The biology and functional morphology of Myochama anomioides Stutchbury, 1830 (Bivalvia : Anomalodesmata : Pandoroidea), with reference to cementation

The small, exclusively Australasian, anomalodesmatan family Myochamidae com prises only two genera; the shallow-burrowing Myadora and the cementing Myo chama. This paper describes the anatomy and cementing behaviour of Myochama anomioides and draws comparisons with Myadora. The anatomy of Myochama anomioides is little different from that previously described for Myadora, except that they are mirror images. Valve inequalit y is not reflected in the organs of the mantle cavity in either taxon. Such differences which are present, for example the reduction of the foot in My ochama, mostly relate to the adoption of a sessile habit. There are few dif ferences in mantle folds of the cementing and non-cementing genus, except t hat in,M. anomioides the right mantle fold, which secretes the cemented val ve, is thicker and less well-developed than the left. During the cementatio n process, the periostracum secreted by the right fold is thinner and has a quilted appearance. Individuals of Myochama anomioides cement by their right valve once they ha ve reached a size of 1.2-3.9 mm. They appear to have a preference for attac hing to the posterior portions of a diversity of living, shallow infaunal b ivalves. The pronounced stereotypic orientation they adopt suggests that th ese hosts are most often alive at the time of colonization and that the myo chamids benefit from the relationship. The relationship, however, is not ob ligate. They are capable of attaching to other shelly or rock debris, but d o so at a larger size, presumably when the pre ferred substrata are not ava ilable. The thin layer of extra-periostracal cement lacks the calcareous cr ystalline nature of oyster cement, instead being largely composed of organi c material. This cement is presumably secreted by glands within the mantle, but these have not been identified. Indeed, the mantle lacks arenophilic g lands which might have been thought a suitable candidate for supplying ceme nt.