Distribution, burrowing, and growth rates of the clam Tridacna crocea on interior reef flats : Formation of structures resembling micro atolls.

Larvae of the burrowing clam Tridacna crocea (Tridacnidae) settle preferentially on top of detached coral heads lying on the surface of the interior reef flat in the Great Barrier Reef province. This species burrows as it grows, eroding the central top surfaces of coral boulders, producing structures that superficially resemble micro-atolls. Storm surges roll these coral heads onto the now flattened surface, killing the live population of clams, and exposing the fresh underside for unimpeded larval settlement. As these clams grow and burrow into the substratum, the coral head becomes progressively flattened and finally breaks apart. Field observations and growthring data documented growth rate; growth rates plus burrow volumes were converted to annual sediment production. At average population densities approximately 140 gm/m2/yr of coral are eroded. Concomitant with erosion is a calcium carbonate increase in the shell of these clams amounting to 60gm/m2/yr. Assuming a stable population structure, with annual mortality equal to annual estimated growth, total sediment production is 200 gm/m2/yr. Clams are usually aggregated at higher densities, however, with numbers regularly exceeding 100 clams/m2. Consequently maximum sediment production rate locally is often 4,500 gm/m2/yr.