PARTIAL MORTALITY IN 3 SPECIES OF REEF-BUILDING CORALS AND THE RELATION WITH COLONY MORPHOLOGY

Partial tissue mortality (lesions) in three coral species with distinc tly different colony morphologies was quantified in a series of field surveys on a shallow reef, Extent and type of partial mortality was re lated to differences in size and morphology of the colonies within and between species. Lesion size-frequency distributions were found to be very skewed to the right, meaning that most partial mortality is smal l in size and well within the regeneration capabilities of all coral s pecies. However, large lesions may make a considerable contribution in terms of surface area to total partial mortality. Within partial mort ality two types of lesions must be distinguished. Type I lesions are c ompletely surrounded by living tissue and their occurrence is mostly r elated to non-bottom associated processes. Type II lesions, at the edg e of a colony, are only partly surrounded by living tissue, and are ca used mostly by bottom associated processes. The ratio of number of typ e II/type I lesions differed between species and was related to the ra tio of colony edge (circumference)/colony surface area in the three sp ecies studied. Type II lesions were larger in size than type I lesions and can make a large contribution to total partial mortality. Species differed in the number of type I and type II lesions per colony and p er unit of tissue area. Type II lesions were almost absent on branched colonies, but very frequent on colonies of massive species. Lesion nu mber increased with colony size in the massive species. Most partial m ortality is caused by bottom related processes and our results show th at ''escape in height'' is a significant feature in the life history s trategies of corals. The surveys showed small colonies to be very vuln erable to partial mortality. Because their circumference/surface area ratio is high, they are very susceptible to colony edge (i.e., bottom- associated) processes that cause mortality. Consequently, small coloni es will often suffer whole colony death. On the other hand, large colo nies, although unlikely to escape partial mortality, will less often s uffer complete mortality. The relationship between this ratio and susc eptibility to partial mortality holds as well within species as betwee n species and suggests an important effect of colony morphology on sur vival. Colony genetic identity also affected susceptibility to partial mortality. Other factors that may influence sensitivity to mortality are discussed. Regeneration capabilities of corals suggest that sclera ctinian corals have become adapted to the very common occurrence of sm all lesions.