Morphological and genetic variation across reef habitats in a broadcast-spawning octocoral

Octocorals, like many other colonial benthic invertebrates, exhibit remarkable levels of morphological variability. The basis for this variability has been largely unexplored. Plexaura flexuosa Lamouroux, a common Caribbean octocoral, is found in virtually all reef habitats, and exhibits habitat-related differences in growth rates and fecundity. Population genetic structure and morphological variability in branch and sclerite characters were examined in nine populations from back reef, shallow forereef, and forereef habitats in the Florida Keys. Discriminant analysis of morphological characters correctly classified 89, 88, and 100% of samples from back, shallow fore, and forereef habitats, respectively. Analysis of randomly amplified polymorphic DNA profiles (RAPDs) showed no evidence of genetic differentiation between shallow back, shallow fore, and the majority of forereef colonies. However, there were two distinct genetic groups of forereef colonies. Much of the habitat-related morphological variability among Plexaura flexuosa is due to plasticity, which is likely an important feature contributing to the broad distribution of this species. However, there may be two genetic subgroups on forereefs.