6 July 2011

Sea urchins are just one big eye

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Sea urchins may use their entire body as a compound eye, throwing shadows with their skeletons to gain directional vision, according to new research.
juvenile urchin

A juvenile urchin with the photoreceptors labelled in red and the nervous system in green. Credit: Sam Dupont

sea urchins

Credit: Sam Dupont

NEWCASTLE: Sea urchins may use their entire body as a compound eye, throwing shadows with their skeletons to gain directional vision, according to new research.

Many echinoderms, a group that includes starfish, sea urchins and sea cucumbers, can react to light despite their lack of eyes. Previous studies have shown that sea urchins have a large number of genes linked to the development of the retina, which is the light-sensitive tissue in the human eye.

New research has now revealed that light-sensing photoreceptors seem to be located on the tip and base of the tube feet that cover the sea urchin’s entire body.

“It has been known for centuries that echinoderms react to light (both examples of escape or attraction),” said co-author Sam Dupont from the Department of Marine Ecology, of the study published in Proceedings of the National Academy of Sciences (PNAS).

“However, there was a lot of debate on how they were doing this. One general idea was the presence of photosensitive cells dispersed over the body. What we show is that it is much more organised and complex than that.”

Seeing the light

Previously, scientists had thought that photoreceptors were present throughout the sea urchins’ bodies, allowing them to react to light, but not necessarily see images.

Genetic studies have shown that sea urchins have several genes for the production of the protein opsin, necessary for light-sensing, or photoreception.

The new study sought to find the opsin and hence the urchins’ light-sensing structures. The presence of opsin on the base and top of the sea urchins’ tube feet confirmed that photoreceptors are dispersed all over the body, and are also integrated with the nervous system.

Filling the evolutionary gap

The study built on the existing genetic research, employing specific antibodies to determine where the types of opsin were appearing. The researchers then combined this structural and molecular information with behavioural clues to understand the urchins’ vision system.

The opsin staining revealed a network of photoreceptors, on the tip and base of tube feet, with the axons of the photoreceptors connected directly to the nervous system.

No screening pigment was found, and the authors suggested that the urchins’ skeletons might be used to screen light, giving them directional vision. Supporting this theory is the fact that urchins don’t start moving as a response to light until their skeletons have fully developed.

Until now, the type of opsin found (rhabdomeric) had not been seen to provide a vision function in deuterostomes, a category that includes both sea urchins and humans. It is, however, used for vision in protosomes, which include insects and flat worms. This implies that it was used for vision by the deuterostomes’ last common ancestor, putting the evolutionary shift to human eyes much later than previously thought.

One large compound eye

It appears that echinoderm vision may be quite complex. In the urchin’s case, the authors suggest that the photoreceptors have evolved into a sophisticated organisation, allowing the information to be integrated by the nervous system into images rather than just the presence of light.

In essence the entire body of a sea urchin could be functioning as one large compound eye.

“Anything that pushes our understanding of the origins of vision is good,” said John Buckeridge, a natural resources engineering professor at RMIT in Melbourne. “The ability of an urchin to determine ‘direction’ based on the way light falls on its photoreceptors is highly probable.”

Investigating ancestor’s eye

Daniel Janies, a computational biologist from the department of biomedical informatics at Ohio State University, commented, “These authors have stitched together many threads with anatomy, immunomicroscopy, and behaviour to provide integrated support for the hypothesis the entire body is a photoreceptor.”

He did however caution that it would be “hard to call it an image forming ‘eye’ without some idea of how the signals are processed”.

Janies also suggested that future work could extend their findings to determine whether the common ancestor of deuterostomes had a cerebral eye or used the entire body as a photoreceptor.

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