The biology and functional morphology of the placental embryo-brooding neolepton salmoneum, a comparison with neolepton subtrigonum (Bivalvia: Cyamioidea: Neoleptonidae), and a discussion of affinities

Abstract

The anatomies of two species of Neoleptonidae are described. Clearly heterodont with cardinal and lateral hinge teeth, the shell also has an internal, parivincular ligament. Shell form suggests a shallow burrowing mode of life in coarse gravels in coastal waters. The labial palps are small and the intestine short further suggesting deposit feeding in well-sorted gravels. A pronounced prodissoconch (II) with a marginal ridge argues for lecithotrophic development further facilitating re-colonization of a narrow niche. In most anatomical respects, the two species are simplified with conjoined inhalant and pedal apertures and few posterior sensory mantle papillae. The ctenidia comprise subequal demibranchs with the outer reduced and the inner modified for internally fertilized embryo brooding. The attachment of each embryo chord to the demibranch filaments is probably from secretions produced by basal glands developed on their abrofrontal surfaces. There may also be secondary external pallial brooding, accounting for the ridged prodissoconch II with attachment achieved via secretions from oil glands in the mantle margin. Comparisons are made with representatives of the earliest-considered affiliates of the Cyamioidea, that is, the Arcticoidea [now rejected], the confamilial Gaimardiidae and Cyamiidae, and the recently suggested Ungulinidae. It is considered that although neoteny has been proposed for the Turtoniidae, Sportellidae, and Neleptonidae to explain cyamioidean small size, anatomical features of the studied species of the latter family herein investigated suggest, rather, that they are simply 'small' there being, contrary to the conclusions of others, little evidence of paedomorphosis. In this scenario, smallness is not a reflection of neoteny. It is the evolutionary selection of a life history trait and opted recipe for success. Notwithstanding, earlier suggestions of an affiliation with the Veneroidea are considered plausible, although no reason is seen for not retaining the superfamily Cyamioidea nor its, as contemporaneously recognized, families, including the Neoleptonidae.