Abstract

The cardiid bivalve taxa Plagiocardium, Maoricardium, Clinocardiinae, Fraginae, Goethemia, Lymnocardiinae, and Tridacninae form a monophyletic group. The most parsimonious cladograms of this group have a topology of (Plagiocardium (Maoricardium (Clinocardiinae (Fraginae (Goethemia (Lymnocardiinae, Tridacninae)))))). The giant clams form a subfamily (Tridacninae) of Cardiidae. In the ontogeny and phylogeny of giant clams, the morphology and position of the muscle scars and hinge changes from that resembling the dimyarian Cerastoderma to the monomyarian morphology displayed by Hippopus and Tridacna, which have lost the anterior half of the shell. This peramorphic trend is the result of differential growth: tremendous growth rate of the posterior portion of the shell and a zero or even negative growth rate of the anterior half of the shell. All tridacnines and several species of fragines are known to harbor photosymbiotic zooxanthellae. These algae transfer excess carbon to its cardiid host. It is known that the chemosymbiotic lucinid and solemyid bivalves reduce or lose many features of their digestive systems. The sulfur-reducing bacteria in lucinids and solemyids provide most of the nutrients that the host clam needs, thus making a full digestive system unnecessary. Most fragine cardiids reduce numerous structures of their digestive systems. Only a few fragines have been examined for the presence of photosymbionts. Based upon their anatomy, it is predicted that most or all of the derived fragines harbor photosymbionts. Tridacnines harbor photosymbionts but do not appreciably reduce their digestive systems. The possession of both photosymbionts and a fully functional digestive system allows tridacnines to be the fastest growing and largest of the extant bivalves.