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

Abstract

The shallow, offshore, hypersaline (∼ 60‰), benthic marine environment of Shark Bay, Western Australia, is dominated by the cockle Fragum erugatum. This small (< ∼ 11 mm long) bivalve occurs in large numbers (4000/m²) and probably completes its life cycle in 1 year. Cyclones with a periodicity of ∼ 50 years have, over the last 5000 years, deposited huge numbers of the shells of F. erugatum onto the shore as windrows, creating the unique Shell Beach, or Hamelin Coquina. In most anatomical respects, F. erugatum is a typical cockle, with a powerful digging foot, siphons formed by fusion of the inner mantle folds only, plicate 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 material and digest it. The labial palps are, however, reduced, allowing the ingestion 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 hypersaline and oligotrophic, so what is the food source that supports such large numbers of filter-feeding bivalves? Fragum erugatum possesses symbiotic zooxanthellae, as do its close relatives the giant clams (Tridacnidae), and probably obtains the same nutritional benefits from the association, which may facilitate survival in the hyperosmotic environment of Shark Bay. The association between giant clams and their zooxanthellae enhances shell growth and results in a long life expectancy and gigantism. Conversely, in F. erugatum the association facilitates a large population size (but short life expectancy), allowing it to dominate a unique, hypersaline, oligotrophic environment. Like giant clams, F. erugatum possesses a zooxanthella tube system 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 haemocoel, to the kidney tubules for lysosomal hydrolysis and eventual expulsion. In the light of this discovery, the functioning of the recently confirmed zooxanthella tube system in the Tridacnidae needs re-examination and re-evaluation.