SOME ASPECTS OF THE FUNCTIONAL-MORPHOLOGY OF THE SHELL OF INFAUNAL BIVALVES (MOLLUSCA)

Measures of streamlining, gapage, umbo position, and pallial sinus dep th were taken from 632 species of Bivalvia in 13 families. Two types o f gapage were measured: exchangeable gapage due to the rocking motion of the shells along a dynamic dorso-ventral axis, and permanent gapage , that portion of the gapage that could not be closed by the rocking m ovement. Models were given to predict changes in shell shape as an ada ptation to infaunal life. Three stages occur in the sequence of shell shapes from shallow to deep infaunal dwellers for the families studied . The first stage is represented by unstreamlined, often sculptured sh ells with complete valve closure. The second, or intermediate stage, c onsists of an increase in streamlining, a loss of sculpture, central p lacement of the umbo, and temporary gapes in the shell for pedal and s iphonal outlets. These gapes may be opened and closed by rocking the s hells along a dorso-ventral axis (exchangeable gapage). Two paths are evident out of the intermediate stage into the third. The myid path re sults in unstreamlined shells with central umbos. The solenid path res ults in streamlined shells with a variable umbo position. Some familie s, such as the Mactridae, have members along both paths. The entry int o this sequence requires a particular set of pre-existing morphologica l conditions. The lack of these conditions in most species studied has resulted in a bottleneck, with few species in the deep infaunal zone. The constraints of bivalve shell geometry have limited the success of that group in otherwise favorable habitats.