Relationship of water motion to the shallow water distribution and morphology of two species of sea urchins

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Abstract
A 13-mo. field and laboratory study was conducted at Santa Catalina Island, California, USA to determine some of the effects of water motion on the shallow water distribution of the sea urchin Centrostephanus coronatus Verrill. The depth distribution of another sea urchin species, Strongylocentrotus franciscanus (A. Agassiz), includes shallow areas of strong water motion (< 2 m depth) from which C. coronatus is absent. Differences in attachment strength and morphology of the 2 spp. were investigated as possible mechanisms affecting the differences in depth distribution. Density of C. coronatus along subtidal transects generally increased with increased depth, ranging from 0 individuals at depths less than 2 m to 1.7/m2 at 16 m depth; the majority of the population occurred deeper than 4.2 m. The shallowest depths of occurrence of C. coronatus are characterized by high levels or sharp increases in water motion. Laboratory wave channel experiments indicate that C. coronatus are dislodged at lower wave amplitudes when exposed to sharp increases in amplitude, than when exposed to more gradual increases. Areas of strong water motion such as noted along the field transects probably directly or indirectly limit the distribution of C. coronatus. The effects of water motion on C. coronatus seem to be relatively direct. C. coronatus are dislodged at lower wave amplitudes than are S. franciscanus. The ability of C. coronatus to withstand dislodgement during exposure to strong water motion was estimated indirectly by comparing the number of podia used in attachment by C. coronatus and S. franciscanus. Although other factors such as differential predation may contribute to the observed field distributions, C. coronatus is probably not well adapted to conditions of strong water motion, and limitations in attachment strength, relative to S. franciscanus, probably limit the shallow water distribution of this species.