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Sea urchins use their entire body as an eye

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Purple sea urchins look like beautiful pincushions. They have no obvious eyes among their purple spines, but they can still respond to light. If you shine a spotlight on one, it will sidle off to somewhere darker. Clearly, the purple sea urchin can see, and over the past few years, scientists have worked out how: its entire body is an eye.

For decades, scientists knew that sea urchins can respond to light, even though they don’t have anything that looks remotely like an eye. The mystery deepened in 2006, when the full genome of the purple sea urchin was published. To everyone’s surprise, its 23,000 genes included several that are associated with eyes. The urchin has its own version of the master gene Pax6, which governs the development of animal eyes from humans to flies. It also has six genes for light-sensitive proteins called opsins.

While these genes are usually switched on in the developing eye, Maria Arnone found that the sea urchin’s versions are strongly activated in its feet. Sea urchins have hundreds of “tube feet”, small cylinders that sway around amid the spines. They can use the feet to move around, to manipulate food, and apparently to see.

Esther Ullrich-Luter – one of Arnone’s collaborators – found that each foot has two clusters of light-sensitive cells: one at the tip and another at its base. Each foot has up to 140 of these cells, giving a total of 200,000 across the entire animal. (For comparison, humans have a thousand times as many.)

The sea urchin has the rhabdomeric type, which is very strange because it’s a deuterostome – it’s more closely related to us than to any fly or spider. There are a few examples of rhabdomeric cells in vertebrates, but they’re not used for vision – they’re mostly used to control body clocks. The purple sea urchin is the exception. It suggests that rhabdomeric light-detectors have been the norm for eyes, throughout much of the animal kingdom’s history. Only in the vertebrates have these cells abandoned their old roles, which were taken up by the ciliary cells.

Via Sea urchins use their entire body as an eye

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Written by Nokgiir

May 5, 2011 at 12:46 am

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