The biology and functional morphology of Pteria brevialata (Bivalvia: Pterioidea), epizoic on gorgonians in Hong Kong

Pteria brevialata characteristically attaches to Hicksonella princeps (Cnidaria: Gorgonoidea) at depths > 10 m in Hong Kong and the South China Sea. Attachment is by a stout byssus to the basal regions of the gorgonian colonies. The shell appears typical of the Pteriidae, but closer inspection shows that the antero-ventral margin, in particular, moulds itself to the form of the cylindrical gorgonian, which also changes its growth form, so that securer attachment is achieved. The margin of the right valve is not nacred so that the flexible outer prismatic layer adpresses firmly against the more extensively nacred left valve when they close; mantle and ctenidia also being withdrawn deeply. This is probably an anti-predation device. There are no mantle fusions, even between posterior inhalant and exhalant streams. The mantle margin is, however, complexly tentaculate. Pteria brevialata is monomyarian with reduced anterior byssal retractor muscles. The anterior face of the shell is thus greatly reduced whereas the posterior is inflated and postero-dorsally 'winged'.
Other features of the anatomy are described, including the ctenidial eyespot and simple photoreceptors in the inner component of the duplicated outer mantle folds, under the periostracum. Similar structures in other pterioideans and arcoideans suggest a close relationship between their respective orders.
It is concluded that gorgonian-associated pterioids can be derived from an epibyssate, crevice-dwelling ancestor, exploiting, as solitary individuals, the currents and clean oceanic waters above the substratum. This has three important advantages: (a) removal from surface-roving predators (the gorgonian providing additional protection); (b) exploitation of a niche hitherto unoccupied by bivalves and (c) removal of the animal from the sea-bed and exclusive exploitation of potential food held in suspension. Current stresses to attachment are avoided by modifications to the shell.