BROODING AND NON-BROODING DACRYDIUM (BIVALVIA, MYTILIDAE) - A REVIEW OF THE ATLANTIC SPECIES

The Atlantic species of the marine bivalve genus Dacrydium are reviewe d, with particular emphasis on their hinge and protoconch characters. The basic groundplan of a Dacrydium comprises a functional primary lig ament, a paired series of primary teeth, and a posterior row of second ary teeth separated from the latter by a secondary ligament; this can be transformed into a single series either by loss of the secondary li gament and merging primary and secondary teeth, or by loss of secondar y teeth and ligament through paedomorphosis. Twelve species are recogn ized, of which eleven are illustrated. One abyssal species is not sepa rable 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, 195 5, is described as a new subspecies D. e. hendersoni. The larvae are b rooded in D. hyalinum (Monterosato, 1875), D. viviparum Ockelmann, 198 3 and D. balgimi. The brooding species have larger larvae (protoconch 210 to 315 mu m long) than the non-brooding (protoconch 120 to 150 mu m 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 analys is of hinge and shell characters as well as the brooding/non brooding character.