The small-subunit rRNA gene sequences of venerids and the phylogeny of Bivalvia

The complete nucleotide sequence of the 18S subunit of ribosomal DNA (rDNA) was determined for the venerid clams Callista chione (Pitarinae) and Venus verrucosa (Venerinae). Comparison of the new sequences with the published sequences of 1 annelid, 2 gastropods, 2 polyplacophorans, and 19 bivalves s howed that when the annelids are used as outgroup the gastropods diverge fr om the bivalves, which form a cluster including the polyplacophorans. When the gastropods alone were compared with the bivalves, the latter split in t wo groups corresponding to the two subclasses of Heterodonta and Pteriomorp ha. The former include two taxa that diverged early, Galeomma and Tridacna, while the Veneridae and Mactridae form two sister groups. In contrast to p revious reports and in line with morphological data, the Ostreidae are incl uded in the Pteriomorphia and form a monophyletic group.