HEAT TOLERANCE AND TEMPERATURE RELATIONSHIPS OF THE FIDDLER CRAB, UCA PUGILATOR, WITH REFERENCE TO BODY COLORATION

¹ Department of Zoology, Newcomb College, Tulane University, New Orleans, Louisiana
² Marine Biological Laboratory, Woods Hole, Massachusetts

1. Upper thermal death points were determined for the fiddler crab, Uca pugilator. In saturated air the lethal temperature for 50% of the crabs, determined graphically from the experimental data, after an exposure of one hour, was 40.7° C. All of the crabs died after one hour at 42° C. In dry air the corresponding temperatures were 45.1° C. and 47° C. for the same time of exposure.

2. Five minutes after having been placed in sunlight the body temperature of dark crabs was 2° C. higher than that of pale crabs. More visible light is reflected from the dorsal surface of the cephalothorax of a pale crab than from a dark crab. The difference is more striking with the rays of longer wave-length which have a greater heating capacity than the rays at the violet end of the visible spectrum. These observations support the hypothesis that the blanching that occurs at high temperatures has a thermoregulatory role.

3. The body temperatures of crabs maintained either in dry air or in air having a relative humidity of 50% were lower than the air temperature, undoubtedly due to transpiration of water. The body temperatures of crabs in saturated air were the same as the air temperature. The cooling that resulted from transpiration was proportional to the decrease in relative humidity.

4. Transpiration from this crab is a passive process. The rate is proportional to the saturation deficit of the air.

5. In their habitat specimens of Uca pugilator exhibit a "feeding-retreat" rhythm that has a frequency of 18-24 minutes. There is no phase interaction between individuals in a population. On hot days the frequent periodic return to the cooler burrows can serve to lower the body temperature.

6. These findings were discussed in relation to the observations of other investigators.