Abstract
The population structure of benthic marine organisms is of central relevance for the conservation and management of these often threatened species as well as an accurate understanding of their ecological and evolutionary dynamics. A growing body of evidence suggests that marine populations can be structured over short distances despite theoretically high dispersal potential. Yet the proposed mechanisms governing this structure vary, and existing empirical population genetic evidence is of insufficient taxonomic and geographic scope to allow strong general inferences. Here we describe the range-wide population genetic structure of an ecologically important Caribbean octocoral, Gorgonia ventalina. Genetic differentiation was positively correlated with geographic distance and negatively correlated with oceanographically-modeled dispersal probability throughout the range. Although we observed admixture across hundreds of kilometers, estimated dispersal is low, and populations can be differentiated across distances <2km. These results suggest that populations of G. ventalina may be evolutionarily coupled via gene flow but are largely demographically independent. Observed patterns of differentiation corroborate biogeographic breaks identified in other taxa (e.g. an east/west divide near Puerto Rico) and also identify population divides not discussed in previous studies (e.g. the Yucatan Channel). Across the range, diversity was positively correlated with latitude, consistent with a source/sink dynamic driven by ocean currents. High genotypic diversity and absence of clonemates indicate that sex is the primary reproductive mode for G. ventalina. A comparative analysis of the population structure of G. ventalina and its dinoflagellate symbiont, Symbiodinium, indicates that the dispersal of these symbiotic partners is not coupled, and symbiont transmission occurs horizontally.