Meeting Abstract
Sea urchin behavioral reactions to light are more complex than previously thought. These animals have a diffuse photoreceptive system with at least two types of opsins expressed throughout their epidermis. Essentially, each urchin functions as a large compound eye. Their dermal light sense facilitates behavioral tasks that even include coarse spatial vision. This is novel, as diffuse dermal photoreception is generally assumed to mediate non-visual tasks. It has been suggested that urchins inhabiting rocky reefs use spatial vision to locate dark crevices to hide from diurnal predators. It is commonly thought that animals have photoreceptive and visual abilities that correlate to the complexity of their light-guided behaviors. The goal of this investigation was to determine the thresholds of urchin photoreception and spatial vision in the context of environmental relevance. Underwater irradiance was measured in the field to confirm environmentally relevant levels of light to test. Laboratory behavioral trials were conducted to establish the lower limits of intensity required for spatial tasks and image resolution of three Eastern Pacific urchin species: Centrostephanus coronatus, Strongylocentrotus purpuratus, and S. franciscanus. Results indicate that higher light intensities and larger targets lead to more accurate visually-guided responses from urchins of all species. In addition, each species exhibited consistent responses to targets above a certain size: C. coronatus > 22°, and S. purpuratus and S. franciscanus > 27°. Describing the function of the urchin as a compound eye allows us to understand how these animals perceive their environments and make choices based on visual cues.
