Herbivory is a major factor that affects plants' turnover and biomass. Corallina pilulifera (Rhodophyta; calcareous algae) is a turf-forming red alga which is often dominant on rocky intertidal and subtidal shores. Corallina algae are thought to avoid herbivory by depositing calcium in their body tissue and making it harder and less nutritious.

However, in the study site in western Kyushu, C. pilulifera was found to be cropped and severely damaged by hermit crabs. Interestingly, hermit crabs did not ingest the calcareous alga, with no trace of algae found in their foreguts. This paradoxical situation of "pseudo-herbivory" was investigated through a combination of laboratory and filed experiments and field research. Density of hermit crab seems to be the key to understanding this situation. In this presentation, we focus on the result of foregut examination of hermit crabs at different densities in a field experiment.