The biology and functional morphology of Nipponoclava gigantea: clues to the evolution of tube dwelling in the Penicillidae (Bivalvia: Anomalodesmata: Clavagelloidea)

Abstract

Nipponoclava gigantea is restricted to the tropical Western Pacific waters of southern Japan at depths of between 5 and 70 m and lives in coarse sand and gravel. In many respects, N. gigantea appears to be similar to other endobenthic penicillids in that there is a true shell, the margins of which, via the intermediary of a narrow ‘saddle’, are united with an adventitious tube. This has a large watering pot anteriorly and pleated ruffles, representing either growth or repair events, posteriorly. Nipponoclava gigantea differs from other penicillids, however, in that its true shell is large, i.e. ∼20 mm high as compared to 2–3 mm, and the angle of valve splay is ∼80° as opposed to 180°. Internally the shell has a wide pallial line and shallow pallial sinus and there are anterior adductor muscle scars. There are possibly posterior adductor muscle scars also. These are not present in the adult animal, however, although the anterior adductor is. Also present are paired anterior and (possibly) posterior pedal retractor muscles, the latter having a nervous union with the visceral ganglia to create simple stretch or tonus proprioreceptors. It is argued that N. gigantea provides clues about how the tube-dwelling lifestyle has evolved in the various taxa of the Penicillidae. It is suggested that through a process of heterochrony, in which the animal produces its adventitious tube at progressively earlier and earlier developmental stages, the true shell has become actually and relatively smaller and smaller and, hence, more splayed in more derived penicillids. Nipponoclava gigantea is thus an important link between an hypothetical lyonsiid ancestor and the more specialized penicillids and helps us to understand the adaptive radiation of this group of tube-dwelling bivalves.