THE ANOMALOUS INFLUENCE OF SALINITY ON TEMPERATURE TOLERANCES OF SUMMER AND WINTER POPULATIONS OF THE COPEPOD EURYTEMORA AFFINIS

¹ Department of Biological Sciences, University of Maryland Baltimore County, Catonsville, Maryland 21228

1. Thermal tolerances of populations of Eurytemora affinis were measured using two basic methods, at various salinities with and without acclimation. Little distinction in tolerances was made using temperature of inactivation. A more useful assay was temperature shocking at 34.5° C, observing time to succumb (TS) and time to recover (TR) over a 30 minute period.

2. Using the shock-recovery assay, there were repeatable and significant differences in tolerance between populations collected in March and August and also between salinities. Average tolerances and differences between populations generally increased with salinity.

3. The distinction between populations in thermal tolerance was greater when the animals were osmotically acclimated for 24 hours.

4. A simple index of tolerance combining TS and TR was suggested. In most cases the proportional variance between populations, using the index, was increased over TS and TR.

5. The seasonal distribution of E. affinis is contrary to that expected from the thermal tolerance-salinity relationship reported here. The explanation offered is that other species, for exampleAcartia tonsa, are at a competitive advantage during the summer and fall because of faster growth rate and greater dependence on phytoplankton.