SEMINAR: Thursday Dec. 14, 11AM in the 6th floor conference room N6s.10 
of the Natcher building (Bldg 45)

SPEAKER: Dr. Jay Schneider

TITLE:   Evolutionary patterns of cardiid bivalves (cockles and giant 
clams): broad agreement between molecules and morphology

Dr. Schneider is a candidate for a position with the NCBI taxonomy 
group. The abstract is appended below.

The bivalve molluscs of the family Cardiidae (cockles and giant clams) 
have a present-day diversity of over 200 species and live in shallow 
marine environments all over the globe.  Both the shell and the anatomy 
of cardiids provide far more characters for phylogenetic analysis than 
most bivalves, therefore making them an excellent system for testing 
the congruency of phylogenetic hypotheses from molecular data versus 
those from morphologic data.

The D2 domain of the 28s gene has been used to produce a phylogenetic 
hypothesis of the cardiids.  Results using preliminary alignments 
produce hypotheses in broad agreement with those derived from 
morphologic data.  The 28s gene in cardiids has been found to be much 
more variable than in any other group of molluscs yet studied. Various 
secondary structure models are being investigated to minimize gaps and 
otherwise improve alignments. In the ontogeny and phylogeny of giant 
clams, the morphology and position of the muscle scars and hinge 
changes from that of the edible cockle Cerastoderma to that displayed 
by the giant clams Tridacna and Hippopus, which have lost the anterior 
half of the shell.  This peramorphic trend is documented by a 
remarkable series of transitional fossils.  The giant clam molecular 
phylogeny (using 16s rDNA) suggests an interesting biogeographic and 
paleontologic pattern: the two rare species of giant clam are the only 
ones which lack a fossil record; they appear to be relict lineages 
which have replaced over almost all of their former ranges by 
congeners, and should eventually be found as fossil.

All giant clams and several species of fragine cardiids harbor 
photosymbiotic zooxanthellae, which transfer excess carbon to their 
molluscan host.  Fragine cardiids tend to have simplified digestive 
systems whereas giant clams do not.  The possession of both 
photosymbionts and a fully functional digestive system allows giant 
clams to be the fastest growing and largest living bivalves.