Hydra "see" the light - but they don't have eyes

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A new study has found that the freshwater hydra can react to light despite having no eyes.

The fresh water polyp, Hydra magnipapillata is diurnal and catches its prey during the day and is also known to move in response to light. The study by scientists form the University of California found that this was because the  stinging cells (cnidocytes) found in hydra tentacles are linked via a simple nervous system to primitive light responsive cells that co-ordinate the animals' feeding behaviour.

The hydra, which is related to the jellyfish and coral, has been around for over 600 million years. They consist of a mouth surrounded by tentacles which contain barbed, poison-containing cnidocytes that they use to stun animals like the water flea, Daphnia, before eating them alive. However, these cnidocytes are also among some of the most complex animal cell types known; both in terms of their structure and by the fact that they can use both chemical and mechanical cues to affect when they discharge.

Up until now, although many studies have demonstrated that both chemical and mechanical aspects can affect the responses of the stinging cells, no studies have discovered any form of receptor cell. The research team lead by Dr David Plachetzki however, discovered that the light sensitive protein opsin found in sensory cells is able to alter the firing of harpoon-like cnidocytes.

They also found that differing light levels had different effects on the intensity of the firing. Light sensitive neurons were also found integrated into arsenals that include the stinging cnidocytes as well as cnidocytes, used to grasp prey, and sticky isorhiza, which help the hydra to somersault at a giddy 10cm/4" a day.

The linking of opsin to cnidocytes explains how hydra are able to respond to light even though they do not have eyes. Dr Plachetzki described how other proteins necessary for phototransduction (turning light into nervous signals) are also present in the sensory cells:

"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. These included… channels required to transfer the signal and a which wipes the phototransduction slate clean for a second signal."

Cnidocyte discharge is regulated by light and opsin-mediated phototransduction David C Plachetzki, Caitlin R Fong and Todd H Oakley BMC Biology 2012, 10:17 doi:10.1186/1741-7007-10-17

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