Brooding and non-brooding Dacrydium (Bivalvia: Mytilidae): A review of the Atlantic species

Abstract

The Atlantic species of the marine bivalve genus Dacrydium are reviewed, with particular emphasis on their hinge and protoconch characters. The basic groundplan of a Dacrydium comprises a functional primary ligament, a paired series of primary teeth, and a posterior row of secondary teeth separated from the latter by a secondary ligament; this can be transformed into a single series either by loss of the secondary ligament and merging primary and secondary teeth, or by loss of secondary teeth and ligament through paedomorphosis. Twelve species are recognized, of which eleven are illustrated. One abyssal species is not separable morphologically from the Indian Ocean D. speculum Poutiers, 1989 and is new to the Atlantic; four new species (D. wareni, D. dauvini, D. filiferum and D. balgimi) are described; a Caribbean form which is hardly distinct from the Eastern Pacific D. elegantulum Soot-Ryen, 1955, is described as a new subspecies D. e. hendersoni. The larvae are brooded in D. hyalinum (Monterosato, 1875), D. viviparum Ockelmann, 1983 and D. balgimi. The brooding species have larger larvae (protoconch 210 to 315 μm long) than the non-brooding (protoconch 120 to 150 μm long), and reach a smaller adult size (1.4 to 3 mm instead of 4.5-5 mm). A phylogenetic reconstruction is attempted using parsimony analysis of hinge and shell characters as well as the brooding/non brooding character.