A Comparative Study of Sea Urchin Visual Ecology

Sea urchins have a diffuse dermal photoreceptive system that allows them to essentially function as large compound eyes. Surprisingly, they are capable of behavioral tasks using low-resolution spatial vision; diffuse dermal photoreception is assumed to mediate non-visual tasks. It has been suggested that urchins inhabiting rocky reefs use vision to find shelter from predators. This project sought to determine the thresholds of urchin spatial vision in the context of environmental relevance. Underwater illuminance was measured in the field to confirm environmentally relevant levels of light. Laboratory behavioral trials were conducted to establish the lower limits of illuminance required for spatial tasks and image resolution of three Southern California urchin species: Centrostephanus coronatus, Strongylocentrotus purpuratus, and S. franciscanus. Results indicate that higher light intensities and larger targets lead to more accurate responses from urchins of all species. Species-specific differences were also significant