Effects of long-term low-salinity on haemolymph osmolality, gill structure and transcriptome in mud crab (Scylla paramamosain)

Scylla paramamosain is an euryhaline species that is extensively distributed in coastal regions along the Indian and Pacific Oceans. Recently, S. paramamosain are considered as species that can be cultured in low-salinity areas to improve the local economy. However, the long-term effects and mechanisms of low salinity on its physiology remain unclear. In this study, S. paramamosain were reared in low-salinity (5 ppt) and normal sea-water environment (25 ppt) for 30 days. We compared haemolymph osmolality, gill structure, and transcriptome changes between the two groups. Results indicated that S. paramamosain subjected to a low-salinity environment showed gill damage such as broken lamellar and reduced haemocytes, some adaptive responses as increased mitochondria in chloride cells were also observed. Even with decreased water ion concentration in lower salinity, the concentrations of Na+, K+, and Cl- in the haemolymph of crab were remained higher than the surrounding water. Transcriptomic analysis revealed 2086 differentially expressed genes, including significant alterations in genes like hexosaminidase and neprilysin. Enriched pathways linked to ion transport were identified, highlighting the main adaptive mechanisms in long-term hypotonic environments. Our findings elucidated the effects and mechanism of long-term low salinity on gill structure and osmoregulation in S. paramamosain, providing theoretical basis for the development of low salinity aquaculture technology of this species.