ASPECTS OF OSMOREGULATION IN TWO SPECIES OF INTERTIDAL CRABS

¹ Department of Zoology, University of British Columbia, Vancouver 8, British Columbia

1. Total osmotic pressure measurements of blood were made on two species of intertidal crabs, Hemigrapsus oregonensis and H. nudus, over a salinity range, 6% to 175% sea water, a temperature range, 5° to 25° C., and at two seasons, summer and winter.

2. Major changes in blood concentration occurred at 48 hours. Both species at either season were hypertonic to all experimental salinities. Below 100% sea water, osmotic regulation was accelerated with decreasing salinity, and in hypotonic media both species showed strong hyper-osmotic regulation.

3. The osmoregulatory abilities of these two species changed seasonally, and these responses resulted from the effect of temperature. Winter crabs maintained greater gradients at sea water concentrations below 100-125% sea water, and were better regulators in hypotonic media, whereas summer crabs were better regulators in hypertonic media, and consequently maintained the greater gradient. Interspecifically, winter H. nudus maintained the greater gradient over the major portion of the salinity range. Summer crabs of both species were similar from 6% to 125% sea water.

4. Blood concentrations of H. oregonensis, when measured at a series of temperatures and at each salinity, showed a general trend, particularly for winter animals. As external sea water concentration decreased (from 75% to 12% sea water) blood concentrations increased significantly with decreasing temperature. Blood concentrations of summer animals measured at the three temperatures and each salinity showed no real difference, but at the lower salinities, blood concentrations of summer animals were significantly lower than those of winter crabs. The same general trend was shown for H. nudus. Temperature had no effect on blood concentrations of summer animals, but the low temperature (5° C.) had a highly significant effect on the blood concentrations of winter animals.

5. No detectable weight changes resulted when animals were subjected to the extreme experimental sea water concentrations. Further, there was no evidence to suggest that size had any effect on osmotic response.

6. Respiratory and osmoregulatory data for both species have been compared at the same temperatures and salinities to determine whether increased osmotic work can be resolved by measuring oxygen consumption. Comparison of these data does not permit such a conclusion.