Presence or absence of the shell aperture: a criterion to identify shell breakage induced by durophagy in Umbonium (Mollusca: Gastropoda: Trochidae)

Abstract

The pattern of shell breakage in the Recent trochid gastropods Umbonium (Suchium) moniliferum (Lamarck) and U. (S.) giganteum (Lesson) was observed under durophagous predation by crabs and in tumbling experiments, and the results of these experiments were applied to recent population samples from the natural environment. The experiments were carried out in order to learn if the pattern of shell breakage by durophagy (shell crushing) is different from that induced by water currents and waves. After predation by the predatory crabs Calappa lophos (Herbst) and Portunus trituberculatus (Miers), shell apertures of U. (S.) moniliferum and U. (S.) giganteum were broken in a characteristic fashion. In contrast, shells tumbled with sand and artificial seawater rarely lost the apertural margins, although other parts of the shells were relatively severely abraded. These results suggest that a shell from the natural environment with a similarly broken apertural margin was probably attacked by a durophagous crab, and that water currents and waves are not agents that cause breakage of shell apertures. The results were then applied to samples of Recent U. (S.) moniliferum from Lake Hamana-ko and fossil U. (S.) costatum (Kiener) from the Pleistocene Jizodo Formation in Japan. Shells with broken apertural margins amounted to 60% of all dead empty shells in the sample of Recent U. (S.) moniliferum, and over 90% in the sample of Pleistocene U. (S.) costatum, suggesting that these shells were extensively preyed upon by durophagous crabs. Shell repair frequencies of Recent U. (S.) moniliferum, calculated as a percentage of individuals with one or more repair scars, were 24–55%. The high repair frequencies indicate that considerable numbers of U. (S.) moniliferum can escape from attacks by crabs. All the results suggest that shell breakage along the apertural margins of gastropods is a good indicator of durophagy and that broken apertures, as well as shell repairs, are a useful tool for the investigation of predatory-prey interaction.