Run and hide: visual performance in a brittle star

Abstract

Spatial vision was recently reported in a brittle star, Ophiomastix wendtii, which lacks discrete eyes, but little is known about its visual ecology. Our aim was to better characterize the vision and visual ecology of this unusual visual system. We tested animal orientation relative to vertical bar stimuli at a range of angular widths and contrasts, to identify limits of angular and contrast detection. We also presented dynamic shadow stimuli, either looming towards or passing the animal overhead, to test for potential defensive responses. Finally, we presented animals lacking a single arm with a vertical bar stimulus known to elicit a response in intact animals. We found that O. wendtii orients to large (≥50 deg), high-contrast vertical bar stimuli, consistent with a shelter-seeking role and with photoreceptor acceptance angles estimated from morphology. We calculate poor optical sensitivity for individual photoreceptors, and predict dramatic oversampling for photoreceptor arrays. We also report responses to dark stimuli moving against a bright background – this is the first report of responses to moving stimuli in brittle stars and suggests additional defensive uses for vision in echinoderms. Finally, we found that animals missing a single arm orient less well to static stimuli, which requires further investigation.

‘Distributed’ vision and the architecture of animal visual systems

“…Both sea urchins and brittle stars have multiple populations of putative photoreceptors, expressing c-or r-opsins, located within skeletal pores, tube feet, spines and radial nerves (Johnsen, 1997;Ullrich-Lüter et al, 2011;2013;Delroisse et al, 2013;Sumner-Rooney and Ullrich-Lüter, 2023). At least the skeletal pore photoreceptors are thought to contribute to vision, and these exhibit substantial oversampling (see Glossary; Kirwan et al, 2018;Sumner-Rooney et al, 2021). Whether this is a universal feature of non-ocular vision is unclear.…”

The sea urchinParacentrotus lividusorients to visual stimuli

“…This is obvious for fast-moving predators and visual specialists with large eyes and centralised brains, such as raptors or octopuses, but is also true of animals with rudimentary vision and simpler visually-guided behaviours. Several species of sea urchin - marine invertebrate grazers lacking discrete eyes or a centralised brain - also possess spatial vision, although with dramatically worse acuity (Kirwan et al 2018; Yerramilli and Johnsen 2010, Sumner-Rooney & Ullrich-Lüter 2023). Their proposed mode of non-ocular vision entails resolving an image using distributed photoreceptors feeding into the animal’s decentralised nervous system.…”

“…Several sea urchin species have been reported to approach dark targets presented along the outer edge of a behavioural arena, which has been interpreted as visual orientation (reviewed by Sumner-Rooney & Ullrich-Lüter 2023). Based on early findings that the body wall of sea urchins is generally photosensitive (Yoshida & Millott, 1959; Millott & Yoshida, 1960) two different mechanisms have been proposed, as to how sea urchins can visually resolve objects.…”