On the monomyarian, Acostaea rivoli and evolution in the family Etheriidae (Bivalvia: Unionacea)

Acostaea rivoli (Desh.) is an etheriid bivalve which occurs, cemented to the substrate, in the basin of the River Magdalena in Colombia. The animal, apart from the shell, is here first described and compared with the two closely related species, the similarly monomyarian Pseudomulleria dalyi (Smith) from Mysore, India and the dimyarian Etheria elliptica Lam. widely distributed in tropical Africa. Attaching at an unusually late stage and by either valve, all three pass from initial bilaterial symmetry to a state of extreme bilateral and– apart from Etheria –also antero‐posterior asymmetry. In Acostaea (and Pseudomulleria ) this involves exclusively posterior and, because settlement is anterior end downward in a depression, topographically upward growth with a change of around 90 in the disposition of the valves as these flatten out over the substrate. This necessitates formation of a new hinge between the anterior spur which is composed of the earlier growth of both valves, and the horizontal extent of the upper valve. With loss of the anterior adductor, the monomyarian condition is achieved in a unique manner. Evidence indicates that the family Etheriidae arose from edentulous Unionidae with the adoption of a pleurothetic cemented habit, this permitting exploitation of a hard substrate often, although not invariably, in rushing water.