The macrobenthic community in intertidal sea urchin pits and an obligate inquilinism of a limpet-shaped trochid gastropod in the pits

Softer rocks in the intertidal zones of southern Japan are occasionally excavated by the rock-boring sea urchin, Echinostrephus molaris, and the pits are often succeeded by non-boring sea urchins, Anthocidaris crassispina and Echinometra tsumajiro after the death of Ec. molaris. Although the rock-boring sea urchin can fold their thin spines and retreat deeply into the pit bottoms, non-boring sea urchins with stouter spines cannot retreat deeply, thus, leaving spaces between their spines and the pit wall. To evaluate the uniqueness of these pits as microhabitats, we conducted an extensive census of biota both inside and outside of the pits occupied by rock-boring and non-boring sea urchins in tidal pools at Shirahama in southern Japan (33°69′51″N, 135°33′58″E). Macrophytes were only observed outside the pits, whereas sessile filter feeders and detritus feeders were found at similar frequencies in all of the microhabitats. The abundance and species richness of algal grazers and carnivores, however, were significantly higher in outside and inside the pits occupied by non-boring sea urchins compared to pits occupied by rock-boring sea urchins. The pit occupied by a non-boring sea urchin was specifically inhabited by a limpet-like trochid snail, Broderipia iridescens, the biology of which is almost completely unknown. Our data suggest that this trochid species is the first example of obligate inquiline with a non-boring pit-inhabiting sea urchin, adapted to the life in the pits, where the limpet benefit by sneaking in the gap between the pit wall and sea urchin spines, escaping from contact with the spines and being protected from attack by predaceous muricid snails.