Evolution of shell flattening and the loss of coiling in top shells (Gastropoda: Trochidae: Fossarininae) on wave-swept rock reefs
Flattening of coiled shells has occurred in numerous gastropod lineages, probably as an adaptation to life in narrow protected spaces, such as crevices or the undersides of rocks. While several genera in the top snail family (Trochidae) have flattened shells, two Fossarininae genera, Broderipia and Roya, are unique in having shells that are limpet-like and zygomorphic, lacking any trace of coiling. The sister genera of these two genera are Fossarina and Synaptocochlea, both of which have coiled shells and live in rock crevices or the vacant shells of sessile organisms. Although Broderipia has recently been identified as living symbiotically in the pits of sea urchins, the habitat and biology of Roya are poorly known. After an extensive search for rare Roya snails on rocky shores of the Japanese Archipelago, we found live Roya eximia snails on intertidal/subtidal rock surfaces exposed to strong waves. The Roya snails crawled swiftly over wave-swept rock surfaces at low tide, while they retreated into the vacant shells of barnacles at high tide, where they adhered firmly to the inner wall. A survey of the macrobenthic communities around the snail habitat showed that Roya snails inhabited only wave-swept rocks of exposed reefs, where the substrata was covered by encrusting red algae and barnacles. Despite the abnormal shell morphology, the radula was similar to other species in the subfamily, and the diet of Roya snails was mainly pennate diatoms. The limpet-like shell of Roya caused loss of coiling and contraction of the soft body, acquisition of a zygomorphic flat body, expansion of the foot sole and loss of the operculum. All of these changes improved tolerance of strong waves and the ability to cling to rock surfaces, and thus enabled a lifestyle split between wave-swept rock surfaces and refugia of vacant barnacle shells.