Meeting Abstract
Vision and photoreception in echinoderms have perplexed researchers for more than a century. In recent years, new experiments and novel techniques have revealed not only large numbers of opsin genes and photoreceptors, but the presence of image formation in sea stars and urchins. Ophiocoma wendtii is a common Caribbean brittle star that undergoes dramatic photoresponsive colour changes and exhibits strong negative phototaxis. It has become an iconic species in vision research, speculatively possessing a unique whole-body compound eye. Hemispherical crystalline calcite structures on the aboral surface are thought to focus incoming light onto sensory nerve bundles beneath, which could be integrated to allow spatial resolution. However, no photoreceptor cells have been identified to date, and behavioural studies in O. wendtii have been limited, despite widespread references to its visual ability. We present the first definitive evidence of an extensive extraocular photoreceptor network, and the first empirical support for low resolution vision, in O. wendtii. Histology, immunolabelling and synchrotron tomography demonstrated that thousands of putative photoreceptors are likely responsible for O. wendtii’s photosensitivity. Target-seeking experiments showed that animals were indeed able to detect large shapes from a distance and move towards them, suggesting they could be capable of integrating activity from the many photoreceptors to contribute to an elementary form of spatial vision. Light-dark choice experiments under manipulated light spectra demonstrate that light evasion behaviour in O. wendtii is likely elicited by blue wavelengths. Furthermore, it appears that a similarly extensive whole-body photoreceptor system could be present in two more congeneric species.
