Extraocular Vision in Echinoderms

Abstract

Scientists have observed light sensitivity in a wide range of echinodermsEchinoderms over several centuries, despite the vast majority of the phylum lacking eyes. Opsin-expressing cells are found scattered across the echinodermEchinoderms body surface and appear to mediate light responses in all five extant classes. Among the eyeless groups, some species nonetheless exhibit what appear to be visual abilities, such as orienting to distant stimuli or responding to the appearance of potential predators. This ability for ``extraocular visionExtraocular vision'' has been the subject of decades of research and – despite substantial progress – remains enigmatic. Although only explicitly demonstrated in two species so far, there is evidence to support extraocular visionExtraocular vision in a range of sea urchins and brittle stars, using photoreceptors spread across the body. Several mechanisms for light channeling, photoreceptor screening, and sensitivity adjustment have been proposed but, so far, the underlying workings of these strange visual systems are elusive. This chapter will synthesize existing work in these groups, including behavioral, morphological, and molecular studies, and evaluate some of the proposed mechanisms that could support extraocular visionExtraocular vision. We also review the challenges posed by such unconventional visual systems and suggest future areas of study.