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

The seaweed Sargassum polyceratium Montagne inhabits a broad spectrum of subtidal and intertidal habitats. Genetic diversity and spatial genetic structure were examined within and among 12 stands using random amplified polymorphic DNA (RAPD) phenotypes. Data were analyzed using analysis of molecular variance (AMOVA) and Shannon's information measure. In both analyses, 60-75% of the variation occurred within stands and 25-40% between stands. These values are consistent with out-crossing, high-dispersal species. Significant differentiation was found among bays ca. 25 km apart (Shannon's G'_st averaged 0.37 and pairwise AMOVA K_st values averaged 0.272) and among stands 150-200 m apart within bays (AMOVA K_st values averaged 0.149). Effects of shore (windward vs. leeward), depth, and bay on population structure were tested. These analyses revealed that the factor depth is confounded with shore, and that bays show significant differentiation from each other but are not completely isolated from one another. Mantel tests for differentiation-by-distance were significant along both sides of the island but stronger along the windward side. A neighbor-joining analysis of genetic distances among stands showed that the effects of currents around both tips of the island were especially important for shallow populations. For S. polyceratium, depth and bay promote population differentiation along shores, yet dispersal around the tips of the island simultaneously connects these populations to varying degrees. This study highlights the importance of investigating the relative contribution of habitat factorsin relation to island-scale population structure.