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Visual Pathways the same, With or Without Eyes
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Visual Pathways the same, With or Without Eyes

Biologists studying the the genetic behaviour of hydra, a freshwater polyp, have made a surprising discovery. Whilst these creatures have no eyes, their tentacles are sensitive to light. This was a sensitivity nobody thought to look for before – precisely because they have no eyes.

When the researchers investigated this phenomenon further, they found not only that light-sensitivity genes are expressed near hydra stinging cells, but that under different light conditions, these cells have different propensities to be fired. The creatures were responding as though they had optic nerve pathways in place, even without eyes attached to them.

Todd Oakley, professor in UCSB's Department of Ecology, Evolution, and Marine Biology was the lead author of the work, and discovered this trait is common to jellyfish, sea anemones, and corals as well as hydras. All of them use tentacles to catch their prey, and all of them actually have optic nerve pathways in the tentacles.

The research found that the light-sensitive protein opsin in sensory cells regulates the firing of the hydra's harpoon-like cnidocytes. These same cells are found in the mechanisms hydra use to grasp prey, and to summersault through the water.

The linking of opsin to the stinging cells helps explain how hydra can respond to light despite the absence of eyes, the scientists said, because the sensory neurons also contain the ion channels and additional proteins required for phototransduction -- the process by which light is converted to electric signals. Phototransduction in humans occurs in the retina.

"I wouldn't call this vision, because as far as we know the hydra are not processing information beyond what's light and what's dark, and vision is much more complicated than that. But these genes that we're studying are the keystones of vision," Oakley said. "For us, as evolutionists, the message is that photoreception can do other things besides just facilitate vision. It can do unexpected things. What good is half an eye? Even without eyes there are other functions for light sensitivity that we may not be thinking of."

It is quite probable that the ability to detect light and dark, aids the tentacles in their tactile search for prey. The evidence is not conclusive yet, but as the image below shows, the tentacles do have precursors of retinal cells scattered in circles around them like suckers.

The Hydra, left, have the same visual pathway as humans. Their tentacles, right, contain active cells shown in red that aid in the movement, and defence of the hydra. Light sensing cells in the active cell rings, are capable of differentiating relative levels of light.

If you think about it, passive senses such as proprioception are not going to work with tentacles, as there are no joints to monitor the status of. So, how do you tell when the tentacle has wrapped around prey, as opposed to another tentacle?

That seems to be what is going on here; a crude form of vision that is capable of differentiating simple shapes only – like the shape of a prey animal swimming nearby, versus that of another tentacle. It may also explain why many of the deeper ocean versions of these creatures have a bio-luminescence, which would work to cast light on the tentacles, and thus aid such a vision.

Turning back to our own uses of such concepts then, it would seem that even supposedly 'blind' or sightless animals need to rely on some form of basic vision system at least in part, in order to utilise touch to the best effectiveness. So, when we are developing probing tendrils for robotic search, if we cover the tendrils in arrays of a photosensitive material, it may well improve their ability to 'grope about in the dark', as it were, by using simple machine vision and pattern recognition to tell if that shape they are feeling is another tendril, or something that feels the same, but isn't.

It may also help avoid tangles in such devices. After all, when working by touch alone, you don't know you've just tied two or more tendrils in a knot, until you have actually done so.

References

UC Santa Barbara Researchers Discover Genetic Link Between Visual Pathways of Hydras and Humans

Cnidocyte discharge is regulated by light and opsin-mediated phototransduction (PDF)

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