GIANT CLAMS IN THE NORTHERN TERRITORY  

Little is known of the biology, ecology, or genetics of giant clams (Tridacnidae) inhabiting the coastal waters of the Northern Territory (NT), Australia. This study used a combination of methods to describe the size, age, habitat association and genetic structure of this group of molluscs at 18 inshore reef sites across the NT coast. A novel and inexpensive underwater video survey system was also developed to assist in this process.

Live giant clams were associated with live hard corals (of the genera Favia, Porites, Favites and Goniastrea), interspersed with substrates composed of sand, rubble, and rock; dead giant clams were typically found in areas dominated by rubble and/or sand, with little or no live coral cover. Shell lengths of giant clams (n = 337) derived from calibrated video footage ranged from 87-464 mm with a mean length of 305 mm. The mean age was estimated at 9.1 years, with 76% of individuals less than 10 years old. Observed densities of giant clams were amongst the highest recorded in the Indo-Pacific region, ranging from 2.5-447.4 clams.ha^-1 (mean 79.5 clams.ha^-1).

Phylogenetic analysis using the COI mtDNA gene confirmed that only a single species of giant clam, Tridacna squamosa, inhabits the inshore reefs of the NT. A new giant clam species, T. ningaloo, was serendipitously discovered amongst samples collected from Ningaloo Reef, Western Australia. Northern Australian T. squamosa populations are significantly different to conspecifics from elsewhere in the Indo-Pacific, including samples from the "Coral Triangle" region. The evolutionary divergence of northern Australian T. squamosa populations is most likely the result of geographical and oceanographic isolation over the last 25 million years.

Giant clams are harvested in the NT as part of a managed fishery. A number of programs are also underway to develop onshore farms and sea ranches in remote coastal Indigenous communities. These programs are aimed at developing an underutilised sustainable natural resource; supply specimens for the aquarium trade; provide fresh food in communities; stimulate economic and employment opportunities for Indigenous communities to participate in the seafood industry.