Large genetic differences were observed among the Great Barrier Reef (GBR), Fiji and Philippine populations of Tridacna derasa (Roding) sampled in 1989 and 1990 (Nei's unbiased genetic distance, D, =0.137 to 0.341). This result contrasted strongly with the low genetic distance (D = 0.032) reported previously for the giant clam T. maxima over similar geographical scales. No significant genetic differentiation was observed among most populations from the GBR (mean D = 0.007), consistent with the high gene exchange expected in this highly connected reef system. However, significant differentiation resulting from differences in the frequencies of less common alleles between the North-Central GBR and South GBR (Swain region) were observed. Historical isolation of blocks of the Central Indo-West Pacific from the GBR and present-day restrictions to gene exchange between the GBR, Fiji and the Philippines as a result of oceanographic current patterns, were thought to be responsible for the high degree of genetic differentiation of T. derasa populations. The relevance of these findings to clam mariculture and reef restocking are briefly discussed. that the relatively few remaining areas with large populations will be used as sources of broodstock. In order to provide a sound basis for the management of these resources, it will be necessary to define their genetic structure before utilizing them for strain improvement or restocking.
There is at present no data on the genetic structure of Tridacna derasa. The only published genetic work on giant clams describes the high degree of genetic variability of T. maxima within single populations sampled in each of the Marshall Islands and the Great Barrier Reef (GBR), but has suggested little genetic differentiation among these widely separated populations (Ayala et al. 1973, Campbell et al. 1975). T. derasa occurs in coral reef habitats over much of the Indo-Pacific, and an investigation of its population structure should also provide important information relevant to patterns of differentiation of marine organisms in this region.
The present paper describes the genetic structure of Tridacna derasa populations from the GBR, Fiji and the Philippines, using data from electrophoretically detectable protein variants. * Contribution No. 197 from the Marine Science Institute, University of the Philippines and Contribution No. 562 from the Australian Institute of Marine Science "16~ ~ "~ '?"~ Escape ~'~') ~j Michaelmas