Biology of modern Fragum erugatum (Mollusca, Bivalvia, Cardiidae) in relation to deposition of the Hamelin Coquina, Shark Bay, Western Australia

Fragum erugatum populations were sampled over two consecutive years in Hamelin Pool, Lharidon Eight and Freycinet Harbour, which are representative of the hypersaline and metahaline regimes of Shark Bay. F. erugatum was widely distributed infratidally between 1.2 m and 6.5 m depth. Dense aggregations of zooxanthellae were present in mantle and gill tissue. Shell shape of the population from Hamelin Pool differs from that of the Lharidon Eight, Freycinet Harbour and Dampier Archipelago populations. F. erugatum was found to be a synchronous hermaphrodite. Settlements of juveniles, and modality of size distributions, indicate a single annual spawning. Production estimates of dry flesh and shell (CaCO3 inclusive) were much lower in Hamelin Pool than in Lharidon Eight, largely because of the lower density of F.: erugatum recorded in Hamelin Pool. Although shells are washed ashore continuously, large-scale deposition of the accumulated infratidal coquinas, largely composed of F. erugatum shells, probably occurs periodically in major storm events, thereby forming the Hamelin Coquina.