There are three main forms of hydras. Brown hydras have the slimmest bodies with tentacles that are three or four times the length of the tube-like body.
American hydras are white or gray with tentacles shorter than their bodies.
Green hydras are similar to American hydras but hold green algae in their tissues. They look grass green in colour. They are partly fed by the algae in their body and partly by dead animal and plant parts that they capture from the water around them. The algae in the bodies of the hyras are given a home by the hydra. The algae are also fertilized by the hydras' left-over food.