Meeting Abstract

P3.34  Jan. 6  Ontogenetic changes in shell microstructure of freshwater gastropods from Lake Tanganyika (Cerithioidea, Paludomidae) ARMGARDT, Emily T*; STRONG, Ellen E; Seattle University; Smithsonian Institution armgardt@seattleu.edu

Lake Tanganyika, an ancient lake located in the Great Rift Valley of East Africa, is known for hosting many endemic species including fishes, crabs, and gastropods. Unlike their fluviatile relatives, the gastropods of Lake Tanganyika have highly decorated and thickened shells. Predator/prey co-evolution between gastropods and shell crushing crabs has been identified as possibly contributing to their unusual shell characteristics. In particular, it has been hypothesized that shells with 3 to 4 crossed-lamellar layers have evolved repeatedly to strengthen the shell in defense against shell-crushing predators. However, past studies included only a small subsample of the species, and did not examine possible ontogenetic variation in shell microstructure. In order to address these issues, the shells of 16 lacustrine species and 2 fluviatile �outgroups� were sectioned longitudinally, embedded in resin and examined with scanning electron microscopy. This survey confirmed that the number of crossed lamellar layers varies among lake species from one to four. Most importantly, significant variation in shell microstructure during ontogeny was found. Early in ontogeny, microstructure is typically rather simple, with one or two layers. In species with many layers, the layers do not persist throughout ontogeny. Instead, shell microstructure becomes simplified again in the adult, usually to only a single comarginal crossed lamellar layer. Assuming that predation pressure is constant throughout ontogeny, this finding is inconsistent with a predator-prey model of coevolution driving repeated origin of numerous crossed lamellar layers to strengthen shells in defense of shell crushing predators.