Inside or Outside the Pits : Variable Mobility in Conspecific Sea Urchin, Anthocidaris crassispina (A. Agassiz)

Abstract

The sea urchin Anthocidaris crassispina (A. Agassiz) lives either in small pits or outside the pits on rock surfaces. Difference in movement between individuals living inside and outside the pits was investigated. Sea urchins living inside the pits never moved out of their pits either in summer or winter, or during the day or night, while urchins living outside the pits moved in both seasons, especially during the night. Among urchins living outside the pits, individuals occupying relatively sheltered microhabitats (hollows or edges; see Fig. 1) exhibited less movement than individuals occupying open flats. On transplantation to an open area without pits, some individuals previously occupying the pits showed slight movement. On the other hand, most of the individuals previously outside the pits, when transplanted into an area with many vacant pits, occupied them and showed reduced movement, although neither transplanted groups of individuals adjusted their movement perfectly to their new microhabitats during the observation period. These results suggest that the movement of the sea urchins is, to some extent, flexible and dependent on microhabitats and that they preferred sheltered microhabitats. A morphological and physiological investigation showed that sea urchins inside the pits had similar gut weight (with the content), shorter lateral spines and heavier gonads than sea urchins outside the pits of similar test diameter. The adaptive significance of pit-dwelling in A. crassispina is discussed.