Gradually elevated salinities alter the haemolymph metabolic profile and nitrogenous wastes of the mature Chinese mitten crab, Eriocheir sinensis (Decapoda, Varunidae)

To understand the metabolic adjustments and nitrogen excretion in haemolymph of mature Eriocheir sinensis during the reproductive migration from fresh water to seawater, haemolymph metabolic variations and nitrogenous wastes were determined after gradual acclimation of females and males from fresh water to different salinities (7, 14, 21, 28 and 35). Total protein content in both female and male crabs was reduced with increased salinity. The contents of lipids including triglyceride and cholesterol significantly decreased at salinities of 0-21, suggesting the lipids of E. sinensis might be mobilized to produce energy for osmoregulation and gonad development at a different salinity. Interestingly, the lipid levels in the females were higher than those in the males, implying the females need to store more lipids to prepare for reproduction. The glucose level was low in this study, which was related to the diet with high protein. Haemolymph urea and uric acid level together decreased at first and then increased, indicating that E. sinensis could regulate the nitrogen excretion pattern by inhibiting or activating ureogenesis and uricogenesis to acclimate to a different osmotic environment. Notably, the females might be more tolerant to salinity increase and have a faster metabolic reorganization than males. These facts could indicate that a different strategy has been employed in mature E. sinensis when encountering salinity stress during the reproductive migration form fresh water to seawater.