Long-distance dispersal by planktonic larvae of shoal-water benthic invertebrates among central Pacific islands

The geological origin of central Pacific islands by volcanic activity at "hot spots" through a pre-existing ocean floor requires that their original colonization by invertebrate organisms must have resulted from long-distance dispersal. It is proposed that among most tropical, shoal-water, sediment-dwelling invertebrates such dispersal was largely accomplished by means of planktonic larvae. Evidence to support this hypothesis comes from plankton samples taken in the area bounded by 140°E to 140°W longitude and from 40°N to 40°S latitude which show that larvae of a wide variety of benthic invertebrates are found in the epipelagic waters of the tropical Pacific, including but not restricted to gastropod and bivalve molluscs, sipunculans, polychaetes, echinoderms and brachyuran decapod crustacea. Data from drift bottles in the South Equatorial Current give a model surface velocity between 20 and 25 km day⁻¹, enough to account for dispersal of teleplanic larvae for at least 2,000–4,000 km over the 3–6 months' period of their pelagic existence. The number of larvae that may be successfully transported between islands will depend on (a) the number that "escape" the influence of the "parent" or donor island, (b) the mortality of larvae during transport, and (c) the chance that larvae that encounter an island survive to reproduce. It is estimated on the basis of drift bottle returns (assuming a 1% survival) that once a larva is entrained in the South Equatorial Current, its chance of encountering an island is about 3 × 10⁻⁴ (ca. 1 in 3,300). Fecundity (usually large insofar as known) and the size of the "parent" population (difficult to estimate) must also importantly influence the frequency of exchange between islands by teleplanic larvae. The relatively low endemism and attenuated Indo-Pacific faunas found on central Pacific islands support the hypothesis that there is not only (a) sufficient capacity for dispersal to colonize new islands as they appear, but also (b) that dispersal and the attendant gene flow between islands is frequent enough to account for the persistence of most shoal-water, sediment-dwelling Indo-Pacific species (rather than the allopatric speciation and high endemism found among many terrestrial forms). Data on the dispersal of particular species in conjunction with information on their genetic variation between islands is required to further test such an hypothesis.