Genetic differentiation in Sargassum polyceratium (Fucales : Phaeophyceae)around the island of Curacao (Netherlands Antilles)

The seaweed Sargassum polyceratium Montagne inhabits a broad spectrum of su btidal and intertidal habitats. Genetic diversity and spatial genetic struc ture were examined within and among 12 stands using random amplified polymo rphic DNA (RAPD) phenotypes. Data were analyzed using analysis of molecular variance (AMOVA) and Shannon's information measure. In both analyses, 60-7 5% of the variation occurred within stands and 25-40% between stands. These values are consistent with out-crossing, high-dispersal species. Significa nt differentiation was found among bays ca. 25 kin apart (Shannon's G'(st) averaged 0.37 and pairwise AMOVA Phi (st) values averaged 0.272) and among stands 150-200 m apart within bays (AMOVA Phi (st) values averaged 0.149). Effects of shore (windward vs. leeward), depth, and bay on population struc ture were tested. These analyses revealed that the factor depth is confound ed with shore, and that bays show significant differentiation from each oth er but are not completely isolated from one another. Mantel tests for diffe rentiation-by-distance were significant along both sides of the island but stronger along the windward side. A neighbor-joining analysis of genetic di stances among stands showed that the effects of currents around both tips o f the island were especially important for shallow populations. For S. poly ceratium, depth and bay promote population differentiation along shores, ye t dispersal around the tips of the island simultaneously connects these pop ulations to varying degrees. This study highlights the importance of invest igating the relative contribution of habitat factorsin relation to island-s cale population structure.