The mud crab, Scylla paramamosain, has abundant nutrients in the ovary, where numerous lipids accumulate during ovarian maturation. However, the mechanism behind the accumulation of lipids in the ovary of mud crab during ovarian maturation is largely unknown. This study conducted a comparative transcriptome analysis of the ovaries of mud crabs at various stages of ovarian maturation. A total of 63.69 Gb of clean data was obtained, with a Q30 of 93.34%, and 81,893 unigenes were identified, including 10,996 differentially expressed genes (DEGs). After KEGG enrichment of these DEGs, MAPK signaling pathway was significantly enriched during vitellogenesis. Moreover, the expression levels of genes involved in carbohydrate, amino acid, and lipid metabolism were found to be higher during vitellogenesis. The two genes (Sp-Eip75B and Sp-Eip78C) that are homologous to the vertebrate gene PPARγ in the PPAR signaling pathway, were identified. Additionally, genes in MAPK signaling pathway might regulate lipid metabolism through PPAR signaling pathway based on Protein-Protein Interaction (PPI) network. These findings suggest that MAPK signaling pathway plays a critical role in lipid metabolism in the ovary during vitellogenesis, which provides new insights into the mechanism of lipid accumulation during ovarian maturation in mud crabs.

To investigate the embryonic development of the mud crab Scylla paramamosain, we analyzed three critical parameters: egg color of, embryo morphology (through conventional and laser scanning confocal microscopy), and the distribution of cell divisions. During embryonic development, the egg color exhibited a progressive transition, shifting from orange to reddish-orange, then to brown, before ultimately darkening to black. Each embryo displayed a spherical shape, measuring approximately 280 μm in diameter, characterized by a smooth surface devoid of any depressions. The embryonic cell division was in the form of mixed oogenesis, comprised of complete division in the early stage, spiral oogenesis in the middle stage and surface division in the late stage. It is noteworthy that the blastopore appeared at the position where the transparent area and cell aggregation just appeared under the microscope, and the blastomere was a characteristic of the embryo entering the gastrulation stage. After entering the gastrulation stage, the cells aggregated towards the blastopore and formed two symmetrical cell clusters, which formed a V-shape with the void of the classic blastopore. When the transparent region occupied approximately 1/5 of the embryo^’s volume, the embryo entered the nauplius stage, and the thoracic and abdominal armor, as well as the optic lobe and abdominal limb primordia, could be clearly distinguished. The appearance of the compound eye pigment band indicated the stage of compound eye pigment formation. At this time, the transparent area accounted for 1/4 of the embryo and a large number of ganglia appeared. The change of the compound eye pigment band from red to black was also one of the reasons for the blackening of the egg color of the crabs. The data obtained through this study have potential applications in the determination of embryonic development status and obtaining of high-quality seeds for S. paramamosain culture.

The sesquiterpenoid hormone methyl farnesoate (MF) plays a vital role during crustacean development, which is mainly evidenced by its varied titers during different developmental stages. However, the biosynthesis pathways of MF remain obscure to some extent. In this study, we identified the complete MF biosynthesis and related pathway genes in Scylla paramamosain, including three involved in acetyl-CoA metabolism, eight in the mevalonate pathway, five in the sesquiterpenoids synthesis pathway, and five in the methionine cycle pathway. Bioinformatics, genomic structure, and phylogenetic analysis indicated that the JH biosynthesis genes might have experienced evolution after species differentiation. The mRNA tissue distribution analysis revealed that almost all genes involving in or relating to MF syntheses were highly expressed in the mandibular organ (MO), among which juvenile hormone acid methyltransferase was exclusively expressed in the MO, suggesting that most of these genes might mainly function in MF biosynthesis and that the methionine cycle pathway genes might play a crucial regulatory role during MF synthesis. In addition, the phylogenetic and tissue distribution analysis of the cytochrome P450 CYP15-like gene suggested that the epoxidized JHs might exist in crustaceans, but are mainly synthesized in hepatopancreas rather than the MO. Finally, we also found that betaine-homocysteine S-methyltransferase genes were lost in insects while methionine synthase was probably lost in most insects except Folsomia candida, indicating a regulatory discrepancy in the methionine cycle between crustaceans and insects. This study might increase our understanding of synthetic metabolism tailored for sesquiterpenoid hormones in S. paramamosain and other closely related species.

Salinity is a pivotal environmental factor that governs crustacean survival and development through its regulatory effects on key physiological processes, including osmoregulation and metabolic homeostasis. In the mud crab Scylla paramamosain, salinity tolerance of the megalopa plays an important role in larval survival rates and aquaculture yield. Here, we employed a combined transcriptomic and proteomic strategy to comprehensively dissect the molecular adaptive mechanisms of S. paramamosain megalopa exposed to acute and prolonged low-salinity stress (8‰) compared to control condition (17‰). Illumina-based transcriptome sequencing generated 81.71 Gb of high-quality clean data, which were assembled into 42,210 unigenes. LC-MS/MS-based proteomic profiling identified 51,390 unique peptides, corresponding to 5909 confidently quantified proteins. Transcriptomic profiling identified 2627 differentially expressed genes (DEGs) under acute low-salinity stress, comprising 1332 upregulated and 1295 downregulated genes compared to the control group. In contrast, a total of 733 DEGs were identified under prolonged low-salinity exposure, including 390 upregulated and 343 downregulated genes. Parallel proteomic analysis identified 199 differentially expressed proteins (DEPs) in the acute stress group, with 105 upregulated and 94 downregulated relative to the control group. Under prolonged stress, 206 DEPs were detected, including 124 upregulated and 82 downregulated proteins compared to the control group. Significant GO term and KEGG pathway enrichments contained metal ion binding, oxidoreductase activity, nucleus, apoptotic process, innate immune response, and amino acid metabolism, suggesting that megalopa employ coordinated regulatory mechanisms involving metabolic reprogramming, immunity system modulation, ion homeostasis maintenance and cell cycle regulation to adapt to hypoosmotic stress. Integrated multi-omics analysis identified 17 genes displaying significant concordant differential expression at both mRNA and protein levels during acute hypoosmotic stress, versus only 5 gene-protein pairs during prolonged stress exposure, indicating extensive post-transcriptional regulation and protein turnover mechanisms in sustained hypoosmotic condition. To the best of our knowledge, this study established the first integrative transcriptome-proteome framework elucidating hypoosmotic adaptation (8‰) mechanisms in S. paramamosain megalopa. The identified molecular signatures offer actionable targets for selective breeding of salinity-tolerant strains and precision management of megalopa culture under suboptimal salinity condition, while fundamentally advancing our mechanistic understanding of osmoregulatory plasticity across decapod crustaceans.

In recent years, with the widespread use of TiO₂–GO nanocomposite in industry, especially in the remediation of water environments, its toxic effects on aquatic organisms have received increasing attention. As molting is extremely important for crustaceans in their growth, in this study, we cloned the full-length cDNA sequences of two key genes related to molting, nuclear hormone receptor E75 (E75) and nuclear hormone receptor HR3 (HR3), in Macrobrachium rosenbergii, examined the gene expression profile, and investigated their toxicological effects on crustacean molting through nanomaterial exposure. The amino acid sequences for E75 and HR3 were respectively determined to encode 1138 and 363 acid residues. Sequence analysis showed that both E75 and HR3 contain a HOLI domain, with the E75 of M. rosenbergii being more closely related to the E75 of Palaemon carinicauda. These two genes were expressed at the highest levels in muscle, followed by hepatopancreas. The results showed that the expressions of E75 and HR3 in hepatopancreas and muscle tissues were significantly decreased after exposure to 0.1 mg/L of TiO₂–GO composite nanoparticles (P < 0.05). This study will serve as a foundation for subsequent research into the evaluation of nanomaterial toxicity on crustacean species.