Brown hydra  (Hydra oligactis)

The brown hydra is common and widespread in Britain (2).

Brown hydras (Hydra oligactis) are freshwater animals that belong to the same group as jellyfish, corals and sea anemones (2). The sac-shaped 'body' is topped with a mouth surrounded by a crown of tentacles that feature stinging cells used to stun prey (2).

The brown hydra is a widespread species, found throughout the northern hemisphere and parts of Australia (3).

The brown hydra is found in freshwater ponds, brooks, rivers, and streams as well as at the edges of lakes, and typically attaches to aquatic plants, stones, twigs and debris (3).

The brown hydra, although largely a sessile species is capable of moving by gliding along on the 'basal disc', the part of the body that attaches to the substrate. They may also move by bending the body around, attaching to the substrate with the tentacles, and then 'somersaulting' or 'looping' forwards (3). This species is carnivorous; it feeds on small aquatic invertebrates that are paralysed by the stinging cells when the prey comes into contact with the tentacles. The prey item is then brought to the mouth by the tentacles and taken into the body of the hydra (3).

Reproduction in hydras typically takes place asexually by a process known as 'budding', in which a bud-like growth on the body of the 'parent' hydra eventually grows into a new individual that becomes separated from the parent. When conditions are harsh, or there is a shortage of food, hydras can reproduce sexually; a single individual may produce both male and female sex cells, which are released into the water where fertilisation occurs. The egg develops into a larva, which is covered in tiny hair-like structures known as cilia. The larva may either settle immediately and develop into a hydra, or become surrounded in a tough outer layer that allows it to survive harsh conditions (4).

The brown hydra is not threatened.

The brown hydra is not listed under any conservation legislation.

For more on the brown hydra:

• Animal Diversity Web brown hydra fact sheet:
  http://animaldiversity.ummz.umich.edu/accounts/hydra/h._oligactis.html