THE WATER AND HEAT RELATIONSHIPS OF FIDDLER CRABS (Uca spp.)

Abstract

The paper reports work on the water and heat relationships of five species of Uca at Inhaca Island, Moçambique. The species urvillei, marionis, chlorophthalmus, annulipes and inversa generally speaking occupy progressively more terrestrial habitats. They stand in this order (except marionis) as regards their sharply defined upper lethal temperatures (40·0°C. for urvillei, 43·3°C. for inversa) and in respect of transpiration rates (inversa being anomalous). Upper lethal temperatures are about 2°C. lower for each species in September than in January. Lower lethal temperatures are less well defined and lie between 7·0° and 8·5°C. for all species. No short-term acclimatization in crabs was found, although it occurs in Oniscus asellus. Measurements by fine thermocouples show that body temperatures are depressed by about 1·5° to 7·0°C. in various conditions. Species which transpire faster generally have lower body temperatures. In direct sunlight the temperatures of living crabs are lower than those of dead, dry specimens, or that of the ground, and this permits crabs to live, in nature, in habitats which might otherwise be lethal. Eleven out of twelve U. annulipes survived for at least 11 days in 50% sea water, 7/12 survived in fresh water, and 3/12 in sea water. The results are discussed in relation to what is known about land isopods and other littoral crabs with reference to the general problem of the origins of a land fauna. The need for further information about environmental conditions and other matters is stressed.