Hydra are tiny freshwater animals belonging to the phylum Cnidaria, the same group as jellyfish and sea anemones. Several species occur in the UK, commonly living in clean ponds, lakes, slow-moving streams, and ditches where they attach themselves to submerged plants, stones, or debris.
A hydra’s body is tubular and only a few millimetres long, with a single opening at the top surrounded by tentacles. These tentacles contain specialised stinging cells called nematocysts, which the hydra uses to capture small prey such as water fleas (Daphnia), tiny insect larvae, and other microscopic aquatic animals.
Hydra are well known for their remarkable regenerative ability—even small fragments of the body can grow into a complete individual. They usually reproduce asexually by budding, where a small outgrowth develops on the parent and eventually detaches, although they can also reproduce sexually under certain environmental conditions.
In UK freshwater ecosystems, hydra play a role as small predators within the microscopic food web, helping regulate populations of tiny invertebrates while also serving as food for larger aquatic organisms. Their presence often indicates relatively good water quality.
Hydra are a favourite food of the great pond snails and 3 spined Stickleback.