Molluscan marginalia: serration at the lip edge in gastropods

The shells of many marine gastropods have ventrally directed serrations (serial projections) at the edge of the adult outer lip. These poorly studied projections arise as extensions either of external spiral cords or of interspaces between cords. This paper describes taxonomic, phylogenetic, architectural and functional aspects of serrations. Cord-associated serrations occur in cerithiids, strombids, the personid Distorsio anus, ocenebrine muricids and some cancellariids. Interspace-associated serrations are phylogenetically much more widespread, and occur in at least 16 family-level groups. The nature of serration may be taxonomically informative in some fissurellids, littorinids, strombids and costellariids, among other groups. Serrated outer lips occur only in gastropods in which the apex points more backward than upward, but the presence of serrations is not a necessary byproduct of the formation of spiral sculptural elements. In hard-bottom gastropods that do not flee from predators, pointed serrations may resist shear when the shell is clamped firmly to the substratum. The functions of serration in other gastropods are less clear, but likely involve defence against predators with soft feeding structures in some cases.