PHYSIOLOGICAL ECOLOGY OF CRABS FROM SAUDI-ARABIAN MANGROVE

This paper presents the results of field studies on the physiology, ec ology and behaviour of 2 species of crab from Avicennia marina mangrov e near Jeddah, Saudi Arabia. One crab, the ocypodid Uca inversa invers a, occurs at the top of the eulittoral zone where it constructs burrow s. Measurements of microclimate show that these burrows provide essent ial protection from environmental extremes. Burrow temperatures (air a nd sediment) were consistently lower and less variable than sediment s urface 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 tha t of the surrounding air. The relative humidity of burrow air was cons istently higher than that outside and the burrows provided a necessary source of standing water. Crab heart rate was monitored in order to a ssess physiological stress. The grapsid Metopograpsus messor occurs th roughout the mangrove and makes strategic use of the mangrove vegetati on in order to minimize the effects of thermal stress, though some cra bs opportunistically utilize burrows. M. messor appears to be more vul nerable to thermal stress than U. inversa.