SMITH, Krister, and GURALNICK, Robert, Museum of Paleontology and Dept. of Integrative Biology, University of California, Berkeley, CA 94720-4780
Understanding how complex, integrated structures change through evolution and the relation of these changes to function has been a focus of both historical and biomechanical approaches in biology. We combine these approaches to examine the molluscan feeding apparatus. The structural elements include the toothed radula, pairs of cartilages, and a system of muscles which connect to the radula and cartilages and power the feeding stroke. Most workers have assumed that these elements are tightly integrated, such that changes in one element are correlated with changes in the others.
An important functional difference in feeding mode, which we focus upon here, is the flexoglossate versus stereoglossate condition. In the flexoglossate condition (polyplacophorans and most gastropods), teeth flex outward and then sweep inward during the feeding stroke, whereas in the stereoglossate condition (patellogastropods), the teeth do not flex. Using a phylogenetic hypothesis of gastropod relationships, we ask: (1) What structural changes, if any, occur simultaneously with major functional shifts in feeding mode? (2) Which of these changes are directly and causally related to change in function? (3) Can similar radular morphologies be used in completely different ways?
We find that the key shift in function, from flexoglossate to stereoglossate, is correlated with the structural dissociation of the radula from the underlying cartilages; this separation forces the change in function. No major changes in musculature facilitate this shift. Polyplacophorans and patellogastropods, which are most similar to each other in their radular dentition, have entirely different feeding modes. Thus, previous hypotheses about adaptive, integrated responses in this particular system are incorrect; changes in different elements have been dissociated during evolution from changes in the others.