SCIENCE IN PICS: How the Eyeless Hydra Sees

March 4, 2012 Updated: September 29, 2015
Epoch Times Photo
Hydra magnipapillata tentacle opsin is shown in blue, cnidocytes and neurons shown in red. (David Plachetzki/University of California)


Epoch Times Photo
This is the fresh water polyp, Hydra magnipapillata. (David Plachetzki/University of California)

A special protein in the stinging cells of aquatic polyps called hydras allows these little creatures to detect light despite lacking eyes.

Hydras belong to the phylum Cnidaria, which also includes jellyfish and corals. Cnidarians are radially symmetric animals with a single mouth surrounded by tentacles carrying specialized cnidocytes for catching prey, as well as protection.

Researchers from the University of California studied a freshwater species, Hydra magnipapillata, which feeds during the day, contracting or moving by somersaulting in response to light.

The stinging cnidocytes on hydra tentacles are connected to light sensitive cells via a simple nervous system to coordinate feeding behavior. Inside these cnidocytes are the protein opsin and other light-sensitive structures that help to regulate the cells’ stinging activity.

“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,” said research leader David Plachetzki in a press release.

“These included cyclic nucleotide gated ion channels (CNG) required to transfer the signal and a hydra version of arrestin, which wipes the phototransduction slate clean for a second signal.”

Light triggers a sensory pathway which causes the stinging cells to discharge.

“We were also able to demonstrate that cnidocyte firing itself is effected by the light environment and that these effects are reversed when components of the phototransduction cascade are turned off,” Plachetzki said.

The findings were published in BioMed Central’s journal BMC Biology on March 4.