Morphology and ecological zonation of Caribbean reef corals: the Montastraea 'annularis' species complex

Estimates of species diversity on coral reefs are extremely high, yet hidden biological diversity makes even these, underestimates. The morphological complexity in the Caribbean reef coral Montastraea 'annularis' was originally interpreted as a single species because colony growth form was highly correlated with depth distribution from the coral reefs surrounding Carrie Bow Cay in Belize. This 'species' has since been re-interpreted from other Caribbean reefs as representing at least 3 separate species based on morphometric, genetic, reproductive, and ecological differences. We revisited the shallow water coral reefs in the Carrie Bow Cay region to test whether the 3 species could be recognized, and if so, what their ecological distribution might be. We have found that the 3 recently described species of the Montastraea 'annularis' species complex can be readily identified using both colony forms observed in the field and morphometric analyses of the corallite wall, and that their abundance distributions vary significantly along depth gradients. Morphometric comparison with both colonies from Panama and results from transplantation experiments in Jamaica show consistent patterns in the morphological characters that differentiate species and growth forms and are useful in understanding the geographic, environmental, and genetic components of variability within the species complex. All 3 species have a broad depth distribution, but each species dominates in a preferred depth zone, suggesting a high degree of niche differentiation. Our results confirm the existence of hidden biological diversity in Caribbean reef corals and call for caution in future estimates of biological diversity on coral reefs.