RETENTION AROUND AND LONG-DISTANCE DISPERSAL BETWEEN OCEANIC ISLANDS BY PLANKTONIC LARVAE OF BENTHIC GASTROPOD MOLLUSCA

Indo-Pacific species of gastropod mollusks often have very wide geogra phic distributions sometimes extending halfway around the world, from the Marquesas or Hawaiian Islands westward throughout the tropical Pac ific and Indian Oceans to the Red Sea and eastern African coast. For a gastropod species to attain such a wide distribution and yet also mai ntain genetic continuity among so large a number of disjunct populatio ns, some sort of dispersal must occur among the many scattered oceanic islands within the species range. One way this can be accomplished is by long-distance dispersal of planktonic larvae. It is proposed that a small fraction of the total veligers produced by a gastropod species on any particular oceanic island, such as the island of Hawaii, will escape the effect of local circulation and as a consequence will have the possibility to be passively transported by oceanic currents to oth er remote islands. Evidence from plankton tows taken in proximity to H awaii, when related to a knowledge of local mesoscale circulation, sho ws how larvae can be dispersed outside the influence of local island c irculation. At the same rime, plankton samples from the tropical centr al and western Pacific Ocean reveal how gastropod veligers are passive ly advected over long distances by ocean currents. Furthermore, drift bottle data illustrate how gastropod veligers, once entrained within t he North Equatorial Current, can encounter other oceanic islands and g ive evidence for the probability of such an island encounter. Notwiths tanding evidence that larvae can be dispersed away from their native i sland (in this instance from the island of Hawaii), it is inferred tha t gastropod species are largely self-sustained by veligers from indige nous populations. For such recruitment from local populations to occur , veligers must (1) be constrained by the local hydrography to remain within the proximity of their natal island, (2) survive the vicissitud es of planktonic life in sufficient numbers and complete development t o the competent stage when settlement and metamorphosis become possibl e, and (3) must be returned passively to a suitable sublittoral enviro nment by the local circulation. Evidence from plankton tows, a knowled ge of mesoscale circulation, and data from drift-bottle returns allow an explanation of how larvae can be retained and how they are ultimate ly returned to their island of origin. Paradoxically, it seems that th e hydrological phenomena that sometimes return larvae to their natal i sland can, in other instances, passively transport veliger larvae out to sea.