Complex spatial patterns of genetic differentiation in the Caribbean mustard hill coral Porites astreoides

In contrast to most Caribbean corals, the mustard hill coral Porites astreoides Lamarck, 1816 has seen its relative coverage increasing across its distribution range in recent years. Interpreting the reasons of such success remains, however, challenging. On the one hand, genetic homogeneity has been observed over large distances, in line with a large effective population size and/or high migration rate. On the other hand, strong genetic differentiation has been recently observed across depths within regions, consistent with some of the life history traits of P. astreoides , such as brooding of larvae followed by a short larval pelagic duration. Based on seven informative microsatellite markers and extensive sampling covering a large part of the P. astreoides distribution in the Caribbean, including six depth transects, we assessed multi-scale genetic connectivity patterns to test whether populations were structured by bathymetry, microenvironments or geography. While genetic diversity was similar in P. astreoides populations, genetic differentiation was observed not only over broad spatial scales, but also at the island scale or within a site along a depth transect. Isolation by distance was observed across the Lesser Antilles populations. However, genetic homogeneity was also observed over large distances, which, in juxtaposition to significant genetic structure within various locations across the Caribbean, has led to a complex spatial pattern of genetic differentiation in P. astreoides for which several likely explanations, including mixed reproductive strategy, are discussed.