Larval development of the Chinese mitten crabEriocheir sinensis H. Milne-Edwards (Decapoda: Grapsidae) reared in the laboratory

The underestimated importance of fertilizer in aquacultural phosphorus budget: Case of Chinese mitten crab

China's reliance on aquaculture has intensified to satisfy the growing human demand for high-quality animal protein, making it the only country whose aquaculture production has greatly exceeded that of capture fishery for a long time. Previous studies have shown that phosphorus (P) is a limiting nutrient for freshwater eutrophication; therefore, the quantification of P flows in freshwater aquaculture is of great importance for improving aquaculture efficiency and reducing environmental pollution. In this study, life cycle assessment (LCA) and substance flow analysis (SFA) are combined to develop a life cycle P flow model for Chinese mitten crab (Eriocheir sinensis) culture and calculate the P inputs, outputs and net change in stock. The results show a relatively low P use efficiency (4 %) in Chinese mitten crab. Among all life-cycle stages, the maximum P input occurs during adult crab cultivation, when feed is continuously added to maintain appropriate nutrition levels and increase body weight. In addition, fertilizer is often neglected in the existing accounts but accounts for 24 % of the total P inputs. On the output side, approximately 86 % of the P accumulates in sediment, indicating the potential of sediment recycling as a nutrient source in agriculture. This study provides an updated quantitative method for describing nutrient flows within freshwater aquaculture systems and will contribute to decision-making in pollution control of intensive freshwater aquaculture activities.

New insights for the regulatory feedback loop between type 1 crustacean female sex hormone (CFSH-1) and insulin-like androgenic gland hormone (IAG) in the Chinese mitten crab (Eriocheir sinensis)

To clarify the hormone control on sex determination and differentiation, we studied the Chinese mitten crab, Eriocheir sinensis (Henri Milne Edwards, 1854), a species with importantly economic and ecological significance. The crustacean female sex hormone (CFSH) and the insulin-like androgenic gland hormone (IAG) have been found to be related to the sex determination and/or differentiation. CFSH-1 of E. sinensis (EsCFSH-1) encoded a 227 amino-acid protein including a signal peptide, a CFSH-precursor-related peptide, and a mature CFSH peptide. Normally, EsCFSH-1 was highly expressed in the eyestalk ganglion of adult female crabs, while the expression was declined in the intersex crabs (genetic females). The intersex crabs had the androgenic glands, and the expression level of EsIAG was close to that of male crabs. During the embryogenesis and larval development, the changes of EsCFSH-1 and EsIAG genes expression in male and female individuals were shown after the zoea IV stage. Next, we confirmed the existence of the regulatory feedback loop between EsCFSH-1 and EsIAG by RNA interference experiment. The feminization function of EsCFSH-1 was further verified by examining the morphological change of external reproductive organs after EsCFSH-1 knockdown. The findings of this study reveal that the regulatory interplay between CFSH and IAG might play a pivotal role in the process of sex determination and/or differentiation in decapod crustaceans.

An environmental gradient dominates ecological and genetic differentiation of marine invertebrates between the North and Baltic Sea

Environmental gradients have emerged as important barriers to structuring populations and species distributions. We set out to test whether the strong salinity gradient from the marine North Sea to the brackish Baltic Sea in northern Europe represents an ecological and genetic break, and to identify life history traits that correlate with the strength of this break. We accumulated mitochondrial cytochrome oxidase subunit 1 sequence data, and data on the distribution, salinity tolerance, and life history for 28 species belonging to the Cnidaria, Crustacea, Echinodermata, Mollusca, Polychaeta, and Gastrotricha. We included seven non‐native species covering a broad range of times since introduction, in order to gain insight into the pace of adaptation and differentiation. We calculated measures of genetic diversity and differentiation across the environmental gradient, coalescent times, and migration rates between North and Baltic Sea populations, and analyzed correlations between genetic and life history data. The majority of investigated species is either genetically differentiated and/or adapted to the lower salinity conditions of the Baltic Sea. Species exhibiting population structure have a range of patterns of genetic diversity in comparison with the North Sea, from lower in the Baltic Sea to higher in the Baltic Sea, or equally diverse in North and Baltic Sea. Two of the non‐native species showed signs of genetic differentiation, their times since introduction to the Baltic Sea being about 80 and >700 years, respectively. Our results indicate that the transition from North Sea to Baltic Sea represents a genetic and ecological break: The diversity of genetic patterns points toward independent trajectories in the Baltic compared with the North Sea, and ecological differences with regard to salinity tolerance are common. The North Sea–Baltic Sea region provides a unique setting to study evolutionary adaptation during colonization processes at different stages by jointly considering native and non‐native species.

Comparative transcriptome analysis explores maternal to zygotic transition during Eriocheir sinensis early embryogenesis

The maternal genome directs almost all aspects of early animal development. As development proceeds, the elimination of maternal gene products and zygotic genome activation (ZGA) occur during the maternal to zygotic transition (MZT). To study the molecular mechanisms regulating this developmental event in Eriocheir sinensis, RNA-Seq technology was applied to generate comprehensive information on transcriptome dynamics during early embryonic stages. In total, 32,088 annotated unigenes were obtained from the transcriptomes of fertilized eggs and embryos at the cleavage (2-4 cell) and blastula stage. A total of 566 maternal genes and 1165 zygotic genes were isolated, among which 103 and 266 genes were predicted conserved maternal transcripts (COMATs) and conserved zygotic transcripts (COZYTs), respectively. The COMATs performed housekeeping gene functions and may be essential for initiating early embryogenesis of the Bilateria. Furthermore, 87, 76 and 117 differentially expressed genes associated with the MZT, morphogenesis and immunity were identified when compared the three transcriptomic datasets. We also unmask that the MZT takes place around the cleavage stage, when the genes involved in the clearance of maternal gene products and the ZGA were significantly up-regulated. Taken together, these datasets provide a valuable resource for understanding the mechanisms of early developmental events in E. sinensis, and facilitate further studies on molecular mechanisms of asynchronous development in crabs.

Transcriptome profiling analysis on whole bodies of microbial challenged Eriocheir sinensis larvae for immune gene identification and SNP development

To study crab immunogenetics of individuals, newly hatched Eriocheir sinensis larvae were stimulated with a mixture of three pathogen strains (Gram-positive bacteria Micrococcus luteus, Gram-negative bacteria Vibrio alginolyticus and fungi Pichia pastoris; 10⁸ cfu·mL^-1). A total of 44,767,566 Illumina clean reads corresponding to 4.52 Gb nucleotides were generated and assembled into 100,252 unigenes (average length: 1,042 bp; range: 201-19,357 bp). 17,097 (26.09%) of 65,535 non-redundant unigenes were annotated in NCBI non-redundant protein (Nr) database. Moreover, 23,188 (35.38%) unigenes were assigned to three Gene Ontology (GO) categories, 15,071 (23.00%) to twenty-six Clusters of orthologous Groups (COG) and 8,574 (13.08%) to six Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways, respectively. Numerous genes were further identified to be associated with multiple immune pathways, including Toll, immune deficiency (IMD), janus kinase (JAK)-signal transducers and activators of transcription (STAT) and mitogen-activated protein kinase (MAPK) pathways. Some of them, such as tumor necrosis factor receptor associated factor 6 (TRAF6), fibroblast growth factor (FGF), protein-tyrosine phosphatase (PTP), JNK-interacting protein 1 (JIP1), were first identified in E. sinensis. TRAF6 was even first discovered in crabs. Additionally, 49,555 single nucleotide polymorphisms (SNPs) were developed from over 13,309 unigenes. This is the first transcriptome report of whole bodies of E. sinensis larvae after immune challenge. Data generated here not only provide detail information to identify novel genes in genome reference-free E. sinensis, but also facilitate our understanding on host immunity and defense mechanism of the crab at whole transcriptome level.

Effects of various salinities on Na(+)-K(+)-ATPase, Hsp70 and Hsp90 expression profiles in juvenile mitten crabs, Eriocheir sinensis

Eriocheir sinensis is a euryhaline crab migrating from sea to freshwater habitats during the juvenile stage. We used quantitative real-time polymerase chain reaction (qRT-PCR) to investigate the gene expression profile of Na(+)-K(+)-ATPase, Hsp70 (heat shock protein 70) and Hsp90 in megalopa exposed to salinities of 0, 2, 5, 10, and 15 parts per thousand. Both low and high salinities markedly stimulated expression of Na(+)-K(+)-ATPase, Hsp70 and Hsp90 genes of Chinese mitten crab megalopa; salinity had different effects on Na(+)-K(+)-ATPase, Hsp70 and Hsp90 levels depending on the duration of salinity stress, implying that Na(+)-K(+)-ATPase, Hsp70 and Hsp90 may play an important role in salinity tolerance in this crab species.

Effects of long-term exposure to different salinities on the location and activity of Na+–K+-ATPase in the gills of juvenile mitten crab, Eriocheir sinensis

The euryhalinity of mitten crab, Eriocheir sinensis, is based on osmoregulation, and thus on the activity of Na(+)-K(+)-ATPase. We studied location and activity of this enzyme in gills of juvenile crabs exposed to 5 per thousand, 25 per thousand, and 40 per thousand salinity. The posterior gills showed always a high number of immunopositive cells (IPC), staining with fluorescent antibody against Na(+)-K(+)-ATPase, covering at 5 per thousand the entire lamellae. At 25 per thousand, they showed fewer IPC which occurred only at the bases of the lamellae. Enzyme activity was consistently higher in posterior than in anterior gills. Low salinity stimulated the activity only in posterior gills. Both histochemical and enzymatic results are consistent with previous ultrastructural observations showing that the epithelial cells of the posterior, but not the anterior gills exhibit typical traits of ionocytes. While an increase in Na(+)-K(+)-ATPase activity at a reduced salinity is consistent with a strong hyper-osmoregulatory capacity in juvenile crabs, a low activity at an enhanced salinity suggests a physiological response, directed towards a reduction of Na(+) uptake. The activity increase of ion-transporting enzymes is directly related to spatial changes in their distribution along the osmoregulatory tissue, i.e. an enhanced number of IPC scattered along the entire lamellae. In juveniles, this allows for successful development and growth at reduced salinities.