The biology and functional morphology of Fragum erugatum (Bivalvia : Cardiidae) from Shark Bay, Western Australia: the significance of its relationship with entrained zooxanthellae

The shallow, offshore, hypersaline (similar to 60 parts per thousand), bent hic marine environment of Shark Bay, Western Australia, is dominated by the cockle Fragum erugatum. This small(< similar to 11 mm long) bivalve occurs in large numbers (4000/m(2)) and probably completes its life cycle in 1 ye ar. Cyclones with a periodicity of similar to 50 years have, over the last 5000 years, deposited huge numbers of the shells of F. evugatum onto the sh ore as windrows, creating the unique Shell Beach, or Hamelin Coquina. In mo st anatomical respects, F. erugatum is a typical cockle, with a powerful di gging foot, siphons formed by fusion of the inner mantle folds only, plicat e ctenidia with a ciliation of type C(1) and powerful rejectory currents on the visceral mass and mantle. It is thus able to collect filterable materi al and digest it. The labial palps are, however, reduced, allowing the inge stion of only the finest particles of food via the acceptance tracts in the ctenidial axes and distal oral grooves. The waters of Shark Bay are hypers aline and oligotrophic, so what is the food source that supports such large numbers of filter-feeding bivalves? Fragum erugatum possesses symbiotic zo oxanthellae, as do its close relatives the giant clams (Tridacnidae), and p robably obtains the same nutritional benefits from the association, which m ay facilitate survival in the hyperosmotic environment of Shark Bay. The as sociation between giant clams and their zooxanthellae enhances shell growth and results in a long life expectancy and gigantism. Conversely, in F. eru gatum the association facilitates a large population size (but short life e xpectancy), allowing it to dominate a unique, hypersaline, oligotrophic env ironment. Like giant clams, F. erugatum possesses a zooxanthella tube syste m linking the digestive diverticula of the stomach with the kidneys. Either accidentally or periodically, ingested zooxanthellae are transferred from the gut, via this tube, along with senescent zooxanthellae from the blood h aemocoel, to the kidney tubules for lysosomal hydrolysis and eventual expul sion. In the light of this discovery, the functioning of the recently confi rmed zooxanthella tube system in the Tridacnidae needs re-examination and r e-evaluation.