Hydra are members of a family of radially symmetric animals (Cnidaria), all of which use specialized cnidocytes to catch prey.
Hydra tentacles contain barbed, poison containing cnidocytes that they use to stun animals like the water flea, Daphnia, before eating them alive, and to protect themselves from attack by other animals.
Researchers from the University of California lead by Dr David Plachetzki have discovered that the light sensitive protein opsin found in sensory cells is able to regulate the firing of harpoon-like cnidocytes.
These light sensitive neurons are found integrated into arsenals that include the stinging cnidocytes as well as desmoneme cnidocytes, used to grasp prey, and sticky isorhiza, which help the hydra to summersault at 10cm a day.
The linking of opsin to cnidocytes explains how hydra are able to respond to light even though they do not have eyes.
Not only did we find opsin in the sensory neurons that connect to cnidocytes in the hydra, but we also found other components of phototransduction in these cells," Dr Plachetzki said.
Known only by its species name, Chrysaora achlyos (kris-AH-oh-rah ACK-lee-us), the creature sported 9-meter (30-foot)-long tentacles riddled with stinging cells called cnidocytes