Anatomical studies on Margaritifera margaritifera and Cumberlandia monodonta (Mollusca: Pelecypoda: Margaritiferidae)

Abstract

The family Margaritiferidae has received little serious anatomical study since the early twentieth centurv. The present paper attempts to provide information, derived from relaxed preserved and living material, on aspects of the mantle anatomy and neuroanatomy of two species, Margaritifera margaritifera and Cumberlandia monodonta. An historical overview of the systematics of the family is provided along with a proposed division of major groups within the genus Margaritifera. The diaphragm of margaritiferids is complete, rather than incomplete as previously interpreted, and is formed bv both the gills and the diaphragmatic septa, the latter of which, in life, cover a portion of the posterior outer gill demibranchs and separate the branchial and suprabranchial chambers. Union of the diaphragmatic septa posteriorly draws the mantle margins together, separating the inhalent and exhalent apertures and thus completing the diaphragm. The general visceral nervous system of each species is consistent with that of other bivalves. Differences from other unionaceans include the possession of a small, as opposed to a large, dorsal supra-anal bifurcation. Also, each of the two species studied lacks the anterior loop of the branchial nerve reported in some other unionaceans.