GENETIC-STRUCTURE OF GIANT-CLAM (TRIDACNA-MAXIMA) POPULATIONS IN THE WEST PACIFIC IS NOT CONSISTENT WITH DISPERSAL BY PRESENT-DAY OCEAN CURRENTS

The Pacific marine biota, particularly species with long planktonic la rval stages, are thought to disperse widely throughout the Pacific via ocean currents. The little genetic data available to date has support ed this view in that little or no significant regional differentiation of populations has been found over large geographical distances. Howe ver, recent data from giant clams has demonstrated not only significan t regional differentiation of populations, but routes of gene flow tha t run perpendicular to the main present-day ocean currents. Extensive surveys of genetic variation at eight polymorphic loci in 19 populatio ns of the giant clam Tridacna maxima, sampled throughout the West and Central Pacific, confirmed that the patterns of variation seen so far in T. gigas were not unique to that species, and may reflect a fundame ntal genetic structuring of shallow-water marine taxa. Populations of T. maxima within highly connected reef systems like the Great Barrier Reef were panmictic (average F-ST < 0.003), but highly significant gen etic differences between reef groups on different archipelagos (averag e F-ST = 0.084) and between West and Central Pacific regions (average F-ST = 0.156) were found. Inferred gene flew was high (N(c)m usually > 5) between the Philippines and the Great Barrier Reef, between the Ph ilippines and Melanesia (the Solomon Islands and Fiji), and between th e Philippines and the Central Pacific island groups (Marshall Islands, Kiribati, Tuvalu and Cook Islands). Gene flow was low between these t hree sets of island chains (N(c)m < 2). These routes of gene flow are perpendicular to present-day ocean currents. It is suggested that the spatial patterns of gene frequencies reflect past episodes of dispersa l at times of lower sea levels which have not been erased by subsequen t dispersal by present-day circulation. The patterns are consistent wi th extensive dispersal of marine species in the Pacific, and with trad itional views of dispersal from the Indo-Malay region. However, they d emonstrate that dispersal along present-day ocean surface currents can not be assumed, that other mechanisms may operate today or that major dispersal events are intermittent (perhaps separated by several thousa nds of years), and that the nature and timing of dispersal of Pacific marine species is more complex than has been thought.