DNA studies do seem to provide some help in settling taxonomic
disagreements.  For example, my DNA data point to a close relationship
between oysters (including Hyotissa) and pearl oysters, with scallops more
distant.  Various authors had proposed either scallop or pearl oyster
affinities for the oysters, based on morphological, paleontological, and
developmental evidence.

Both morphological and DNA evidence point to the tridacnids as being an
offshoot of the cockles.  This has been at least guessed at for a long
time, but only recently has there been any formal analysis of the
morphological data.  Maruyama,T., Ishikura,M., Yamazaki,S. and Kanai,S.
Molecular phylogeny of zooxanthellate bivalves Biol. Bull. 195, 70-77
(1998) has the DNA data, and various papers by Jay Schneider have the
morphological analyses (he's also working on DNA currently).  Part of the
issue here is what qualifies as a family.  Is it OK to have one family
(Tridacnidae) to be a specialized offshoot from within another family, or
should it be demoted?

The impact on the number of species is not yet clear.  Some similar-looking
shells have distinct DNA, whereas other, more different-appearing forms
seem to overlap in DNA features.

David Campbell

"Old Seashells"

Department of Geological Sciences
CB 3315 Mitchell Hall
University of North Carolina at Chapel Hill
Chapel Hill NC 27599-3315
USA

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919-962-0685
FAX 919-966-4519

"He had discovered an unknown bivalve, forming a new genus"-E. A. Poe, The
Gold Bug