Hail Hydra

Home Grown. We took some pond water and brought it inside last fall (2020). We had tadpoles in October and wanted to save some from the cold weather.

The 1st step was preparing their home. A number of cool organisms came along in the water. The hydra was one. It took a few days to get to this size. And we had a forest.

Hydrozoa viridissima

Hyrdrozoa comes from the myth of the nine-headed monster Hydra. Virisissma means “greenest”.

H. viridissima is green because it has algae as a symbiote. Why symbiosis? Well, it benefits both organisms. The Hydra gets fed from the algae, and the algae get protection from predators. The hydra controls how much algae is present. After eating, low amount of algae; before eating (hungry), high amount of algae.

The Hydra is a predator. It is no shrinking violet, though it can contract. It hunts with its branches, or “heads”.


  • Hydra are a member of the phylum Cnidaria (guess who else belongs to this phylum?)
  • One of the key attributes for animals in this phylum is the presence of Cnidocytes (a cell that contains a nematocyst; who in blue blazes has a nematocyst?)
  • The nematocyst fires and impales prey with a toxic barb
  • The Hydra then reels in its dinner and feasts (sometimes it catches another prey whiles its reeling in its first!)
The orange cell is the cnidocyte. Click on the image to visit Gordon Ramel’s great site!
Nematocysts firing in a jellyfish. The same thing happens on the arms of the hydra.
Watch a hydra catch prey when it deploys its nematocysts!

Bottom line: Hydrozoa viridissima are freshwater jellyfish!

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