A sea water barrier to coral gene flow

Land is not the only barrier to dispersal encountered by marine organisms. For sedentary shallow water species, there is an additional, marine barrier, 5000 km of uninterrupted deep‐water stretch between the central and the eastern P acific. This expanse of water, known as the ‘ E astern P acific B arrier’, has been separating faunas of the two oceanic regions since the beginning of the C enozoic. Species with larvae that cannot stay in the plankton for the time it takes to cross between the two sides have been evolving independently. That the eastern Pacific does not share species with the rest of the P acific was obvious to naturalists two centuries ago (Darwin 1860). Yet, this rule has exceptions. A small minority of species are known to straddle the E astern P acific B arrier. One such exception is the scleractinian coral P orites lobata (Fig. [Figure 1. A colony of Porites lobata at Wolf Island, Galapagos, ...]). This species is spread widely throughout the I ndo‐ P acific, where it is one of the major reef‐builders, but it is also encountered in the eastern P acific. Are eastern and central P acific populations of this coral connected by gene flow? In this issue of Molecular Ecology , Baums et al . (2012) use microsatellite data to answer this question. They show that P . lobata populations in the eastern P acific are cut off from genetic influx from the rest of the P acific. Populations within each of the two oceanic regions are genetically connected (though those in the H awaiian islands are also isolated). Significantly, the population in the C lipperton A toll, the westernmost island in the eastern P acific, genetically groups with populations from the central P acific, suggesting that crossing the E astern P acific B arrier by P . lobata propagules does occasionally occur.