Brown hydra (Hydra oligactis)
|Size||Length of tentacles: up to 25 cm (2)|
Body length: 20-30 mm (2)
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:
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- Asexually: of asexual reproduction: reproduction that does not involve the formation of sex cells ('gametes'). In many species, asexual reproduction can occur by fission (or in plants 'vegetative reproduction'); part of the organism breaks away and develops into a separate individual. Some animals, including vertebrates can develop from unfertilised eggs, this process, known as parthenogenesis gives rise to offspring that are genetically identical to the parent.
- Carnivorous: feeding on flesh.
- Fertilisation: the fusion of gametes (male and female reproductive cells) to produce an embryo, which grows into a new individual.
- Invertebrates: animals with no backbone, such as insects, crustaceans, worms, molluscs, spiders, cnidarians (jellyfish, corals, sea anemones), echinoderms, and others.
- Larva: stage in an animal's lifecycle after it hatches from the egg. Larvae are typically very different in appearance to adults; they are able to feed and move around but usually are unable to reproduce.
National Biodiversity Network Species Dictionary (Jan 2003):
- Olsen, L., Sunesen, J., & Pedersen, B. V. (1999) Small freshwater creatures. Oxford University Press, Oxford.
Animal Diversity Web: Brown Hydra (March 2003):
- Nichols, D., Cooke, J. & Whiteley, D. (1971) The Oxford Book of Invertebrates. Oxford University Press, Oxford.