Abstract
This paper presents the results of field studies on the physiology, ecology and behaviour of 2 species of crab from Avicennia marina mangrove near Jeddah, Saudi Arabia. One crab, the ocypodid Uca inversa inversa, occurs at the top of the eulittoral zone where it constructs burrows. Measurements of microclimate show that these burrows provide essential protection from environmental extremes. Burrow temperatures (air and sediment) were consistently lower and less variable than sediment surface temperatures or air temperatures just above the sediment. Crab body temperature closely followed burrow temperature. When out of the burrow, evaporative cooling maintained body temperature lower than that of the surrounding air. The relative humidity of burrow air was consistently higher than that outside and the burrows provided a necessary source of standing water. Crab heart rate was monitored in order to assess physiological stress. The grapsid Metopograpsus messor occurs throughout the mangrove and makes strategic use of the mangrove vegetation in order to minimize the effects of thermal stress, though some crabs opportunistically utilize burrows. M. messor appears to be more vulnerable to thermal stress than U. inversa.