Meeting Abstract
Sea urchins lack eyes, yet several species have been shown to have spatial vision via behavioral assay. It has been suggested that their spatial acuity is determined by the density of spines across the body. The spines determine the angle of acceptance of light on an area of their skin, which is generally photosensitive. By combining signals from many areas, the animal may therefore function analogous to a large compound eye. In all published studies to date the acuities predicted by spine density and determined by behavioral assay are similar. In visual ecology, it has been suggested that an animal’s visual abilities, including acuity, should match its environmental needs. However, in sea urchins, which lack discrete visual structures (eyes/eye spots), this assumption may not hold. Sea urchins inhabit nearly all marine habitats worldwide, from the intertidal to the deep sea and from the equator to the poles. They are therefore an excellent group in which to examine traits that are suggested to vary by ecology and habitat. If acuity matches environmental needs, we may expect spatial acuity to be high in well-lit habitats with high spatial complexity and low in dim and low spatial-complexity habitats. We performed an extensive survey of species from all 24 extant families of sea urchins (Class Echinoidea). For each species, acuity based on spine density was estimated from photographs from The Echinoid Directory (Natural History Museum, London) and mapped against a published phylogeny and habitat data (including depth range and substrate type). Only species for whom reliable habitat data and high quality photos were available were included. We were therefore able to analyze whether phylogenetic or environmental factors are more likely to constrain the trait (spine density) that most likely determines visual acuity in sea urchins. Results will be discussed.
