Biology and functional morphology of Myochama anomioides Stutchbury, 1830 (Bivalvia: Anomalodesmata: Pandoroidea), with reference ot cementation

Abstract

The small, exclusively Australasian, anomalodesmatan family Mychamidae comprises only two genera; the shallow-burrowing Myadora and the cementing Myochama. This paper describes the anatomy ad 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 inequality 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 Myochama, mostly relate to the adoption of a sessile habit. There are few idfferences in mantle folds of the cementing and non-cementing genus, except that in M. anomioides the right mantle fold, which secretes the cemented valve, is thicker and less well-developed than the left. During the cementation 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 have recahed a size of 1.2—3.9 mm. They appear to have a preference for attaching to the posterior portions of a diversity of living, shallow infaunal bivalves. The pronounced stereotypic orientation they adopt suggests that these hosts are most often alive at the time of colonization and that the mychamids benefit from the relationship. The relationship, however, is not obligate. They are capable of attaching to other shelly or rock debris, but do so at a lager size, presumaby whe the preferred substrata are not available. The thin layer of extra-periostracal cement lacks the calcereous crystalline nature of oyster cement, instead of being largely composed of organic material. This cement is presumably secreted by glands within the mantle, but these have not been identified. Indeed, the mantle lacks arenophilic glands which might have been thought a suitable candidate for supplying cement.