CONVERGENCE, PARALLELISM, AND FUNCTION OF EXTREME PARIETAL CALLUS IN MARINE GASTROPODS

This study finds evidence for convergent evolution of the extreme parietal callus in caenogastropods. EPC is defined as a thickening of the spire and axial calluses that covers more than 50% of the ventral shell surface. We used SEM imaging to examine shell microstructure of gastropod primary shell, callus, and spines to characterize the microstructures that comprise each shell layer. We made comparisons of species that develop EPC and a congener or confamilial with normal callus as a control species. We compared species from four caenogastropod families: Volutidae, Pseudolividae, Olividae, and Ancillariidae, with a fifth family, Strombidae as an outgroup. We observed a range of microstructures including homogenous, simple prismatic, spherulitic prismatic, crossed lamellar (CL), complex crossed lamellar, weakly ordered fibrous, and irregularly ordered fibrous.

Normal shell microstructure (commonly CL) requires an energetically expensive organic framework for highly-organized, high-density crystal growth. In contrast, EPC was commonly constructed of low-density, poorly-organized microstructures. Poorly-organized microstructure requires less metabolic energy for crystal deposition which suggests EPC might function to rapidly increase shell thickness and body size. We find that caenogastropods evolved EPC convergently at least three times and through parallelism, a subset of convergence, two times. Species within the two closely related families Ancillariidae and Pseudolividae likely used parallel developmental pathways to construct EPC as did two species within the genus Olivancillaria. Species with EPC in the distantly related Strombidae, Volutidae, and Olivancillaria constructed EPC convergently using different microstructures.

We also conducted a wave tank and a drop tank study on a species with EPC and a congener without EPC to test two functional hypotheses. In the wave tank we found that the test specimens with EPC remained stationary while the control specimens continued to move throughout the trial. This suggests that EPC might function as ballast to improve gastropod stability when moving through soft substrates or engaging in swimming behavior. We did not find significant support that EPC helps reorient the shell when or decrease free fall time when dropped through the water column.