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Starfish Spot The Way Home With Eyes On Their Arms

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.

Credit: Garm & Nilsson, 2013. Royal Society.
Credit: Garm & Nilsson, 2013. Royal Society.

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.

Credit: Garm & Nilsson, 2103. Royal Society.

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.

Credit: Garm & Nilsson, 2013. Royal Society.
Credit: Garm & Nilsson, 2013. Royal Society.

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

2 thoughts on “Starfish Spot The Way Home With Eyes On Their Arms

  1. Starfish and eyes — tickled my memory…
    23 August 2001
    Nature 412, 819 – 822 (2001) © Macmillan Publishers Ltd.
    Calcitic microlenses as part of the photoreceptor system in brittlestars

    Photosensitivity in most echinoderms has been attributed to ‘diffuse’ dermal receptors. Here we report that certain single calcite crystals used by brittlestars for skeletal construction are also a component of specialized photosensory organs, conceivably with the function of a compound eye. The analysis of arm ossicles in Ophiocoma showed that
    in light-sensitive species, the periphery of the labyrinthic calcitic skeleton extends into a regular array of spherical microstructures that have a characteristic double-lens design.
    These structures are absent in light-indifferent species. Photolithographic experiments in which a photoresist film was illuminated through the lens array showed selective exposure of the photoresist under the lens centres. These results provide experimental evidence that the microlenses are optical elements that guide and focus the light inside the tissue. The estimated focal distance (4-7 µm below the lenses) coincides with the location of nerve bundles–the presumed primary photoreceptors. The lens array is designed to minimize spherical aberration and birefringence and to detect light from a particular direction. The optical performance is further optimized by phototropic
    chromatophores that regulate the dose of illumination reaching the receptors. These structures represent an example of a multifunctional biomaterial that fulfills both mechanical and optical functions.

    Here’s the layman-friendly Nature Science Update report on those starfish… it also brings out the amazing, “sophisticated” engineering involved.

    Engineers envy brittlestar bones’ built-in lenses.
    23 August 2001

    A relative of the starfish and sea urchin has turned its
    skeleton into an all-seeing eye. Near-perfect microscopic
    lenses in brittlestars’ bones are more sophisticated than
    anything humans can produce, say engineers keen to copy
    the trick.

    Also somewhat related: http://www.sciencedaily.com/releases/1999/11/991105073420.htm “New To Science, A Novel Insect Eye Could Be A Very Old Way Of Seeing” — “An unusual type of eye — resembling a tiny raspberry and possibly following a design principle that vanished with the extinction of trilobites hundreds of millions of years ago — lives today in a parasitic insect, Cornell University biologists report in the Nov. 5 issue of the journal “Science”. …”

  2. I wonder if ocean acidification is causing some sort of blindness or other visual distortion that explains why starfish are literally tearing themselves apart. While primarily reported on the US west coast, there are reports on the east coast of sea star wasting disease.

    “The arms crawl in opposite directions, until they tear away from the body and their insides spill out. And unlike most starfish, the arms don’t regenerate.”


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