Responses to salinity in larvae from three local populations of the freshwater shrimp, Macrobrachium nipponense.

Abstract

Responses to variations in salinity were examined in larvae of the freshwater plaemonid shrimp Macrobrachium nipponense derived from females collected in Lake Kasumigaura, Lake Biwa, and the Shimanto River. Zoeas from Kasumigaura females adjusted most successfully to seawater dilutions of 30-50%, while those from Shimanto females to 40-70% sea water. Zoeas from Biwa females showed rather inconsistent survival patterns, but their response to salinity was on the whole not very different from that of Kasumigaura zoeas. The duration of the free-swimming larval stage was much longer in Shimanto zoeas than in Kasumigaura and Biwa zoeas. In each local sample the duration tended to be shorter in optimum salinity levels and longer in unfavorable salinities. The carapace length of lst-stage zoeas was 0.30-0.38mm for Kasumigaura zoeas and 0.23-0.25mm for Shimanto zoeas. These results suggest that the duration of the free-swimming larval stage, as well as the number of zoea stages, decreases as the shrimp under-goes adaptation to fresh water, and that M. nipponense inhabiting Lakes Kasumigaura and Biwa is in a more advanced position than that occurring in the Shimanto River with regard to the degree of adaptation to fresh water.