Physiological responses to salinity stress in the flatback mud crab Eurypanopeus depressus

Estuaries in Southwest Florida experience highly altered freshwater inflow resulting in part from anthropogenic activities. To gage possible effects of altered salinity regimes on species in these systems, physiological responses to salinity stress were investigated in the flatback mud crab, Eurypanopeus depressus, a dominant member of oyster-reef communities in the region. Haemo-lymph osmolality and oxygen consumption were measured for animals acclimated to 5ppt, 15ppt, 25ppt, and 35ppt over a period of four weeks in a laboratory setting. Haemolymph osmolality varied significantly with respect to salinity, with measured osmolalities at each salinity treatment being significantly different (p < 0.0001) from one another. Oxygen consumption also varied significantly with salinity, with values recorded for the lowest salinity treatment (5ppt) being generally higher (p < 0.0001) than all other treatments over the length of the study. Oxygen consumption did not vary over the first three weeks of the study for animals acclimated to 5ppt but declined significantly by week four. This general pattern of increased oxygen consumption in response to dilute salinity levels is a trend that has been documented previously in several estuarine species. The results of this study suggest that osmoregu-lation requires greater energy expenditure by E. depressus at subnormal salinity levels. Having to expend additional energy to meet osmoregulatory demands can result in reduced amounts of energy available for processes such as growth and reproduction, thereby affecting organism health. Suboptimal salinity levels resulting from altered freshwater inflow can thus significantly affect species abundance and distribution, specifically in the upper reaches of estuaries.