
Hydras resemble tiny, delicate, elongated sea anemones. They have a columnar or trunklike body, which is structurally a bag (with the mouth at the top). The mouth is rimmed with several long tentacles. Hydra bodies are only two cell layers thick. A sticky secretion at the “foot” enables hydras to cling to a surface.
Hydras can glide slowly on their foot, or they can bend over and turn slow-motion somersaults.
Colors vary with species; they can be gray, brown, tan, cream-colored, green, orange, whitish, pinkish, or “clear.”
Some species include the brown hydra (Hydra oligactis), the green hydra (Hydra viridissima), and the freshwater polyp (Hydra vulgaris).
Length: stretched out fully, most are about ½ to 2 inches; when contracted, only the size of a speck.

Statewide.
Habitat and Conservation
Hydras can be found in virtually every kind of aquatic habitat, though they are most common in fairly quiet waters such as sunlit pools, where they attach to submerged vegetation and other objects, stretch out their tentacles, and “hunt.”
They can occasionally grow to large numbers, and when this happens in fish hatcheries, they can be a serious threat to newly hatched fry.
Food
Hydras wave their tentacles slowly in the water. When food touches a tentacle, special stinging cells discharge to help subdue the prey. The tentacles draw the food into the hydra’s mouth. Any tiny animal is fair game, including worms, small crustaceans, newly hatched fish fry, young insects, and larval mollusks.
One species, the green hydra (Hydra viridissima), like the saltwater corals and anemones, holds symbiotic algae in its tissues and derives nutrients from the algae.
Life Cycle
Hydras usually reproduce asexually by budding — a new hydra starts as a “bud” forming on the side of a larger hydra’s body; it grows and eventually breaks away as a genetic clone of the original.
Some species reproduce sexually, releasing sperm into the water that can reach eggs on another hydra. The eggs form a tough coating and can survive drought and freezing. Sexual reproduction usually only happens in harsh environmental conditions or right before winter.
Human Connections
The name of genus Hydra comes from Greek mythology, where the Hydra, a many-headed creature, was defeated by the hero Heracles. In the myth, the beast was able to regenerate a severed body part. Members of genus Hydra actually do have the ability to regenerate injured or severed tissues.
Hydras, though very small, can live in aquariums if they have access to the small worms, water fleas, and miniature, planktonic prey they need. Many fish will eat hydras, if they can see them. In most cases, hydras show up in aquariums as a surprise, having been introduced on live plants or water from another tank. They are tiny, but they may become more numerous and noticeable if you have been feeding baby brine shrimp to small fish, and inadvertently feeding the hydras as well.
Ecosystem Connections
Most of us know that ecosystems are based on the tiny plants and animals that form the base of food webs. Hydras form an important link between the tiny animals they eat, and their own predators — fish, crayfish, and aquatic insects.
Hydras have several amazing adaptions for survival in their environment. Researchers have been studying hydras' genetics and capacity for self-renewal, and some have described these "simple" animals as never aging and essentially immortal. For example, a hydra that is cut in half can regenerate the parts that were cut away, meaning that the mouth and tentacles can regrow a new foot, and the section with just the foot can regrow a new mouth and tentacles.



