Brittlestars change their body colour to seeHeather StewartBrittlestars, marine animals that look a little like starfish, may see without eyes by changing the colour of their bodies. While we already knew that brittlestars have photoreceptors all along their bodies, we didn’t know exactly how they worked until now. The discovery could help explain how other…
Brittlestars, marine animals that look a little like starfish, may see without eyes by changing the colour of their bodies.
While we already knew that brittlestars have photoreceptors all along their bodies, we didn’t know exactly how they worked until now. The discovery could help explain how other related marine creatures, like sea urchins, are also able to see without eyes.
Lauren Sumner-Rooney at the University of Oxford and her colleagues looked at two closely related species of brittlestars: one that can orient itself towards light, Ophiocoma wendtii, and one that can’t, O. pumila. Both species are highly light averse and spend nearly all their time hidden beneath rocks.
Sumner-Rooney wanted to know whether these brittlestars could determine the contrast of a scene, rather than just differentiate between light and dark. This would involve an ability akin to vision.
By looking at the two closely related species, Sumner-Rooney and her colleagues were able to surmise that O. wendtii was able to orientate itself towards differing contrasts of light, but O. pumila could not. Sumner-Rooney says this skill comes in handy as “it makes it easier to find somewhere to hide” in complex visual environments like a coral reef.
The researchers put individuals from the two species in a 60-centimetre-diameter cylindrical tank. They coloured a narrow band of the tank’s wall black with a white border, and left the rest of the tank’s wall a uniform grey.
Because the black and white bands were so close to one another, the light that reflected off them clashed to create a light intensity identical to that from the grey parts of the wall. This means an animal that can simply sense light wouldn’t be able to identify the black band. O. wendtii did tend to recognise the black band and crawl towards it to seek shelter, but O. pumila did not.
Why one species has this ability and the other doesn’t may come down to the fact that O. wendtii changes colour, unlike O. pumila. O. wendtii is a deep, red-brown colour in the light and a pale beige in the dark.
Using a combination of microscopic observations and RNA sequencing, the researchers speculate that in light, the animal’s pigment-containing cells constrict the photoreceptors. This means that they can only receive light from one direction, giving the brittlestars more detailed information about the contrast of their surroundings.
Sea urchins that can “see” without eyes also have these pigment-containing cells.
Journal reference: Current Biology, DOI: 10.1016/j.cub.2019.11.042
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