Mass Extinctions and Genetic Polymorphism in the 'Killer Clam,' Tridacna

Mass extinctions of marine invertebrates have been attributed to genetic depauperation in specialized lineages. Tridacna maxima is a plausible modern analog of the lineages that were commonly associated with mass extinctions; it is restricted to a relatively stable biogeographic province, lives in shallow water, is highly specialized, and is associated with reef communities. Our studies show, however, that it is highly polymorphic and heterozygotic, and thus fails to support the depauperate gene-pool hypothesis of mass extinction.