ARONOWSKY, A.: Trends in clade-level morphology: a geometric morphometric study of the Naticidae (Mollusca: Caenogastropoda)
Naticidae is a taxonomically diverse clade of marine caenogastropods that is notoriously conservative in shell morphology. Ten taxa from four clades were examined using geometric morphometrics. Thirteen landmarks were identified in apertural view, with the coiling axis used as a baseline. All taxa were found to be significantly different from each other in pairwise comparisions of Bookstein coordinates using Hotelling’s T2 test. Within-species variance was higher in taxa with wider, more inflated apertures. These taxa also partially internalize their shells and some have completely lost the ability to retract into the shell. Shell internalization, therefore, may put greater soft-tissue pressure on the growing edge of the shell, leading to erratic deformation. Principle components analysis deformation vectors mapped onto a well-supported phylogenetic hypothesis revealed repeated trends in the more speciose clades examined. A clade including species of the genus Natica and a Polinices-dominated clade both contain basal taxa with laterally-contracted apertures and high-spired shells. Crownward progression in each clade leads to a lateral expansion of the aperture and a general widening and shortening of the shell. However, this shell shortening is accomplished through different growth mechanisms. The repetition of this general trend suggests the possibility of an underlying functional cause. Increased mobility associated with a wider aperture is one good candidate for future experimental testing.
