David Plachetzki of the University of California, Davis and Caitlin Fong and Todd Oakley from the University of California, Santa Barbara have found that Hydra magnipapillata, a creature similar to a jellyfish, has light sensitive neurons. This might not be so interesting if it weren't for the fact that hydras have no eyes. They use light to regulate their stinging cells rather than for vision.
Hydra are tiny freshwater polyps in the phylum Cnidaria. Like other members of this group, they have stinging cells called cnidocytes. These cells contain harpoon-like structures that can fire in less than a microsecond when activated (see left). As the barb hits the Cnidarian’s prey, a toxin is injected. This is what makes contact with a jellyfish’s tentacles so painful. Although this system is highly effective, it does have one drawback: each harpoon can only be used once. Thus, it’s in the animal’s best interest not to waste a discharge.
Plachetzki and his colleagues have found that H. magnipapillata’s cnidocytes are connected to light-sensitive neurons that use the same components as the light-sensitive neurons in our eyes. And in fact, experiments show that the hydra’s stinging cells fire much less under bright than under dim lights.
How could information about ambient light conditions help an eyeless predator? The authors have a few ideas. For one thing, the amount of light in the environment might naturally coincide with the abundance of prey. Perhaps prey is scarce during the day so it’s not worth the creature’s while to bother wasting its stingers. Alternatively, the light-sensitive cells might be able to detect the shadow of a prey item, helping to align the stinging cells for maximal effect.
Below, you can watch a tough little hydra (though a different species) taking on tiny planktonic crustaceans called Daphnia.