Physiology and ecology of Stylophora pistillata and Echinopora gemmacea from the Red Sea

1. Some aspects of the biology and nutritional physiology of the corals Stylophora pistillata and Echinopora gemmacea from the Red Sea are described in this thesis. The two species were selected on the basis of their differing growth form and nutritional strategies. The main objective was to compare their nutritional energy budgets and to examine the effects upon these of environmental factors associated with differing depths on the reef and with different seasons. 2 . S. pistillata is a branching coral, with small polyps. In shallow water (1m) the coral has compact club-shaped branches, whereas at 10m it has an open growth form with slender branches. E. gemmacea has large polyps, and both encrusting and lightly branching growth forms. It was not found at depths of less than 3m. 4 . Histological examination showed that the coenosarc of S. pistillata is characterised by holotrich nematocysts and mucus-gland cells containing neutral mucopolysaccharides. By contrast, the epidermis of E. gemmacea contains spirocysts and large numbers of mucus-gland cells secreting an acid mucopolysaccharide. The gastrodermis of the tentacles of S. pistillata, which are expanded during the day time are packed with zooxanthellae. In E. gemmacea the tentacles are withdrawn during the day and are devoid of zooxanthellae. In both species, lipid stores are concentrated in the gastrodermal layers only, mainly in the lower half of the polyps. 4 . Four types of nematocyst are present in both species : spirocysts, microbasic P-mastigophores, microbasic B-mastigophores and holotrichs. Spirocysts are most common on the oral discs and mainly at the tentacle tips. Microbasic P-mastigophores are more common in the tentacles of E. gemmacea suggesting a more important role for prey capture in this species. In both species the mesenterial filaments are armed with microbasic P-mastigophores and holotrichs, indicating that they are important for aggression and defence. 5 . S. pistillata displays a weak ability to deal with particulate food when presented as Artemia nauplii or eggs. There is an extensive development of ciliary currents on the outers epidermis, but these appear to be used only for cleansing of the outer surface. E. gemmacea expands its tentacles at night and feeds on particulate organic matters trapped on the surface mucus net. Large particles, including fish faecal pellets, may be ingested. The expansion of the tentacles of S. pistillata during the day time suggests a nutritional economy which is more dependent upon the symbiotic zooxanthellae. 6 . Both species are hermaphrodite with protogynous gametogenesis. S. pistillata has an extended breeding cycle. Oogenesis began in May, whilst spermatogenesis was first detected in August. Fertilisation takes place in the coelenteron and the first planulae were observed in December. E. gemmacea has a short annual cycle of gametogenesis. Oogenesis was first observed in June and spermatogenesis in August. Broadcast-spawning occurred in November. 7 . The distribution of the two species on a shallow fringing reef in Sharm Ubhur, a small creek to the north of Jeddah, is described from a transect survey. At the study site, the lowest mean water temperature of 25.5