Hydra Vision: Implications for Evolution of the Eye. The freshwater polyp, Hydra (a relative of jellyfish) catches prey with a potent sting from its waving tentacles. The tips of these tentacles were known to be sensitive to touch and to chemicals, and now, to light. But the Hydra has no eye. Or does it?
• Researchers found the basic components of our visual signaling mechanism in the tentacle cells (Image 2): the photosensitive protein opsin, the cyclic-nucleotide-gated (CNG) ion channel, and arrestin (a protein that shuts off opsin). They then showed that firing of the stinging cells (cnidocytes) was inhibited by bright light, and that a drug that blocked the CNG channel reversed this light inhibition (Image 3). Possible functions of light sensitivity could be to regulate feeding patterns or to sense shadows cast by approaching prey.
• This means that the biochemical pathway central to vision in our highly complex eye is found in this boneless, brainless animal. Studying primitive “vision” is like looking back in time to the early steps in evolution that predate the development of our eyes .