The most well-known animals from the Coelenterata family are the marine anemones, coral and jellyfish, and most of the family does make it's home in the oceans of the world. There are however, a number of these animals that live in freshwater. These are much smaller and less colorful than their more popular marine relatives. Most are less than an inch tall.
Hydra look very much like miniature sea anemones. They can be white brown or geen. Hydra have a saclike body and a circle of tentacles at around the body opening. This opening is used for both taking in food and eliminating wastes. Hydra don't have eyes but they can sense light.
Hydra occasionally can show up in the aquarium where they attach to almost anything, the plants, decorations, gravel, the aquarium itself. They will also reproduce easily, either asexually, by forming buds or by laying a single egg.
Hydra are carnivous and will eat anything they can manage to catch including single celled animals, small crustaceans, worms, insects, and other tiny animals. They are also reported to eat small fish fry in the aquarium. Hydra use their tentacles for catching prey. The tentacles have special cells called nematocysts. To kill their prey hydra shoot the nematocysts in to the prey which paralyzes and kills it. It is then digested by the hydra.
Hydra can do well in the aquarium if there's a lot of small live foods. This can be a devastating combination for some fish breeders; with lots of food for the fry feeding the hydra, and then the fry themselves becoming hydra food. I sometimes see hydra in my tanks but they have never become a real problem for me. Gouramis and some other fish are reported to eat hydra. There are also several medications available commercially that are reported to kill hydra. In most aquariums, just keeping the tank clean and not overfeeding, should keep hydra from multiplying to the point of them becoming so much of a problem that medications are needed.
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