Loss of genetic variation in a strongly isolated Azorean population of theedible calm, Tapes decussatus

We used allozyme electrophoresis to compare the genetic variation of an int roduced and strongly isolated population of the edible clam Tapes decussatu s in the Azores (Lagoa de Santo Cristo, Sao Jorge) with populations from th e main range of the species (Ria and Thau). Observed and expected heterozyg osity values, number of polymorphic loci, and mean number of alleles per lo cus in the main-range populations fall within the limits reported for T. de cussatus and other Venerid clams. In contrast to previous studies on Veneri d clams, we observed no heterozygote deficiencies. In the introduced Lagoa population, we observed a strong reduction of allelic diversity and expecte d heterozygosities and an effective population size of only 5.30. The Lagoa population is only slightly differentiated from populations from the speci es' main range and may thus be of low "biological value." Exploitation of T . decussatus could therefore be allowed to continue but must follow strict collection guidelines, especially given that only 15% of the area is suitab le for exploitation. Otherwise, a unique component of the Azorean fauna tha t also serves as a fishery resource may be lost.