Most starfish have eyes on the tips of their arms. They’re hard to see and even if you spot them, you might not recognise them as eyes. But they can see you (as long as you’re not moving too fast).
The starfish in the top image is an Indo-Pacific species called the blue star (Linckia laevigata). Here’s a close-up of one of its arms. That groove runs up the entire underside of the arm and contains thousands of tube feet, which the animal uses to crawl about. The eye sits at the end of the groove, where the white arrow is pointing.
It’s a tiny red nub, barely half a millimetre wide. It’s usually exposed, but the starfish can retract it into the arm if danger threatens.
Scientists have known about starfish eyes for ages, and they’ve assumed (reasonably) that the animals use these organs to see. But no one has properly tested this, or probed the limits of their vision. Anders Garm from the University of Copenhagen and Dan-Eric Nilsson from Lund University are the first.
They started with a detailed study of the blue star’s eyes. Each one sits on the end of a modified tube foot, and contains 150 to 200 separate light-collecting units called ommatidia. You can see them below, stained with a red dye. The set-up is similar to an insect’s compound eye, but with one big difference: insect ommatidia have lenses to focus light onto the underlying cells, while starfish lack lenses of any sort.
Each eye has a fairly large visual field that extends over 210 degrees horizontally and 170 degrees vertically—a slightly wider range than what your eyes can cover. And since the starfish has five of these eyes at the end of its flexible arms, it can probably see in every direction at once. Alternatively, it might be able to narrow its view to certain directions by moving the flexible black tube feet that surround each eye, and using them as blinders.
In the wild, blue stars live on coral reefs. When Garm and Nilsson placed them in front of a real reef, the starfish walked around randomly at first. But when they got within 2 metres of coral, they made a bee-line for it.
They needed their eyes for that. If Garm and Nilsson dissected out the eyes (don’t worry—they grow back after a few weeks), the blinded starfish couldn’t navigate home. They walked at the same speed, but they couldn’t find the coral. This is clear evidence that the blue star sees with its eyes, and uses them to navigate.
However, its vision is rather poor. It’s colour-blind, and sees the world only in shades of light and dark. Its light-detecting cells work very slowly, so fast-moving objects are invisible to it. And it has poor spatial resolution, so it can’t see fine detail.
But none of this matters. Garm and Nilsson suspect that the starfish doesn’t need to see colours, details, or speedy objects, because it mostly uses its eyes to detect coral reefs. Which are large environmental shapes that don’t move. The reefs would also appear as dark splotches against a bright ocean, in the starfish’s monochrome view of the world.
“When vision first evolved, it must have supported some tasks,” says Garm. “What behaviours could you control with a very crude image? Detecting large predators? They move fast and you’d need fast vision.” And to process images quickly, you’d need a decent amount of brainpower—something that the owners of the first eyes probably lacked.
An alternative, and one that Nilsson has championed, is that the first eyes evolved to detect large, stationary objects, so that animals could recognise their habitats and find their way home. “To do that, you only need poor eyesight,” says Garm. A starfish, guided back to a reef by its five arm-tip eyes, could be giving us an example of what the first visual systems were used for.
Reference: Garm & Nilsson. 2013. Visual navigation in starfish: first evidence for the use of vision and eyes in starfish. Proc Roy Soc B http://dx.doi.org/10.1098/rspb.2013.3011