulation of crabs remains unknown. RNA interference analysis showed that EsFOXO participates in the positive regulation of the expression of two pancreatic lipases, three anti-lipopolysaccharide factors, and three crustins. Results from our research suggest that two strategies are adopted by crabs with HPND for survival under starvation: on the one hand, the synthesis of antimicrobial peptides is increased to improve the innate immunity; on the other hand, the expression of enzymes correlated with lipid catabolism is up-regulated that mobilizes the fat in the crab, going through catabolism. Our study provides more evidence for an in-depth understanding of the survival mechanism of crabs with HPND.

Microsporidian Pathogens of Aquatic Animals

Around 57.1% of microsporidia occupy aquatic environments, excluding a further 25.7% that utilise both terrestrial and aquatic systems. The aquatic microsporidia therefore compose the most diverse elements of the Microsporidia phylum, boasting unique structural features, variable transmission pathways, and significant ecological influence. From deep oceans to tropical rivers, these parasites are present in most aquatic environments and have been shown to infect hosts from across the Protozoa and Animalia. The consequences of infection range from mortality to intricate behavioural change, and their presence in aquatic communities often alters the overall functioning of the ecosystem.In this chapter, we explore aquatic microsporidian diversity from the perspective of aquatic animal health. Examples of microsporidian parasitism of importance to an aquacultural ('One Health') context and ecosystem context are focussed upon. These include infection of commercially important penaeid shrimp by Enterocytozoon hepatopenaei and interesting hyperparasitic microsporidians of wild host groups.Out of ~1500 suggested microsporidian species, 202 have been adequately taxonomically described using a combination of ultrastructural and genetic techniques from aquatic and semi-aquatic hosts. These species are our primary focus, and we suggest that the remaining diversity have additional genetic or morphological data collected to formalise their underlying systematics.

Protective effects of dietary icariin on lipopolysaccharide-induced acute oxidative stress and hepatopancreas injury in Chinese mitten crab, Eriocheir sinensis

To investigate the effects of dietary icariin (ICA) supplementation on acute oxidative stress and hepatopancreatic injury induced by lipopolysaccharide (LPS) injection in Eriocheir sinensis, an 8-week feeding trial of crabs was conducted using 4 diets with different supplementation levels of ICA (0, 50, 100, and 200 mg/kg diet weight, respectively), and then challenged with LPS of 400 μg/kg body weight for 6 h. Results showed that 100 mg/kg ICA supplementation increased the antioxidant capacity, reduced the stress-related indicators in haemolymph, strengthen the mitochondrial membrane potential, and reduce apoptosis compared to the single LPS-treated crabs. The expressions of apoptosis-related genes and proteins were also evaluated to further understand the effects of dietary ICA pretreatment on LPS-induced cell apoptosis. As a result, dietary 100 mg/kg diet weight ICA pre-addition significantly down-regulated the expression of HSP60, HSP70, Caspase 3c, Caspase 8, Caspase 3, Caspase 9, P38, and Bax (P < 0.05), and alleviated the suppressed expression of PI3K, AKT, MEK, and Bcl-2 (P < 0.05) in crabs challenged with LPS. Overall, this research reveals that ICA supplementation of 100 mg/kg diet weight could enhance the resistance to oxidative damage and apoptosis in E. sinensis facing LPS challenge.

The effects of ammonia-N stress on immune parameters, antioxidant capacity, digestive function, and intestinal microflora of Chinese mitten crab, Eriocheir sinensis, and the protective effect of dietary supplement of melatonin

Ammonia nitrogen pollution seriously affects the economic benefits of Chinese mitten crab (Eriocheir sinensis) farming. In this study, we first evaluated the protective effects of melatonin (MT) on immune parameters, antioxidant capacity, and digestive enzymes of E. sinensis under acute ammonia nitrogen stress. The results showed that ammonia-N stress significantly decreased the antibacterial ability of crabs, nevertheless MT could significantly improve it under ammonia-N stress (P < 0.05). Ammonia-N group hemolymph antioxidant capacity indicators (T-AOC, T-SOD, GSH-Px) were significantly decreased than control (p < 0.05), while the MT ammonia-N group hemolymph T-SOD activity significantly increased than ammonia-N group (p < 0.05). For hepatopancreas, ammonia-N group GSH-PX activity significantly decreased than control group, but MT ammonia-N group was significant increased than ammonia-N (p < 0.05). Ammonia-N stress has significantly increased the content of MDA in hemolymph and hepatopancreas (p < 0.05), but MT ammonia-N treatment significantly decreased than ammonia-N group (p < 0.05). Compared with the control group, ammonia-N significantly reduced the activities of Trypsin in the intestine and hepatopancreas (p < 0.05), while MT ammonia-N group can significantly improve the intestinal trypsin activity than ammonia-N (p < 0.05). The intestinal microbiota of E. sinensis results showed that ammonia-N stress significantly decreased the relative abundance of Bacteroidetes (p < 0.05). Ammonia-N stress significantly decreased the Dysgonomonas and Rubellimicrobium, and the Citrobacter significantly increased. In summary, melatonin has a protective effect on E. sinensis under ammonia-N stress. Acute ammonia-N stress may lead to the decrease of probiotics and the increase of pathogenic bacteria, which may be closely related to the impairment of digestive function and immune function.

Transcriptome analysis reveals the molecular mechanism of long-term exposure of Eriocheir sinensis to low concentration of trichlorfon

Trichlorphon, a common organophosphorus pesticide (OPs), is widely used in aquaculture to prevent aquatic insects from infecting cultured objects as well as to control the excessive proliferation of plankton in water bodies. However, its repeated use time can contaminate water bodies and impart direct/indirect toxicity to beneficial aquatic species. However, the underlying mechanism regarding toxicity and cellular metabolism remains unclear. Understanding the mechanism would enable the standardized use and management of OPs and their use in the aquatic environment. Here, low concentration of trichlorphon (5 × 10^-5 g/L) was used to construct a hepatopancreatic transcriptional library 30 d, 60 d and 90 d after exposure using RNA-Seq. We detected 649, 148, and 2949 DEGs in the hepatopancreas of E. sinensis for the Tri01 vs. Ctr01, Tri02 vs. Ctr02 and Tri03 vs. Ctr03 library, respectively. The results of KEGG pathway enrichment analysis showed that DEGs were mainly enriched in signal transduction, carbohydrate metabolism, transport and catabolism, endocrine system, and digestive system. Also, under trichlorfon stress, DEGs of E. sinensis were enriched in thyroid hormone signaling pathways, protein digestion and absorption, cancer pathways, etc. The significant DEGs were mainly related to metabolism and the apoptosis and autophagy pathways. This study lays a foundation for further revealing the effects of long-term trichlorfon stress on E. sinensis as well as the potential physiological toxicity. The relevant transcriptome data could provide a reference for the molecular toxicological evaluation of trichlorfon in aquaculture.

Characterizing the Xenoma of Vairimorpha necatrix Provides Insights Into the Most Efficient Mode of Microsporidian Proliferation

Microsporidia are a group of obligated intracellular parasites that can infect nearly all vertebrates and invertebrates, including humans and economic animals. Microsporidian Vairimorpha necatrix is a natural pathogen of multiple insects and can massively proliferate by making tumor-like xenoma in host tissue. However, little is known about the subcellular structures of this xenoma and the proliferation features of the pathogens inside. Here, we characterized the V. necatrix xenoma produced in muscle cells of silkworm midgut. In result, the whitish xenoma was initially observed on the 12^th day post infection on the outer surface of the midgut and later became larger and numerous. The observation by scanning electronic microscopy showed that the xenoma is mostly elliptical and spindle with dense pathogen-containing protrusions and spores on the surface, which were likely shedding off the xenoma through exocytosis and could be an infection source of other tissues. Demonstrated with transmission electron microscopy and fluorescent staining, the xenoma was enveloped by a monolayer membrane, and full of vesicle structures, mitochondria, and endoplasmic reticulum around parasites in development, suggesting that high level of energy and nutrients were produced to support the massive proliferation of the parasites. Multiple hypertrophic nuclei were found in one single xenoma, indicating that the cyst was probably formed by fusion of multiple muscle cells. Observed by fluorescence in situ hybridization, pathogens in the xenoma were in merongony, sporogony, and octosporogony, and mature stages. And mature spores were pushed to the center while vegetative pathogens were in the surface layer of the xenoma. The V. necatrix meront usually contained two to three nuclei, and sporont contained two nuclei and was wrapped by a thick membrane with high electron density. The V. necatrix sporogony produces two types of spores, the ordinary dikaryotic spore and unicellular octospores, the latter of which were smaller in size and packed in a sporophorous vesicle. In summary, V. necatrix xenoma is a specialized cyst likely formed by fusion of multiple muscle cells and provides high concentration of energy and nutrients with increased number of mitochondria and endoplasmic reticulum for the massive proliferation of pathogens inside.

Evaluation of pathology and environmental variables contributing to hepatopancreatic necrosis syndrome of Chinese mitten crab, Eriocheir sinensis

Hepatopancreatic Necrosis Syndrome (HPNS) severely impacts the Chinese mitten crab (Eriocheir sinensis) industry. However, little knowledge of the aetiology and pathogenesis of the disease causes significant difficulties in its prevention and control. In this study, we conducted a pathological analysis of HPNS through time-integrated large-volume sampling, to clarify the disease characteristics and mechanism of HPNS-afflicted crabs; besides, animal models were constructed to verify the pathological diagnosis. The results showed that the hepatopancreas was the principal target organ of HPNS; multiple correspondence analysis revealed that the main histopathological characteristics included non-interstitial atrophic hepatopathy diseases such as hepatic tubule atrophy, dilated hepatic tubules, and hepatic tubule necrosis. Additionally, the muscles also showed signs of disease, including myofibre atrophy, necrosis, and inflammation. Ultrastructural studies showed prominent apoptosis and autophagy-like alterations in the hepatopancreas of HPNS-afflicted crabs. Further, the establishment of animal models revealed that the double variate stimulation of environmental variables such as abamectin/sewage with nutrition deficiency could result in HPNS-similar lesions. Based on these studies, we concluded that HPNS is a chronic hepatopancreas-initiated energy-consumed disease with a low likelihood of pathogen but a high probability of environment and nutrition.

Metatranscriptomic Analysis Reveals an Imbalance of Hepatopancreatic Flora of Chinese Mitten Crab Eriocheir sinensis with Hepatopancreatic Necrosis Disease

Simple Summary

The cause of Chinese mitten crab Eriocheir sinensis hepatopancreas necrosis disease (HPND) remains a mystery. In this study, metatranscriptomics sequencing was conducted to characterize the changes in the structure and gene expression of hepatopancreatic flora of crabs with and without typical symptoms of HPND; an imbalance of hepatopancreatic flora can be found in the crab with HPND, and the detected microbial taxa decreased, whereas the prevalence of Spiroplasma eriocheiris significantly increased in the hepatopancreatic flora of crabs with typical symptoms of HPND, and the relative abundances of the virus and microsporidia in crabs with HPND were very low and did not increase with disease progression. The differentially-expressed genes (DEGs) in hepatopancreatic flora between crabs with and without HPND were enriched ribosome, retinol metabolism, and biosynthesis of unsaturated fatty acid KEGG pathways. These results suggested that an imbalance of hepatopancreatic flora was associated with crab HPND, and the enriched pathways of DEGs were associated with the pathological mechanism of HPND.

Abstract

Hepatopancreas necrosis disease (HPND) of the Chinese mitten crab Eriocheir sinensis causes huge economic loss in China. However, the pathogenic factors and pathogenesis are still a matter of dissension. To search for potential pathogens, the hepatopancreatic flora of diseased crabs with mild symptoms, diseased crabs with severe symptoms, and crabs without visible symptoms were investigated using metatranscriptomics sequencing. The prevalence of Absidia glauca and Candidatus Synechococcus spongiarum decreased, whereas the prevalence of Spiroplasma eriocheiris increased in the hepatopancreatic flora of crabs with HPND. Homologous sequences of 34 viral species and 4 Microsporidian species were found in the crab hepatopancreas without any significant differences between crabs with and without HPND. Moreover, DEGs in the hepatopancreatic flora between crabs with severe symptoms and without visible symptoms were enriched in the ribosome, retinol metabolism, metabolism of xenobiotics by cytochrome P450, drug metabolism—cytochrome P450, biosynthesis of unsaturated fatty acids, and other glycan degradation. Moreover, the relative abundance of functions of DEDs in the hepatopancreatic flora changed with the pathogenesis process. These results suggested that imbalance of hepatopancreatic flora was associated with crab HPND. The identified DEGs were perhaps involved in the pathological mechanism of HPND; nonetheless, HPND did not occur due to virus or microsporidia infection.

Comparison on nutritional quality of adult female Chinese mitten crab (Eriocheir sinensis) with different colored hepatopancreases

White hepatopancreas syndrome has recently emerged in Chinese mitten crab (Eriocheir sinensis) aquaculture, causing considerable economic loss. The hepatopancreas color of diseased crabs becomes gradually lighter, turning from yellow to yellow-white to white. Therefore, this study was conducted to investigate the changes in nutrient composition in three edible parts (hepatopancreas, ovaries, and muscle) of adult females with different colored hepatopancreases. Three groups were assessed in this study, including a yellow hepatopancreas group (control, L * = 63.92, a * = 22.14, b * = 60.95), a yellow-white hepatopancreas group (YWHG, L * = 65.06, a * = 22.35, b * = 57.80), and a white hepatopancreas group (WHG, L * = 65.72, a * = 10.70, b * = 30.52). No statistically significant differences in average weight, tissue indices, and total edible yield were observed among the three crab groups (P >0.05). The moisture content of the hepatopancreases and ovaries in WHG was 56.12% and 9.23% higher than the control values (P <0.05), whereas hepatopancreas crude fat and ovary crude protein levels were 62.23% and 11.45% lower than the control values (P < 0.05). The total carbohydrate levels of the three edible tissues were significantly higher and the crude protein content of ovaries was significantly lower in YWHG (P < 0.05). Most amino acid levels in the WHG muscle and ovaries were significantly lower than the control (P < 0.05). Moreover, the hepatopancreas levels of total polyunsaturated fatty acids (PUFA) and n-6PUFA in WHG were 24.88% and 31.83% lower than in control group, whereas the hepatopancreas levels of total PUFA and n-6PUFA in YWHG were also 21.88% and 23.20% lower compared to the controls (P < 0.05). Overall, the growth and the edible parts were not affected in YWHG and WHG. Moreover, YWHG crabs exhibited few effects on nutritional value; however, the fatty acid composition of crabs was significantly changed. In contrast, WHG crabs exhibited poor nutritional quality. Nonetheless, the consumption of crabs with yellow-white or white hepatopancreases is not recommended since the animal also referred to as diseased crabs.

Proteomic Analysis of the Hemolymph After Metschnikowia bicuspidata Infection in the Chinese Mitten Crab Eriocheir sinensis

The “milky disease” of the Chinese mitten crab, Eriocheir sinensis, is a highly lethal fungal disease caused by Metschnikowia bicuspidata infection. To elucidate the immune responses of the hemolymph of E. sinensis to M. bicuspidata infection, a comparative analysis of the hemolymph of E. sinensis infected with M. bicuspidata and that treated with phosphate buffered saline was performed using label-free quantitative proteomics. A total of 429 proteins were identified. Using a 1.5-fold change in expression as a physiologically significant benchmark, 62 differentially expressed proteins were identified, of which 38 were significantly upregulated and 24 were significantly downregulated. The upregulated proteins mainly included cytoskeleton-related proteins (myosin regulatory light chain 2, myosin light chain alkali, tubulin α-2 chain, and tubulin β-1 chain), serine protease and serine protease inhibitor (clip domain-containing serine protease, leukocyte elastase inhibitor, serine protein inhibitor 42Dd), catalase, transferrin, and heat shock protein 70. Upregulation of these proteins indicated that phenoloxidase system, phagocytosis and the ROS systems were induced by M. bicuspidata. The downregulated proteins were mainly organ and tissue regeneration proteins (PDGF/VEGF-related factor protein, integrin-linked protein kinase homing pat-4 gene) and hemagglutination-associated proteins (hemolymph clottable protein, hemocyte protein-glutamine gamma-glutamyltransferase). Downregulation of these proteins indicated that M. bicuspidata inhibited hemocyte regeneration and hemolymph agglutination. Fifteen differentially expressed proteins related to immunity were verified using a parallel reaction monitoring method. The expression trend of these proteins was similar to that of the proteome. To the best of our knowledge, this is the first report on the proteome of E. sinensis in response to M. bicuspidata infection. These results not only provide new and important information on the immune response of crustaceans to yeast infection but also provide a basis for further understanding the molecular mechanism of complex host pathogen interactions between crustaceans and fungi.

Morphological and molecular characterization of a new microsporidium, Janacekia tainanus n. sp. from the adipose tissue of Kiefferulus tainanus (Diptera: Chironomidae) in China

We reported a new microsporidium Janacekia tainanus n. sp. from the adipose tissue of the midge Kiefferulus tainanus Kieffer, 1912 collected from a eutrophic pond in Daye city, Hubei Province, China. Infected chironomid larvae with hypertrophied adipose tissue exhibited porcelain-white. All developmental stages possessed large nuclei. The earliest stages observed were diplokaryotic meronts which were in direct contact with the host adipocyte cytoplasm. Diplokaryotic meronts developed into sporonts with the deposition of electron-dense coagulum on their surface. Multinucleate sporogonial plasmodia developed into uninucleate sporoblasts by the rosette-like division. Mature spores were oval and monokaryotic, measuring 6.14 ± 0.27 (5.65-6.67) µm long and 3.71 ± 0.12 (3.43-3.98) µm wide. Bipartite polaroplast consisted of a narrow anterior lamella and a wide posterior lamella. Isofilar polar filaments coiled 13-17 turns and arranged in one row. The exospore was thin and of no stratification, but remarkably covered with tubular secretions. The electron-lucent endospore was thick and measured 145-352 nm wide. Phylogenetic analysis based on the obtained SSU rDNA sequence indicated that the present species clustered closely with Jirovecia sinensis, a species with rod-shaped mature spores isolated from the coelomocytes of Branchiura sowerbyi. Consistent with the previous result, the monophyletic clade of Jirovecia-Bacillidium-Janacekia was sister to Pseudonosema clade and then collectively nested within Clade V of Class Aquasporidia sensu Vossbrinck and Debrunner-Vossbrinck (2005). The novel species did not form an independent monophyletic lineage with the congener, Janacekia debaisieuxi. Based on the morphological characters and ultrastructural features, as well as SSU rDNA-inferred phylogenetic relationships, a new species in the genus Janacekia, Janacekia tainanus n. sp. was designated. This is the first report of aquatic arthropod-infecting microsporidia in China.

Lipid metabolism disorders contribute to the pathogenesis of Hepatospora eriocheir in the crab Eriocheir sinensis

The microsporidia Hepatospora eriocheir has been identified as an emerging pathogenic agent in the commercial crab Eriocheir sinensis. Histology analysis indicated that hepatopancreas was a significant target for H. eriocheir infection. However, the functional consequences of such tissue tropism remain poorly studied. Considering that hepatopancreas was a centre for lipid metabolism and energy supply, we furtherly investigated the comparative lipid metabolism profiles between the control and H. eriocheir-infected hepatopancreas by liquid chromatography-mass spectrometry (LC-MS)-based lipidomics approach. Results confirmed that H. eriocheir induced apparent alterations of lipid metabolic phenotypes in hepatopancreas. Sixty-seven lipids, including triglyceride (TG), diglyceride (DG), sphingomyelin (SM), phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylglycerol (PG), phosphatidylinositol (PI), ceramide (CER), hexosyl ceramide (HEX CER) and (o-acyl)-1-hydroxy fatty acid (OAHFA), were significantly changed and could be determined as effective biomarkers. TG and DG accounted for the largest proportion (58.2% and 11.9%, respectively). Notably, over 94% of the distinguished lipids presented a similar modified trend with profoundly reduced contents, implying blatant energy exploitation of the parasite. These lipids were involved in pathways of energy and lipid metabolism and signal regulation. Such information suggests that H. eriocheir possibly "starves" the host via destructing hepatopancreas tissue together with appropriate host energy resources, leading to the corresponding alterations of lipid metabolism and a decrease in the colour of the hepatopancreas.

The protective effects of melatonin on survival, immune response, digestive enzymes activities and intestinal microbiota diversity in Chinese mitten crab (Eriocheir sinensis) exposed to glyphosate

Glyphosate is one of the most widely used pesticides, which can cause toxicity to aquatic animals. In this study, the survival rate, immune response, digestive enzyme activities, and the intestinal microbiota diversity of Chinese mitten crab (Eriocheir sinensis) were evaluated after 14 days of exposure to glyphosate (48.945 mg/L from 50% 96 h LC50 value) and melatonin feeding (80 mg/kg). The results showed that MT significantly improved the survival rate, antibacterial capacity of E. sinensis (P < 0.05). After exposure to glyphosate, the expression of Hsp60, Hsp70 and Hsp90 in cranial ganglia and thoracic ganglia was decreased significantly, but MT significantly raised the expression of these proteins (P < 0.05). Glyphosate significantly decreased lipase activity compared with the control group (P < 0.05), while melatonin significantly increased the lipase, amylase and trypsin activities (P < 0.05). Melatonin significantly increased the Chao1 and Shannon index and the relative abundance of Proteobacteria and Bacteroidetes (P < 0.05). This study shows that melatonin has a protective effect on the glyphosate exposed E. sinensis.

Metschnikowia bicuspidate associated with a milky disease in Eriocheir sinensis and its effectitve treatment by Massoia lactone

The pathogenic yeast strain LIAO causing the milky disease in the Chinese mitten crab belonged to one member of Metschnikowia bicuspidate which could grow well at different temperatures from 28 to 4 °C. It was also found that the pathogenic yeast strain LIAO could grow in the extracts of the muscle, gill, heart tissues, intestinal tracts of the healthy Chinese mitten crabs by using the reducing sugars, amino acids and other nutrients in them. Massoia lactone released from liamocins produced by Aureobasidium melanogenum had high anti-fungal activity against the pathogenic yeast strain LIAO and M. bicuspidate WCY isolated from the diseased marine crabs. The minimal inhibitory concentrations (MIC) and the minimal fungicidal concentration (MFC) in the liquid culture against the pathogenic yeast strain LIAO were 0.15 mg/mL and 0.34 mg/mL, respectively. Massoia lactone as a bio-surfactant could damage the cell membrane, even break the whole cells of the pathogenic yeast strain LIAO and cause cellular necrosis of the pathogenic yeast LIAO. Therefore, Massoia lactone could be used to effectively kill the pathogenic yeast strains and as an effectitve treatment for milky disease in the Chinese mitten crab.

Morphological and molecular characterization of a new species, Agglomerata daphniae n. sp. from the hypoderm of Daphnia magna (Crustacea: Daphniidae)

A new microsporidian species was described from the hypoderm of Daphnia magna sampled from gibel carp (Carassius auratus gibelio) ponds located in Wuhan city, China. The infected cladocerans generally appeared opaque due to numerous plasmodia distributed in the host integument. The earliest stages observed were uninucleate meronts that were in direct contact with the host cell cytoplasm. Meronts developed into multinucleate sporogonial plasmodia enclosed in sporophorous vesicles. Sporoblasts were produced by the rosette-like division of sporogonial division. Mature spores were pyriform and monokaryotic, measuring 4.48 ± 0.09 (4.34-4.65) µm long and 2.40 ± 0.08 (2.18-2.54) µm wide. The polaroplast was bipartite with loose anterior lamellae and tight posterior lamellae. Polar filaments, arranged in two rows, were anisofilar with two wider anterior coils, and five narrower posterior coils. The exospore was covered with fibrous secretions and was composed of four layers. Phylogenetic analysis based on the obtained SSU rDNA sequence, indicated that the present species clustered with three unidentified Daphnia pulicaria-infecting microsporidia with high support values to form a monophyletic lineage, rather than with the congener, Agglomerata cladocera. The barcode motif of the internal transcribed spacer (ITS) region of the novel species was unique among representatives of the "Agglomeratidae" sensu clade (Vávra et al., 2018). Based on the morphological characters and SSU rDNA-inferred phylogenetic analyses, a new species was erected and named as Agglomerata daphniae n. sp. This is the first report of zooplankton-infecting microsporidia in China.

Relationship Between the Fatty Acid Profiles and Gut Bacterial Communities of the Chinese Mitten Crab (Eriocheir sinensis) From Ecologically Different Habitats

The gut microbiota plays an important role in a variety of physiological functions such as intestinal digestion, metabolic homeostasis, immune response, and responses to disease treatment. Whether there is a relationship between gut microbial communities and fatty acid (FA) profiles of Chinese mitten crab is unclear. Hence, we analyzed the relationship between FA profiles and the gut bacterial communities of six Chinese mitten crab (Eriocheir sinensis) populations from different lakes. The crabs were sampled from six different lakes in Jiangsu Province, China. The FA profiles of these crab populations were compared and clustered, and then used to determine the relationship between geographic location and FA composition. We also characterized the gut microbial communities of these crabs using 16S rRNA high-throughput gene sequencing. The FA profiles varied significantly (P < 0.05) between crabs from different geographical locations. A similar trend was also observed in the gut microbial communities, which also varied significantly based on their geographical origin (P < 0.05). Furthermore, alpha diversity, cluster analysis, and matching bacterial community structures with specific locations revealed patterns that significantly linked FA profiles to the gut microbiota. Further analysis of FA profiles and gut microbial community generated patterns that linked the two parameters. Hence, it was observed that the gut microbial community seems to contribute significantly to the FA composition of the Chinese mitten crab. However, further studies need to be conducted to investigate the interactions between gut microbial communities and the biochemical composition of the Chinese mitten crab, which will ultimately unravel the complexity of microbial ecosystems for potential applications in aquaculture and species conservation.

Transcriptome analysis of hepatopancreas of Eriocheir sinensis with hepatopancreatic necrosis disease (HPND)

Hepatopancreatic necrosis disease (HPND) is a newly emerging disease in the Chinese mitten crab, Eriocheir sinensis, which has resulted in large economic losses. However, the underlying cause of this disease remains unclear. To better understand the pathogenesis and pathogenic mechanism of HPND, we compared the transcriptome differences of the hepatopancreas of E. sinensis with and without HPND. The analysis yielded > 30 million reads for each sample of three test (with HPND) and three control groups (without HPND). We observed 978 downregulated genes and 644 upregulated genes. Among the gene ontology categories "biological process," "cellular component," and "molecular function", the subcategories cellular process, single-organism process, biological regulation, metabolic process, cell part, organelle, organelle part, binding, and catalytic were enriched. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis showed that "metabolism of xenobiotics by cytochrome P450," "drug metabolism-cytochrome P450," "chemical carcinogenesis," and "material metabolism" were the "five" most significantly enriched pathways in the hepatopancreas of E. sinensis with HPND. The results revealed that material metabolic abnormalities and drug effects from the external environment might be associated with HPND in the Chinese mitten crab. Considering the wide use of pyrethroids for pond cleaning in Xinghua city, we speculated that pyrethroids might cause HPND in the Chinese mitten crab. Our study provided useful information about the cause and pathogenetic mechanisms of HPND and could help to prevent this disease in production practice.

Arginine supplementation improves growth, antioxidant capacity, immunity and disease resistance of juvenile Chinese mitten crab, Eriocheir sinensis

To investigate the effects of arginine (Arg) on the growth, antioxidant capacity, immunity and disease resistance of juvenile Chinese mitten crab, three diets containing Arg levels at 1.72% (control), 2.73% and 3.72% were formulated and fed to Chinese mitten crab (0.22 ± 0.03 g) for eight weeks. The weight gain, ecdysterone and growth hormone in the serum, relative expression of insulin-like growth factor 2 in the hepatopancreas significantly increased in crabs fed the 2.73% and 3.72% Arg diets. The protein and lipid contents significantly increased in crabs fed the 3.72% Arg diet. The feed conversion ratios in crabs fed the diets with Arg additions were lower than in the control. Arg supplementation also enhanced the antioxidative capacity by increasing the activities of superoxide dismutase, catalase and the relative expression of Kelch-like ECH-associated protein 1 gene in the hepatopancreas, which subsequently decreased malondialdehyde content in the hepatopancreas. Besides, Arg also decreased nitric oxide content in the serum and the activity of nitric oxide synthetase in the hepatopancreas. The relative mRNA levels of crustin, relish, lysozyme and cryptocyanin genes were significantly upregulated by Arg supplementation. The activities of acid phosphatase and alkaline phosphatase in the serum significantly increased in crabs fed the 3.72% Arg diet than those in the control. Similarly, the relative mRNA levels of crustin, cryptocyanin and proPO genes were significantly upregulated in crabs fed the 2.73% Arg diet after lipopolysaccharide challenge, and in crabs fed the 3.72% Arg diet after the Poly (I:C) challenge. The crabs fed the 2.73% and 3.72% Arg diets had higher survival rate after bacterial infection than those fed the control diet. This study indicates that the addition of Arg to the diet at 2.7-3.7% can improve the growth, survival, antioxidant capacity, immunity and disease resistance in juvenile Chinese mitten crab.

Effects of the glyphosate-based herbicide roundup on the survival, immune response, digestive activities and gut microbiota of the Chinese mitten crab, Eriocheir sinensis

Glyphosate is one of the most widely used pesticides in the world and can be transported easily by surface runoff, air, and rivers, potentially affecting aquaculture. In this study, the survival rate, intestinal and hepatopancreatic immune and digestive functions, and the intestinal microbial diversity of Chinese mitten crab (Eriocheir sinensis) were evaluated after 7 days of exposure to glyphosate (48.945 mg/L from 1/2 96-h LC50 value). The results showed that glyphosate significantly reduced the survival rate of E. sinensis. After exposure to glyphosate, the totoal antioxidant capacity (T-AOC) in the midgut and hindgut of E. sinensis was significantly decreased, and malondialdehyde (MDA) content in the midgut was significantly increased (P < 0.05). After glyphosate exposure, the activities of digestive enzymes (including lipase and amylase) in the intestinal tract were significantly decreased and trypsin was significantly increased, while three enzymes in the hepatopancreas were significantly increased (P < 0.05). Using high-throughput sequencing analysis of the gut microbiota, the results showed that glyphosate significantly decreased the diversity of E. sinensis gut microbiota, while significantly increasing the taxonomic richness of Bacteroidetes and Proteobacteria (P < 0.05). This study suggested that these bacteria may be involved in glyphosate effects on survival by regulation of immune and digestive function.

Effects of dietary supplementation with icariin on growth performance, antioxidant capacity and non-specific immunity of Chinese mitten crab (Eriocheir sinensis)

We investigated the effects of icariin (ICA) on growth performance, antioxidant capacity and non-specific immunity in Chinese mitten crab (Eriocheir sinensis). A total of 200 healthy crabs (average weight: 33.58 ± 0.05 g) were randomly assigned to four treatments with five replicates, each with ten individuals per pool. There were four dietary treatments: the control group (fed with the basal diet), the ICA 50 group, the ICA100 group, and the ICA 200 group (fed with the basal diet supplemented with 50, 100, and 200 mg/kg ICA, respectively). These diets were provided for 8 weeks. Results indicated that ICA100 crabs had higher weight gain (WG), specific growth rate (SGR) and survival rate (SR) than the controls. Protein carbonyl content (PCC) and malondialdehyde (MDA) concentrations in the haemolymph and hepatopancreas of ICA100 crabs were significantly lower than in the control group, while the superoxide dismutase (SOD) and glutathione peroxidase (GPx) activities were significantly higher. The activities of PO, LZM, ACP and AKP were significantly enhanced with ICA supplementation at 50 and 100 mg/kg, yet decreased subsequently at 200 mg/kg. Furthermore, supplementation of 100 mg/kg ICA up-regulated the mRNA expression of prophenoloxidase (proPO), catalase (CAT), mitochondrial manganese superoxide dismutase (mtMnSOD), thioredoxin-1 (Trx1) and peroxiredoxin 6 (Prx6), while the mRNA expression of toll like receptors (TLRs), NF-κB-like transcription factor Relish and lipopolysaccharide-induced TNF-α factor (LITAF) were down-regulated in the hepatopancreas (P < 0.05). These findings indicate that dietary ICA supplementation at an optimum dose of 100 mg/kg may be effective in improving growth performance, antioxidant capability and non-specific immunity of Chinese mitten crab.

Effect of dietary L-tryptophan on the survival, immune response and gut microbiota of the Chinese mitten crab, Eriocheir sinensis

This study investigated the influence of L-tryptophan (L-trp) on the survival, immune response and gut microbiota of the Chinese mitten crab, Eriocheir sinensis (with an average weight of 16.58 ± 2.20 g). After 30 days of feeding with diets supplemented with L-trp at 0.36%, 0.47%, 0.73% and 1.05% (groups 1, 2, 3 and 4, respectively), the survival rate and bacterial challenge (Aeromonas hydrophila) were evaluated, the activities of antioxidant and phosphatase enzymes in the serum were assessed, and the gut microbiota were measured via high-throughput Illumina sequencing. The results showed that the supplementation of L-trp significantly improved the survival rate of crabs (P < 0.05). After feeding for 7 days, it was observed that a high L-trp diet significantly increase the survival rate relative to a basal diet after a 96-h post-challenge with A. hydrophila (P < 0.05). The activity of CAT and AKP in the serum were increased by the addition of L-trp. The activity of CAT and AKP in the serum in group 4 were higher than those in group 1 (P < 0.05). Furthermore, we observed that adjunction of the L-trp can significantly increase the richness and diversity of the gut microbiota. The dominant phylum in the intestine of the Chinese mitten crab were Tenericutes, Proteobacteria, Firmicutes, Chloroflexi and Actinobacteria. The L-trp in the diets increased the richness of Proteobacteria, Firmicutes and Actinobacteria in the intestine significantly. These bacteria were all dominant bacteria and had a specific role in promoting the immunity of E. sinensis. Therefore, it could be inferred that L-trp supplementation is beneficial in the diet of E. sinensis. Based in these results, the dietary 0.47% or 0.73%L-trp supplemented is found to be optimum to improve E. sinensis survival.

An integrated metabolic consequence of Hepatospora eriocheir infection in the Chinese mitten crab Eriocheir sinensis

Despite the economic and evolutionary importance of aquatic host-infecting microsporidian species, at present, limited information has been provided about the microsporidia-host interactions. This study focused on Hepatospora eriocheir, an emerging microsporidian pathogen for the Chinese mitten crab Eriocheir sinensis. Hypertrophy of hepatopancreas cells was a common feature of H. eriocheir infection. More importantly, mitochondria of the hepatopancreas were drawn around the H. eriocheir, most likely to aid the uptake of ATP directly from the host. To better understand the crab anti-microsporidian response, de novo transcriptome sequencing of the hepatopancreas tissue was furtherly proceeded. A total of 47.84 M and 57.21 M clean reads were generated from the hepatopancreas of H. eriocheir infected and control groups respectively. Based on homology searches, functional annotation with 6 databases (Nr, Swiss-Prot, KEGG, KOGs, Pfam and GO) for 88,168 unigenes was performed. 2619 genes were identified as differently up-regulated and 2541 genes as differently down-regulated. Prominent functional categories enriched with differentially expressed genes (DEGs) were "ATP binding", "mitochondrion and extracellular region", "oxygen transporter activity", "oxidoreductase activity", "alanine, aspartate and glutamate metabolism", "carbohydrate metabolic process", "starch and sucrose metabolism" and "fatty acid biosynthesis". These results confirmed a parasite external energy supply and an integrated metabolic stress. In addition, simple sequence repeats (SSRs) and single nucleotide polymorphisms (SNPs) were also identified from the gene library. Taken together, these findings allow us to better understand the underlying mechanisms regulating interactions between H. eriocheir and the crab E. sinensis.

Hepatopancreas transcriptome analysis of Chinese mitten crab (Eriocheir sinensis) with white hepatopancreas syndrome

White hepatopancreas is a syndrome that has recently emerged in aquaculture of Chinese mitten crab (Eriocheir sinensis). High lethality of the disease caused large economic loss, which drew considerable attention of fish farmers and scientific researchers. In this study, hepatopancreas reference transcriptome was de novo assembled and differential expression analysis was conducted between white hepatopancreas and normal (yellow) hepatopancreas of E. sinensis. A total of 90,687 transcripts were assembled, and 27,387 were annotated. Transcriptomic comparison revealed 69 differentially expressed genes between individuals featuring white hepatopancreas and yellow hepatopancreas. Genes associated with immune response and cell death, include thioredoxin-related transmembrane protein 1, hemocytin, methuselah-like 1, and E3 ubiquitin-protein ligase, and they were up-regulated, whereas titin and 5-formyltetrahydrofolate cyclo-ligase, which are genes related to cell proliferation, were down-regulated in E. sinensis with white hepatopancreas syndrome. Our study provides novel insights into genetic causes of formation and novel gene markers for detection of white hepatopancreas syndrome in aquaculture of E. sinensis.

Development of In situ hybridization and real-time PCR assays for the detection of Hepatospora eriocheir, a microsporidian pathogen in the Chinese mitten crab Eriocheir sinensis

A microsporidian parasite, Hepatospora eriocheir, is an emerging pathogen for the Chinese mitten crab Eriocheir sinensis. Currently, there is scant information about the way it transmits infection in the crustacean of commercial importance, including its pathogenesis, propagation and infection route in vivo. In this study, chromogenic in situ hybridization (ISH) and quantitative real-time PCR (qPCR) assays were developed to address this pressing need, and we provided an advance in the detection methods available. Pathogens can be seen in situ with associated lesions using ISH. Positive hybridization signals were noted inside the epithelial cells of the hepatopancreas, and putative free parasite spores were observed within the tubule lumen, which were associated with lesions detected by electron microscopy and haematoxylin and eosin (H&E) analysis. qPCR allows the determination of parasite loads in infected tissues, which is important for understanding disease progression and transmission. The hepatopancreas displayed the biggest statistical copy numbers among different tissues of infected crabs, confirming a tissue-specific pathogen infection characteristic. The qPCR assay also proved to be suitable for the diagnosis of asymptomatic carrier crabs. Combination of the two methods could facilitate the study of H. eriocheir infection mechanism in E. sinensis, enhance the early diagnosis of the pathogen and improve the management of microsporidian diseases in commercial crustaceans.

A Meta-Transcriptomics Survey Reveals Changes in the Microbiota of the Chinese Mitten Crab Eriocheir sinensis Infected with Hepatopancreatic Necrosis Disease

Infection of the freshwater Chinese mitten crab Eriocheir sinensis with hepatopancreatic necrosis disease (HPND) has been a major problem in the crab-cultivated Chinese Province of Jiangsu since 2015. To explore the etiology of HPND, meta-transcriptomic libraries of the hepatopancreata from crabs with and without HPND were constructed. Comparison analyses showed that there were no statistically significant differences in viral and microsporidial communities in the hepatopancreata of diseased and healthy crabs. Bacteroidetes, Proteobacteria, and Firmicutes were the most dominant bacterial phyla in the hepatopancreata of healthy crabs, with a combined prevalence of 93%. However, a decrease in bacterial diversity and a striking shift in the microbial composition were found in the hepatopancreata of crabs infected with HPND. Tenericutes was the most prevalent bacterial phylum in diseased crabs (31.82%), whereas its prevalence was low in healthy crabs (0.02%). By contrast, the prevalence of Bacteroidetes was significantly lower in crabs with HPND (3.49%) than in crabs without HPND (41.04%). We also found that the prevalence of Actinobacteria was higher in crabs with HPND (16.70%) than in crabs without the disease (4.03%). The major bacterial family within the Tenericutes phylum in crabs with HPND was detected by polymerase chain reaction and determined to be Mycoplasmataceae. In conclusion, there were striking changes in the microbiota of diseased and healthy crabs. Specifically, the prevalence of bacteria belonging to Tenericutes and Actinobacteria phyla increased, whereas the prevalence of bacteria belonging to the Bacteroidetes phylum decreased in crabs with HPND, clearly pointing to an association with HPND.

Biochemical changes and tissue distribution of Enterocytozoon hepatopenaei (EHP) in naturally and experimentally EHP-infected whiteleg shrimp, Litopenaeus vannamei (Boone, 1931), in India

Stunted growth in pond-reared Litopenaeus vannamei was observed in different farms located in Tamil Nadu and Andhra Pradesh, India. No mortality was associated with stunted growth. PCR assay on these samples revealed the presence of Enterocytozoon hepatopenaei (EHP) in stunted shrimp. Tissue distribution of EHP in naturally and experimentally infected shrimp was studied by PCR and histology. Histological examination revealed the presence of EHP in hepatopancreas and gut, but not in other organs. The PCR assay revealed the presence of EHP in all the organs tested in both naturally and experimentally infected shrimp. Healthy shrimp were challenged with E. hepatopenaei by intramuscular injection and oral route, and no mortality was observed in both routes after 30 days post-challenge. Different developmental stages of the microsporidian parasite were observed in the hepatopancreatic epithelial cells. Biochemical parameters such as total protein, albumin, aspartate transaminase (AST), alanine transaminase (ALT) and alkaline phosphatase were measured in the haemolymph of naturally and experimentally EHP-infected shrimp. All biochemical parameters mentioned were found to be significantly higher in EHP-infected shrimp when compared to normal shrimp. This is the first report relating AST and ALT levels to EHP infection in naturally and experimentally infected shrimp.

Gliocyte and synapse analyses in cerebral ganglia of the Chinese mitten crab, Eriocheir sinensis: ultrastructural study

The Chinese mitten crab Eriocheir sinensis is an economically important aquatic species in China. Many studies on gene structure, breeding, and diseases of the crab have been reported. However, knowledge about the organization of the nerve system of the crab remains largely unknown. To study the ultrastructure of the cerebral ganglia of E. sinensis and to compare the histological findings regarding the nerve systems of crustaceans, the cerebral ganglia were observed by transmission electron microscopy. The results showed that four types of gliocytes, including type I, II, III, and IV gliocytes were located in the cerebral ganglia. In addition, three types of synapses were present in the cerebral ganglia, including unidirectional synapses, bidirectional synapses, and combined type synapses.

Single and multi-gene phylogeny of Hepatospora (Microsporidia) - a generalist pathogen of farmed and wild crustacean hosts

Almost half of all known microsporidian taxa infect aquatic animals. Of these, many cause disease in arthropods. Hepatospora, a recently erected genus, infects epithelial cells of the hepatopancreas of wild and farmed decapod crustaceans. We isolated Hepatospora spp. from three different crustacean hosts, inhabiting different habitats and niches; marine edible crab (Cancer pagurus), estuarine and freshwater Chinese mitten crab (Eriocheir sinensis) and the marine mussel symbiont pea crab (Pinnotheres pisum). Isolates were initially compared using histology and electron microscopy revealing variation in size, polar filament arrangement and nuclear development. However, sequence analysis of the partial SSU rDNA gene could not distinguish between the isolates (~99% similarity). In an attempt to resolve the relationship between Hepatospora isolated from E. sinensis and C. pagurus, six additional gene sequences were mined from on-going unpublished genome projects (RNA polymerase, arginyl tRNA synthetase, prolyl tRNA synthetase, chitin synthase, beta tubulin and heat shock protein 70). Primers were designed based on the above gene sequences to analyse Hepatospora isolated from pea crab. Despite application of gene sequences to concatenated phylogenies, we were unable to discriminate Hepatospora isolates obtained from these hosts and concluded that they likely represent a single species or, at least subspecies thereof. In this instance, concatenated phylogenetic analysis supported the SSU-based phylogeny, and further, demonstrated that microsporidian taxonomies based upon morphology alone are unreliable, even at the level of the species. Our data, together with description of H. eriocheir in Asian crab farms, reveal a preponderance for microvariants of this parasite to infect the gut of a wide array of decapods crustacean hosts and the potential for Hepatospora to exist as a cline across wide geographies and habitats.