Historical and biomechanical analysis of integration and dissociation in molluscan feeding, with special emphasis on the true limpets (Patellogastropoda : Gastropoda)
R. Guralnick et K. Smith, Historical and biomechanical analysis of integration and dissociation in molluscan feeding, with special emphasis on the true limpets (Patellogastropoda : Gastropoda), J MORPH, 241(2), 1999, pp. 175-195
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.