Article: The structure of ontogenetic variation in the shell of Pecten
Spafford C. Ackerly
Shell growth in Pecten conforms closely to a model where the shell margin is a sector of an ellipse (nearly circular), and where shell growth reflects an expansion and migration of the margin away from the shell apex. Detailed ontogenetic measurements on seven species (n = 23) indicate that all specimens are allometric, as reflected in both (1) a curvature of the ribs on the flattened left valve, and (2) a doming or vaulting of the coiled right valve. Patterns of covariation between growth parameters indicate that shell morphologies converge towards specific adult geometries. For example, juvenile shells with small length to height (L/H) ratios tend to become more elongate during growth (L/H increases) whereas juveniles with large L/H ratios tend to become shortened during growth (L/H decreases). These covariation patterns may represent morphogenetic processes which constrain adult morphologies to specific functional geometries.