EVIDENCE OF A BIOGEOGRAPHIC BREAK BETWEEN POPULATIONS OF A HIGH DISPERSAL STARFISH - CONGRUENT REGIONS WITHIN THE INDO-WEST PACIFIC DEFINEDBY COLOR MORPHS, MTDNA, AND ALLOZYME DATA

Both mtDNA variation and allozyme data demonstrate that geographic gro upings of different color morphs of the starfish Linckia laevigata are congruent with a genetic discontinuity between the Indian and Pacific Oceans. Populations of L. laevigata sampled from Thailand and South A frica, where an orange color morph predominates, were surveyed using s even polymorphic enzyme loci and restriction fragment analysis of a po rtion of the mtDNA including the control region. Both allozyme and DNA data demonstrated that these populations were significantly genetical ly differentiated from each other and to a greater degree from 23 popu lations throughout the West Pacific Ocean, where a blue color morph is predominant. The genetic structure observed in L. laevigata is consis tent with traditional ideas of a biogeographic boundary between the In dian and Pacific Oceans except that populations several hundreds kilom eters off the coast of north Western Australia (Indian Ocean) were gen etically similar to and had the same color morphs as Pacific populatio ns. It is suggested that gene flow may have continued (possibly at a r educed rate) between these offshore reefs in Western Australia and the West Pacific during Pleistocene falls in sea level, but at the same t ime gene flow was restricted between these Western Australian populati ons and those in both Thailand and South Africa, possibly by upwelling s. The molecular data in this study suggest that vicariant events have played an important role in shaping the broadscale genetic structure of L. laevigata. Additionally, greater genetic structure was observed among Indian Ocean populations than among Pacific Ocean populations, p robably because there are fewer reefs and island archipelagos in the I ndian Ocean than in the Pacific, and because present-day surface ocean currents do not facilitate long-distance dispersal.