Historical and biomechanical analysis of integration and dissociation in molluscan feeding, with special emphasis on the true limpets (Patellogastropoda : Gastropoda)

Modifications of the molluscan feeding apparatus have long been recognized as a crucial feature in molluscan diversification, related to the important process of gathering energy from the environment. An ecologically and evol utionarily significant dichotomy in molluscan feeding kinematics is whether radular teeth flex laterally (flexoglossate) or do not (stereoglossate). I n this study, we use a combination of phylogenetic inference and biomechani cal modeling to understand the transformational and causal basis for flexur e or lack thereof. We also determine whether structural subsystems making u p the feeding system are structurally, functionally, and evolutionarily int egrated or dissociated. Regarding evolutionary dissociation, statistical an alysis of state changes revealed by the phylogenetic analysis shows that ra dular and cartilage subsystems evolved independently. Regarding kinematics, the phylogenetic analysis shows that flexure arose at the base of the Moll usca and lack of flexure is a derived condition in one gastropod clade, the Patellogastropoda. Significantly, radular morphology shows no change at th e node where kinematics become stereoglossate. However, acquisition of ster eoglossy in the Patellogastropoda is correlated with the structural dissoci ation of the subradular membrane and underlying cartilages. Correlation is not causality, so we present a biomechanical model explaining the structura l conditions necessary for the plesiomorphic kinematic state (flexoglossy). Our model suggests that plesiomorphically the radular teeth must flex late rally as they pass over the bending plane as a result of the mechanical res trictions in the flexible but inelastic subradular membrane and close assoc iation between subradular membrane and cartilages. Relating this model to t he specific character states of the clades, we conclude that lack of flexur e in patellogastropods is caused by the dissociation of the subradular memb rane and cartilage supports. (C) 1999 Wiley-Liss, Inc.