The sea urchin Paracentrotus lividus orients to visual stimuli

Though lacking eyes, some sea urchins can see: Several species exhibit resolving vision, as distinct from mere light detection. How and where light is captured in the eyeless sea urchins, and how this information is integrated to elicit visual behaviour, remains a fascinating enigma. We assessed the spatial resolution of the sea urchin Paracentrotus lividus in laboratory experiments using fifty adults from the Bay of Naples. This keystone species is an important grazer of the NE Atlantic and Mediterranean and a model system to study development. We carried out behavioural trials in which individuals were placed in a submerged cylindrical arena to determine if they orient towards a visual stimulus on the arena wall, under diffuse, downwelling light. We adopted a novel isoluminant stimulus, necessitating vision of a given resolving power around the horizon to be detected. We tested individuals at five stimulus widths, including a uniform control. Animals oriented (upon clearing an obstacle) only to the widest stimuli (45 deg and above). This acuity may suffice for tasks such as finding nearby shelters or distant patches of habitat. We modelled the visual and neuronal processes to recapitulate these responses in P. lividus, by fine-tuning the model of Li et al. (2022), as applied to the sea urchin Diadema africanum. While these species differ morphologically, the model robustly predicts angular sensitivity in keeping with the behavioural experiments. We find that P. lividus (and likely many Echinacea) possesses coarse spatial vision and that the neurosensory model applies broadly to sea urchins.