Mesanophrys sp. is reported to be highly pathogenic to marine crustaceans. This study presents the first report of Mesanophrys sp. infection in the mud crab (Scylla paramamosain). In this study, we first recorded the survival rates of an experimentally infected group and a control group; the cumulative survival rate in the infected group was significantly lower compared to the control group after 72 h (73.20% vs. 94.19%), while the highest mortality of S. paramamosain occurred within the first 24 h post-infection. Then, we investigated the dynamic distribution and tissue tropism of the Mesanophrys sp. in the infected S. paramamosain by a quantitative real-time polymerase chain reaction (qPCR). The result showed that a significant increase in the number of Mesanophrys sp. could be detected in all tested tissues (obtained from the eyestalks, gills, heart, nerves, muscles and hepatopancreas) at 3 h post-infection. The numbers of Mesanophrys sp. in the gill, eyestalk and nerve tissues were relatively higher than in the other tissues. The gill tissue showed the highest numbers from 6 to 48 h. Histopathological observation found a severe collapse in the filament structure, which indicated tissue-specific pathogen infection. Furthermore, the antioxidant enzyme activity of superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) in three representative tissues (gill, muscle and hepatopancreas) were compared between the infected and control groups, and a significant increase in enzyme activity was observed in all three tested tissues in the infected group, indicating a relatively strong innate immune defense reaction that could have been induced by Mesanophrys sp. infection. These results will be helpful to Mesanophrys sp. pathogenicity-related research and the control of this pathogen in S. Paramamosain in the future.

Weight traits serve as key economic indicators for assessing growth performance and commercial quality in the mud crab Scylla paramamosain, yet the genetic basis of these traits remains poorly characterized. Here, we performed whole-genome resequencing on 323 individuals and conducted genome-wide association studies (GWAS) on five weight-related traits: (1) body-related traits, including body weight (BW), trunk weight (TruW), and weight excluding chelae (WEC); (2) appendage-related traits, containing appendage weight (AppW) and cheliped weight (CheW). Significantly associated SNPs were primarily enriched on chromosomes 15, 22, 25, and 36. For body-related traits, we identified 45 shared candidate SNPs and 175 common candidate genes; appendage-related traits revealed 71 shared candidate SNPs, and 229 common genes were identified; and across all five traits, there were 9 shared candidate SNPs and 49 common genes. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses indicated that shared functional terms/pathways among the five traits were mainly related to metabolism, development, and immunity. Body-related traits exhibited more unique GO terms and KEGG pathways associated with metabolism and immunity, whereas appendage-related traits showed some unique GO terms and KEGG pathways involved in development and morphogenesis. Among the candidate genes, we identified multiple genes associated with growth and development, metabolism, and immune responses. For example, the CCHa1R gene, common to carapace-related traits, is linked to feeding; the DCX-EMA gene, which is common to appendage-related traits, is connected to movement, and the MSTO1 gene is pertinent to muscle development. Among the candidate genes shared by all five traits, there are a series of genes concerning growth and development (such as NVD, CYP307A1, FGF1, NF2, ANKRD52) and immune responses (RGS10). These findings advance our understanding of the genetic architecture underlying decapod crustacean growth and provide valuable insights for optimizing sustainable breeding strategies in S. paramamosain.

The insulin-like androgenic gland (IAG) gene is considered a crucial factor in the process of sexual differentiation in decapod crustaceans. In this study, we characterized the structural features and ontogenetic expression profile of the IAG gene in the swimming crab P. trituberculatus (Pt-IAG). To further elucidate its biological roles, we performed long-term RNA interference (RNAi) to evaluate the physiological impacts of sustained Pt-IAG suppression. Results showed that the gene structure of Pt-IAG is conserved with the characteristic B-C-A polypeptide arrangement that is representative of decapod IAGs. Transcripts of Pt-IAG were detectable as early as the zygote stage, which may imply its potential role during early-stage development. Although no completesex reversal was observed, continuous knockdown of Pt-IAG resulted in a decrease in body weight and a delay in gonad development in both sexes, underscoring its pleiotropic role in coordinating growth and maturation. Furthermore, the silencing of Pt-IAG led to a significant downregulation of insulin-like growth factor-binding protein (Pt-IGFBP), and a decrease in insulin-receptor (Pt-IR) expression was observed in males. Notably, the Pt-IR gene was not detected in female ovaries. These findings suggest that IAG may modulate male development through an insulin-like signaling axis; however, the absence of ovarian Pt-IR expression implies the potential existence of alternative regulatory pathways in females. Collectively, this study identifies a dual regulatory role of IAG in both growth and reproduction, providing a specific theoretical basis for developing sex-control and growth-optimization strategies in crab aquaculture.

The paddle-shaped dactyl of the fifth pereiopod in the mud crab Scylla paramamosain is a specialized appendage, but the genetic basis and genomic prediction potential of its traits remain poorly understood. Using whole-genome resequencing data from 323 individuals, we performed a genome-wide association study (GWAS) and genomic prediction analyses for bilateral dactyl length, width, area and first principal component (PC1). Heritability estimates ranged from 0.184 to 0.538. GWAS identified 116 and 70 significant SNPs for left and right dactyl length, 73 and 68 for the corresponding width traits, 102 and 76 for the corresponding area traits, and 91 for PC1. Major signals occurred on chromosomes 19 and 28 for length, chromosomes 19 and 23 for width, and chromosomes 19, 21 and 30 for composite traits. Two SNPs, Chr16:23074328 and Chr30:357279, were shared across all seven phenotypes and showed consistent additive effects on bilateral length and width. Chr30:357279 was annotated as a missense variant in an SRSF6-like gene. GBLUP based on GWAS-prioritized SNP subsets improved accuracy over whole-genome SNPs, with the FDR = 0.05 subset performing best (0.39–0.47). These results provide new insights into the genetic dissection and genomic prediction of functional morphological traits in S. paramamosain.