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Hydra viridissima (also known as Hydra viridis and Chlorohydra viridissima), or the green hydra, is a species of freshwater hydroid polyp in the genus Hydra found widely dispersed in the northern temperate zone. It is a common organism found in still waters from early spring to late autumn. The characteristic green colour comes from cells of the unicellular alga Chlorella within the cells of the gastrodermis. Because of this, H. viridissima are generally less predacious than aposymbiotic hydra species.
It is most commonly found attached to the stems of water plants and the undersides of leaves including duckweed. When disturbed it retracts to a small green blob which is easily overlooked. Gently sweeping through a clean weedy pond and allowing the collected water and leaves to stand in a jar will often reveal H. viridissima emerging after only a few minutes. H. viridissima in captivity will tend to congregate towards the light.
When in feeding mode, the extended tentacles are generally only as long as, or shorter than the body. It lives in water and resembles a fine delicate seaweed.
- "Hydra viridissima Pallas, 1766". World Register of Marine Species.
- Matthias Habetha et al. The Hydra viridissima / Chlorella symbiosis.
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