The first steps in the evolution of an eye may be evident in the hydra’s stinging tentacles, say scientists who have discovered that the tentacles are light sensitive and operate in fundamentally the same way as the eyes in animals. The findings, in the journal BMC Biology, indicate that even in the absence of complex eyes, some creatures display a light-sensitivity that uses the same visual pathway that allows humans to see.
The findings are based on the stinging mechanism in the tiny, brainless and eyeless freshwater polyp Hydra magnipapillata. The hydra is part of a group of animals called cnidarians that includes sea anemones, corals and jellyfish. A hydra, say the University of California (UC) researchers, is essentially a mouth surrounded by tentacles armed with stinging cells (cnidocytes).
The researchers – David Plachetzki from UC Davis, with Caitlin Fong and Todd Oakley from UC Santa Barbara – discovered the stinging nerve cells express a set of genes including opsin, a light-sensitive pigment; cyclic nucleotide gated ion channels; and arrestin. These components are basically the same as those in the light-detecting pathway in animals with eyes.
“Hydra stinging cells were already known to be touch sensitive and taste sensitive, but no one had ever thought before to look for light sensitivity – probably because they don’t have eyes,” Oakley said. “We’re the first to have found that. And we 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 researchers speculate that the reaction to light might be because prey are more active at dusk and after sunset. “Light could be acting as a daily, rhythmic cue that tells hydra when, and when not, to sting,” said Oakley.
“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,” he added. “For us, as evolutionists, the message is that photoreception can do other things besides just facilitate vision. It can do unexpected things… other functions for light sensitivity that we may not be thinking of.”
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