Hydra (genus)

Hydra (genus)

name = "Hydra"

image_width = 230px
image_caption = "Hydra" species
regnum = Animalia
subregnum = Eumetazoa
subphylum = Medusozoa
classis = Hydrozoa
subclassis = Leptolinae
ordo = Anthomedusae
subordo = Capitata
familia = Hydridae
genus = "Hydra"
genus_authority = Linnaeus, 1758
subdivision_ranks = Species
subdivision = "Hydra americana" "Hydra attenuata" (or "Hydra vulgaris") "Hydra canadensis" "Hydra carnea" "Hydra cauliculata" "Hydra circumcincta" "Hydra hymanae" "Hydra littoralis" "Hydra magnipapillata" "Hydra minima" "Hydra oligactis" "Hydra oregona" "Hydra pseudoligactis" "Hydra rutgerensis" "Hydra utahensis" "Hydra viridis" "Hydra viridissima"

"Hydra" is a genus of simple, fresh-water animals possessing radial symmetry. Hydras are predatory animals belonging to the phylum Cnidaria and the class Hydrozoa. They are Multicellular. They can be found in most unpolluted freshwater ponds, lakes and streams in the temperate and tropical regions by gently sweeping a collecting net through weedy areas. They are usually a few millimeters long and are best studied with a microscope. Biologists are especially interested in hydras due to their regenerative ability. Hydras appear to be unique among animals in that they do not undergo senescence (aging). [Martinez, D.E. (1998) "Mortality patterns suggest lack of senescence in hydra." "Experimental Gerontology" 1998 May;33(3):217-225. [http://www.ucihs.uci.edu/biochem/steele/PDFs/Hydra_senescence_paper.pdf Full text.] ]

"Hydra" is a small animal with a body length ranging from 1 mm to 20 mm when fully extended. It has a tubular body secured by a simple adhesive foot called the basal disc. Gland cells in the basal disc secrete a sticky fluid that allows for its adhesive properties. At the free end of the body is a mouth opening surrounded by one to twelve thin, mobile tentacles. Each tentacle, or cnida (plural: cnidae), is clothed with highly specialised stinging cells called cnidocytes. Cnidocytes contain specialized structures called nematocysts which look like miniature light bulbs with a coiled thread inside. At the narrow outer edge of the cnidocyte is a short trigger hair. Upon contact with prey, the contents of the nematocyst are explosively discharged, firing a dart-like thread containing neurotoxins into whatever triggered the release. To humans, this poses a nuisance at worst; however, to some prey, this strike can be paralyzing.

Hydras mainly feed on small aquatic invertebrates such as Daphnia and Cyclops. All species of "Hydra" exist in a mutual relationship with various types of unicellular algae. The "Hydra" affords the algae protection from predators and in return, photosynthetic products from the algae are beneficial as a food source to the "Hydra" .


"Hydra" have two main body layers separated by mesoglea, a gel-like substance. The outer layer is the epidermis and the inner layer is called the gastrodermis. The cells making up these two body layers are relatively simple.

The nervous system of the "Hydra" is a nerve net, which is structurally simple compared to mammalian nervous systems. "Hydra" does not have a recognizable brain or true muscles. Nerve nets connect sensory photoreceptors and touch-sensitive nerve cells located in the body wall and tentacles.

Respiration occurs by diffusion through the epidermis. Some excretion and transportation also occurs in this manner.

Many members of the Hydrozoa go through a body change from a polyp to an adult form called a medusa.However, all hydras remain as a polyp throughout their lives.

19th century biologists reported that the "Hydra" was such a simple animal that it was possible to force one through gauze to separate it into individual cells; if the cells were then left to themselves, they would regroup to form a hydra again.

Motion and locomotion

If a "Hydra" is alarmed or attacked, the tentacles can be retracted to small buds and the body column itself can be retracted to a small gelatinous sphere. Hydras generally react in the same way, regardless of the direction of the stimulus and this may be due to the simplicity of the nerve net,Hydras are generally or sessile, but do occasionally move quite readily, especially when hunting. They do this by bending over and attaching themselves to the with mouth and tentacles and then release the foot, which provides the usual attachment. The body then bends over and makes a new place of attachment with the foot. By this process of "inch-worming" or "somersaulting," a hydra can move several inches (c. 100 mm) in a day. Hydras may also move by amoeboid motion of their bases, or by simply detaching from the substrate and floating away in the current.


When food is plentiful, many hydras reproduce asexually by producing buds in the body wall which grow to be miniature adults and simply break away when they are mature. When conditions are harsh, often before winter or in poor feeding conditions, sexual reproduction occurs in some hydras. Swellings in the body wall develop into either a simple ovary or testes. The testes release free swimming gametes into the water and these can fertilise the egg in the ovary of another individual. The fertilized eggs secrete a tough outer coating and, as the adult dies, these resting eggs fall to the bottom of the lake or pond to await better conditions, whereupon they hatch into miniature adults. Hydras are hermaphrodites and may produce both testes and an ovary at the same time.


When feeding, hydras extend their body to maximum length and then slowly extend their tentacles. Despite their simple construction, the tentacles of hydras are extraordinarily extensible and can be four to five times the length of the body. Once fully extended, the tentacles are slowly maneuvered around waiting for contact with a suitable prey animal. Upon contact, nematocysts on the tentacle fire into the prey and the tentacle itself coils around the prey. Within 30 seconds most of the remaining tentacles will have already joined in the attack to subdue the struggling prey. Within two minutes, the tentacles will have surrounded the prey and moved it into the opened mouth aperture. Within ten minutes, the prey will have been enclosed within the body cavity and digestion will have started. The hydra is able to stretch its body wall considerably in order to digest prey more than twice its size. After two or three days, the indigestible remains of the prey will be discharged by contractions through the mouth aperture.

The feeding behavior of the hydra demonstrates the sophistication of what appears to be a simple nervous system. A Hydra is part of the Polyp group.


The hydra undergoes morphallaxis (tissue regeneration) when injured or severed. See the morphallaxis article for more details.


It has often been assumed that hydras are unique among animals in that they do not undergo senescence (aging), and so are biologically immortal. Evidence for this was provided by Martinez (1998). [Martinez, D.E. (1998) "Mortality patterns suggest lack of senescence in hydra." "Experimental Gerontology" 1998 May;33(3):217-225. [http://www.ucihs.uci.edu/biochem/steele/PDFs/Hydra_senescence_paper.pdf Full text.] ]


*Gilberson, Lance, "Zoology Lab Manual", 4th edition. Primis Custom Publishing. 1999
*Solomon, E., Berg, l., Martin, D., "Biology" 6th edition. Brooks/Cole Publishing. 2002

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