Effects of trimethylamine N-oxide supplementation on low-salinity tolerance of the mud crab Scylla paramamosain relative to Na+/K+-ATPase activity and haemolymph osmolality

Mud crabs (Scylla paramamosain) are important marine organisms. Typhoons and other events lead to sudden decreases in salinity, which substantially influence the survival and growth of mud crabs. This study aimed to enhance the low-salinity tolerance capacity of mud crabs through nutritional regulation of trimethylamine N-oxide (TMAO). A seven-day experiment explored the impact of dietary supplementation with five concentrations of TMAO, 0 mg/kg (CK), 100 mg/kg (T1), 200 mg/kg (T2), 400 mg/kg (T4), and 800 mg/kg (T8), on the survival rate and physiological response of juvenile mud crabs under acute low-salinity conditions. The results showed that a diet supplemented with TMAO significantly impacted the survival rate, structural integrity of the gill tissue, osmolality of haemolymph, ion content (Na+, Cl-, K+, and Ca2+), activity of Na+/K+-ATPase, and gene expression of crucial transporters and enzymes (such as nkcc, nka, clc, and gdh) in crabs following acute exposure to low-salinity conditions. In particular, 400 mg/kg TMAO supplementation, resulted in a significant improvement in the survival rate of crabs after 168 h of low-salinity exposure, and a survival rate of 89 %, in comparison to the CK group, which only achieved a survival rate of 27 % (P < 0.01). Additionally, 400 mg/kg TMAO supplementation mitigated damage to posterior gill tissue during low-salinity stress, with no observed difference in gill filament diameter before and after low-salinity stress. Furthermore, crabs treated with TMAO manifested enhanced haemolymph osmolality, increased Na+/K+-ATPase activity in the posterior gills, increased TMAO accumulation within the haemolymph and muscle tissues, and upregulated gene expression associated with ion transporters and enzymes under low salinity. Overall, these results suggest that dietary supplementation with 400 mg/kg TMAO increases the survival rate of mud crabs when acutely exposed to low-salinity stress.