16. Gene Flow in Coral Reef Organisms of the Tropical Eastern Pacific

Gene flow can provide cohesion between conspecific populations. In order to obtain an indirect measure of gene flow between coral reef species in the eastern tropical Pacific (ETP) and between these populations and those of the rest of the Pacific we compiled available data from sequences of DNA and microsatellites for corals, gastropods, echinoderms and fishes, and calculated F_ST statistics. The ETP consists of a narrow strip of continental shelf along the coast of the Americas and a deeper water gap between the coast and the outer eastern Pacific Islands; a large expanse of deep ocean separates the ETP and the closest islands in the central Pacific. We have, therefore, compared populations in four major directions: (1) between the eastern and the central Pacific, (2) between the coast and the outer islands, (3) among the outer islands, and (4) along the coast and nearshore islands. The available data are biased in favor of showing high levels of gene flow because they contain an excess of transpacific species, which are a minority among ETP biota. Despite this bias, shallow water populations of the ETP are isolated from the rest of the world’s oceans. Occasional breaching of the expanse of water between the ETP and the Central Pacific by some species is also possible. Gene flow between the outer eastern Pacific islands and the mainland coast is variable, depending on the species examined. Gene flow among populations at the outer eastern Pacific islands is high except for those at Easter Island (Rapa Nui), in which all but one sampled species show large and significant values of F_ST in comparisons with populations from all other islands. Gene flow rates among populations along the ETP coast are high. There is no evident genetic break resulting from the Central American Gap (southern Mexico to the Gulf of Fonseca, Honduras) in any of the sampled species. A trend of isolation by distance along the coast is evident in corals and fishes.