Blood osmotic concentration of blue crabs (Callinectes sapidus Rathbun) found in fresh water

Conclusions

Mill Creek exemplifies a somewhat unusual ecotone between lacustrine and estuarine habitats. While its fauna is largely freshwater in character, several estuarine species are apparently able to survive there, albeit for unknown periods. The most conspicuous estuarine species is the blue crab, Callinectes sapidus, which seasonally invades the upper reaches of the stream in large numbers. Invasion by the population is apparently restricted to a period of a few weeks each year, sometime between the middle of August and early October, and the length of time spent there by an individual crab is unknown. The blood osmotic concentration of 13 crabs caught in Mill Creek is higher than one would predict from literature on osmotic adjustment to freshwater under laboratory conditions. But it approaches values for crabs acclimatized for prolonged periods to natural freshwaters.

Unfortunately, the time course of acclimation is not known. Perhaps it is no coincidence that invasion occurs when the water temperature is at its maximum, since crabs can make osmotic adjustments at 28–30C that they cannot do at temperatures only five degrees lower (Ballard and Abbott, 1969).