Physiological Adaptation of the Amphibious Rocks Crab Grapsus tenuicrustatus from the Red Sea

The rate of weight specific oxygen consumption in the Red Sea supra littoral crab Grapsus tenuicrustatus was examined under aerial and aquatic conditions. This indicates that the smallest crabs had the highest weight specific oxygen consumption, and the rate of aquatic respiration was lower than that in air. The effect of temperature on the rate of oxygen consumption and heart beat was also investigated. The Q10 values for both oxygen consumption and heart rate were often < 2 and were relatively constant over a wide range of temperature. This suggests the G. tenuicrustatus is well adapted over a wide range of temperature. Gill area and brachial volume were also examined in a size range of crabs. The species had fewer gills and less gill area than aquatic crabs. These observations enable further understanding of adaptations which allow Grapsus tenuicrustatus to occupy the high supra littoral zone. Introduction The family Grapsidae including the genus Grapsus comprises crabs that can be commonly found occupying fringe area of the littoral zone, under stones, in wreckage, and in crevices (Barnard, 1950; Banerjee, 1960; Forest and Guinot, 1961; Crosnier, 1965; and Eshky, 1980). After the construction of the coastal highway (Al-Cornich in 1980), in the Red Sea area around Jeddah city, the lava rocks and boulders along the highway provide the grapsid crab Grapsus tenuicrustatus and other associated species with a good protection not only from predators but also from environmental extremes. Therefore Grapsus tenuicrustatus has now become the most conspicuous member of the new environment. During night hours most of its time is spent in crevices under the rocks surface close to the water surface between rocks. It emerges to forage during daylight hours, scavenging amongst rocks and feeding on encrusting organisms exposed on the rocks faces by falling tide. Grapsus tenuicrustatus is primarily an air breather and can withstand short periods of immersion (Eshky, 1992a&b). Respiratory studies on brachyuran crabs have largely concentrated on extremes of adaptation , investigating either terrestrial species (review by McMahon and Burggren, 1988) or the aquatic species not showing activity in the air (Burnett, 1988; DeFur, 1988; and McMahmon, 1988). Both J. KAU: Mar. Sci., vol. 10, pp. 109-124 (1419 A.H. / 1999 A.D.)