EFFECT OF TEMPERATURE AND SALINITY ON HEAT TOLERANCE IN TWO GRAPSOID CRABS, HEMIGRAPSUS NUDUS AND HEMIGRAPSUS OREGONENSIS

¹ Department of Zoology, University of British Columbia, Vancouver, British Columbia, Canada

1. The influence on heat tolerance was determined of seasonal change and laboratory acclimation to various temperature-salinity combinations, for two species of grapsoid crabs, Hemigrapsus nudus and H. oregonensis.

2. There was a seasonal change in 50 per cent survival in both species when base lines from summer and winter were compared.

3. A definite species difference in tolerance to high temperatures was found to exist, but both species reacted similarly to any particular temperature-salinity combination.

4. Acclimation to a high temperature generally increased the resistance to lethal temperatures whereas acclimation to low salinity generally decreased it. High temperature, high salinity was the most favourable combination to withstand the high test tolerance temperatures.

5. Gain in heat tolerance whether the salinity was low or high was rapid, less than one week.

6. Winter tolerances with both low and high salinities in the low temperature series were not demonstrated in the laboratory with summer animals acclimated to these same conditions. Various reasons are suggested which might explain this apparent discrepancy.

7. Moulting during the test tolerance experiments adversely affected the resistance. The number of animals per dish at each test temperature had a pronounced effect on tolerance. The sex of the crabs did not affect the survival, but smaller animals appeared to be slightly more resistant.