THE SALINITY TOLERANCE OF SOME ESTUARINE PLANKTONIC COPEPODS

The salinity tolerance of three species of Acartia was investigated in the laboratory by recording the survival of female copepods at various salinities. The salinity tolerance of A. tonsa, A. discaudata, and A. bifilosa captured during the autumn, winter, and spring, respectively, depended on the temperature. Thermal prehistory of copepods affected their survival in diluted seawater, all species being most tolerant when the experimental and environmental temperatures were close and least tolerant when the experimental and field temperatures differed markedly. The following order of salinity tolerance was established: A. tonsa > A. bifilosa > A. discaudata. Reinvestigation of the temperatures at which copepods had shown poorest survival in diluted seawater demonstrated that temperature acclimation increased salinity tolerance. The results imply that cooling of the environment might not necessarily lower the tolerance of the autumn, warmwater species, A. tonsa, provided that the change was gradual and that slow warming might not cause a decline in the tolerance of the spring, cold‐water species, A. bifilosa. Salinity acclimation increased the salinity tolerance of A. bifilosa. The recovery of A. bifilosa in full‐strength seawater after short exposure to low salinities depended on the length of exposure.