![]() We also obtained functional data on the physiology and spatial resolution of their eyes, and correlated this with the behavioural data. We tested the hypothesis that they use vision to find their way back to the reef if displaced. ![]() Here, we studied the eyes and behaviour of Linckia laevigata, a circum tropical species of starfish closely associated with coral reefs. Still, no behaviour has ever been directly associated with the eyes and image formation, and today it is still unknown why they possess such prominent eyes. In the 1960s, Yoshida & Ohtsuki made experiments studying their phototaxis and found some indications that the eyes are possibly involved in this behaviour. Relatively few behavioural experiments have been conducted with starfish and almost none dealing with their visual performance. From immunocytochemistry and physiology, it has been indicated that vision in these eyes is opsin based as in all other known animal eyes. The eye is presumably supporting true image vision, but this has not been verified. Depending on species, the fully grown starfish eyes are composed of 50–200 ommatidia, each holding several photoreceptors. The behavioural significance of the image formation in sea urchins has not yet been established.Īlthough certainly convergently evolved, the compound eyes of starfish resemble the compound eyes of arc clams, which also lack lenses in contrast to the more advanced compound eyes of arthropods. Interestingly, there is evidence of image formation without eyes in sea urchins possibly using directional shading of epidermal opsin by the spines. Another example is found within echinoderms, where only starfish and a single species of holothurian possess true eyes with putative image-forming capacity. One example of assumed independently evolved eyes is found in the cnidarian lineage peaking in complexity in the image-forming lens eyes in cubozoans. ![]() Though still debated, there are many indications of image-forming eyes having evolved several times within Metazoa. Our results provide an example of an eye supporting only low-resolution vision, which is believed to be an essential stage in eye evolution, preceding the high-resolution vision required for detecting prey, predators and conspecifics. We also show that the spectral sensitivity optimizes the contrast between the reef and the open ocean. ![]() This is, to our knowledge, the first report of a function of starfish compound eyes. Further, our behavioural experiments reveal that only specimens with intact eyes can navigate back to their reef habitat when displaced, demonstrating that this is a visually guided behaviour. The eyes are capable of true image formation although with low spatial resolution. Here, we show that the eyes of the coral-reef-associated starfish Linckia laevigata are slow and colour blind. There are indications that they are involved in negative phototaxis but this may also be governed by extraocular photoreceptors. Although these compound eyes have been known for about two centuries, no visually guided behaviour has ever been directly associated with their presence. Most known starfish species possess a compound eye at the tip of each arm, which, except for the lack of true optics, resembles an arthropod compound eye.
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