GENE FLOW AMONG GIANT CLAM (TRIDACNA-GIGAS) POPULATIONS IN PACIFIC DOES NOT PARALLEL OCEAN CIRCULATION

Tridacna gigas (L.) populations were sampled from 13 locations through out the west Pacific in 1990-1491 and screened for allozyme variation at seven polymorphic loci. Significant genetic differences were found between the few large stocks of T. gigas remaining in the world. Clust er analysis identified two major regional groups of populations: a ''C entral Pacific'' group consisting of Kiribati and the Marshall Islands , and a ''West Pacific'' group comprising all other populations. Withi n the latter group, the Great Barrier Reef populations were significan tly differentiated from those in the Solomon Islands. The main routes of gene flow did not run parallel with known major surface currents as might be expected, but crossed them. Gene exchange was limited east-w est between the Central and West Pacific groups and the GBR and the So lomon Islands. The lack of correspondence between the major surface cu rrents and the routes of gene flow suggests that the genetic structure of T. gigas reflects historical patterns of migration that no longer occur, rather than present-day dispersal. These findings raise fundame ntal questions concerning the origin and maintenance of genetic divers ity in Pacific faunas, and have important implications for the maricul ture and conservation of T. gigas.