Hydras feed by stinging smaller animals that happen to encounter their tentacles while passing by and then drawing them to their mouths. Some hydra also form symbiotic relationships with photosynthetic algae and gain some of their energy from them in the form of sugars in exchange for the nutrients in prey.
The stinging cells of hydras are known as nematocysts, which are tiny chambers containing sharp, poisoned threads. These shoot into anything the hydras' tentacles touch. Once body fluids begin leaking from the prey, the hydras' tentacles shorten, their mouths open and they engulf the prey. The prey is digested over several hours, and then any indigestible parts are regurgitated. Prey items are mostly invertebrates, ranging anywhere up to the size of the hydra itself, including annelid worms, insect larvae and, most commonly, crustaceans.
Hydras are small relatives of jellyfish, corals and sea anemones, all part of a primitive group of animals known as Cnidaria. Hydras are the only members of the group that can live in freshwater, although like their relatives, they mostly live in the oceans. They live attached to aquatic plants and other surfaces without moving for long periods of time, glued on with discs of mucus. When they need to move, however, they can do so in a somersaulting motion, going tentacles to base over and over until they reach a new site.