Partial mortality in three species of reef-building corals and the relation with colony morphology

Partial tissue mortality (lesions) in three coral species with distinctly different colony morphologies was quantified in a series of field surveys on a shallow reef, Extent and type of partial mortality was related 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 small in size and well within the regeneration capabilities of all coral species. However, large lesions may make a considerable contribution in terms of surface area to total partial mortality. Within partial mortality two types of lesions must be distinguished. Type I lesions are completely surrounded by living tissue and their occurrence is mostly related to non-bottom associated processes. Type II lesions, at the edge of a colony, are only partly surrounded by living tissue, and are caused mostly by bottom associated processes. The ratio of number of type II/type I lesions differed between species and was related to the ratio of colony edge (circumference)/colony surface area in the three species 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 per unit of tissue area. Type II lesions were almost absent on branched colonies, but very frequent on colonies of massive species. Lesion number increased with colony size in the massive species. Most partial mortality is caused by bottom related processes and our results show that ''escape in height'' is a significant feature in the life history strategies of corals. The surveys showed small colonies to be very vulnerable 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 colonies will often suffer whole colony death. On the other hand, large colonies, although unlikely to escape partial mortality, will less often suffer complete mortality. The relationship between this ratio and susceptibility to partial mortality holds as well within species as between species and suggests an important effect of colony morphology on survival. 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 scleractinian corals have become adapted to the very common occurrence of small lesions.