The biology and functional morphology of Dianadema gen. nov. multangularis (Tate, 1887) (Bivalvia: Anomalodesmata: Clavagellidae)

Abstract

The anomalodesmatan Clavagellidae can be separated from the Penicillidae on the basis of ligament and adventitious tube or crypt structure. The Clavagellidae includes species of Clavagella (and Dacosta), Bryopa and Stirpulina. Two species, first described as Clavagella torresi Smith, 1885 and Aspergillum multangularis Tate, 1887, have consistently been placed in the genus Clavagella, but both are herein located in the new genus Dianadema. Because more material is available, this is primarily a study of Dianadema multangularis (Tate) which is designated as the type species of the new genus. Dianadema multangularis is unlike Clavagella, Bryopa and Stirpulina which are, respectively, recessing, boring and endobenthic representatives of the Clavagellidae, in that it is cemented and possesses a crown of dorsal tubules, in addition to anterior and ventral ones. As in all clavagellids (sensu stricto) the left valve is cemented into the fabric of an adventitious crypt while the right valve is free inside it. There is also a posterior multi-angular siphonal tube. The anatomy of D. multangularis is described and it is, in most respects, a typical clavagellid. The species differs from other clavagellids in the inferred aeration of the crypt via patterns of water flow, the crown of crypt tubules, and its unique cemented lifestyle. There are three (known) genera of clavagelloids endemic to southern and eastern Australia, i.e. Dacosta and, now, Dianadema (Clavagellidae) and to which the third, Humphreyia (Penicillidae), is similar in that the latter two are both cemented, albeit convergently. They also provide a surprising picture of convergent evolution of the cemented lifestyle even to the extent that both possess a unique pair of pericardial proprioreceptors, each derived differently, to monitor body tonus. Moreover, two other anomalodesmatan genera are also cemented, i.e. Cleidothaerus (Cleidothaeridae) and Myochama (Myochamidae), and these too are restricted to eastern Australia (and New Zealand). What forces promoted cementation in these four antipodean anomalodesmatan families are unknown.