Hydra

Presentation

The hydra is a small freshwater animal belonging to the cnidarians, related to jellyfish and corals. Its tubular body measures from a few millimeters to about one centimeter in length and bears a crown of tentacles around the mouth. It lives attached to a surface by a basal disc, in the still waters of ponds, marshes, and rivers. This genus is especially known for its exceptional regenerative abilities and for the apparent absence of cellular aging observed in laboratory conditions. It captures small prey using its stinging tentacles.

The hydra's body is a simple tube made up of two cell layers, with no differentiated internal organs or centralized nervous system. It has between four and twelve tentacles lined with cnidocytes, stinging cells used to capture prey and for defense. It can move slowly by creeping or by successive somersaults. Its transparency and small size make it difficult to observe without magnification.

Characteristics

Indicative values: they vary by subspecies, sex and region.

Habitat and distribution

Hydra lives in calm, clean freshwater environments such as ponds, marshes, ditches, and shallow areas of rivers and lakes. It prefers habitats rich in aquatic vegetation where it can attach and find prey. It generally avoids heavily polluted or turbulent waters. It is often found clinging underneath leaves or on the stems of submerged plants.

Behaviour

The hydra is a sedentary animal that typically remains attached to a plant, a stone, or submerged debris. It responds to mechanical and chemical stimulation by retracting its tentacles. It can occasionally detach and move by slow creeping or by a kind of somersault using its tentacles. Its behavior is essentially reflex-based, without complex centralized coordination.

Reproduction

Hydra reproduces mainly through asexual reproduction by budding, where a small individual forms on the parent's body before detaching. It can also reproduce sexually by producing gametes, especially under unfavorable conditions such as cold or lack of food. Some species are hermaphroditic while others have separate sexes. Fertilized eggs can enter a dormant state before hatching.

Peculiarities

Hydra is famous in biology for its remarkable capacity for near-complete regeneration: a severed fragment can regenerate into a whole individual. Studies suggest that it does not show the classic signs of aging thanks to continuous cell renewal from stem cells. Its name refers to the Lernaean Hydra from Greek mythology, because of its ability to regenerate severed parts. It is an important model organism in research on regeneration and aging.

Threats and conservation

Hydra is not considered threatened and is not subject to any specific conservation evaluation because of its wide distribution and rapid reproductive capacity. Freshwater pollution and destruction of wetlands can locally affect populations. It has no specific protection status.

References

Profile updated 01 September 2026 · Reviewed by the TunisieVet editorial team.

CONSERVATION STATUS

Data deficient (DD)

IUCN source, latest published assessment.

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