Hydra oligactis

Geographic Range

The brown hydra is commonly found in the Northern Hemisphere and parts of Australia.


Hydras are are well known as the faunas of ponds, spring brooks, unpolluted streams, rivers, and the littoral zone of lakes.

  • Aquatic Biomes
  • lakes and ponds
  • rivers and streams

Physical Description


The common asexual method of reproduction by hydras is budding. Buds originate at the junction of the stalk and gastric regions. The bud begins as a hemispherical outpouching that eventually elongates, becomes cylindrical, and develops tentacles. The bud then pinches off and a new individual becomes independent. Buds are produced every two to three days under favorable conditions. Following unfavorable conditions, such as injuries or periods of scarce resources, hydras occasionally reproduce through transverse and longitudinal fission.

  • Parental Investment
  • no parental involvement


Hydra oligactis is mostly sessile. Hydras attach to stones, twigs, vegetation, or debris. The brown hydra rarely is found at depths exceeding 1.5 m. Spontaneous movements are few. When the hydra remains undisturbed, its body is extended and the tentacles spread. For no apparent reason, contractions and expansions of the body occur at intervals and the tentacles are constantly in motion. Free hydras can move from place to place by basal gliding. The hydras usually move by looping and somersaulting, by attaching the tentacle ends and pulling themselves along. When there is an insufficient supply of oxygen, hydras move to regions of higher oxygen content. In general, the behavior is characterized by its mechanical nature, great independence of parts, lack of integration, and lack of exact responses.

Food Habits

Hydra oligactis, as in all Cnidaria, are strictly carnivorous and eat many different kinds of small metazoans, including annelids, copepods, cladocerans, and insects. Hydra capture their food by paralyzing and killing the food organism by means of nematocysts, which are discharged into the prey. The prey is brought to the mouth (proctostome) by the tentacles, a response that is induced glutathione. This is considered the key mechanism in digestion. The organism is then taken in through the mouth, which is star-shaped or circular. Hydras have been known to feed on the organic material of the substrate when the food supply is insufficient. This behavior, however, is not considered normal. Digestion is both extra and intracellular.

Economic Importance for Humans: Negative

Hydra oligactis is a hatchery nuisance because it may kill fish fry.

Other Comments

Hydras have well developed powers of regeneration and grafting. Very little is known about the length of life under normal conditions. In the lab, hydras have been known to live for three to twelve months or more. Males outnumer females, an unusual situtation in most fresh water invertebrates.


Jana D. Weinberger (author), University of Michigan-Ann Arbor.



Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

World Map


living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map


animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

native range

the area in which the animal is naturally found, the region in which it is endemic.

radial symmetry

a form of body symmetry in which the parts of an animal are arranged concentrically around a central oral/aboral axis and more than one imaginary plane through this axis results in halves that are mirror-images of each other. Examples are cnidarians (Phylum Cnidaria, jellyfish, anemones, and corals).


Hyman, Libbie Henrietta. 1940. The Invertebrates: Protozoa through Ctenophora. McGraw Hill Book Co., New York.

Pearse, Vicki and John and Mildred and Ralph Buchsbaum. 1987. Living Invertebrates. Blackwell Scientific Publications, Palo Alto.

Brusca, Richard C. and Gary J. Brusca. 1990. Invertebrates. Sinauer Associates, Inc., Sunderland.

Pennak, Robert W. 1989. Invertebrates of the United States: Protozoa to Mollusca. John Wiley and Sons, Inc., New York.