This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (May 2012)

Hydra ( /ˈhaɪdrə/) is a genus of simple fresh-water animal possessing radial symmetry. Hydras are predatory animals belonging to the phylum Cnidaria and the class Hydrozoa.^[2][3] They can be found in most unpolluted fresh-water ponds, lakes, and streams in the temperate and tropical regions and can be found by gently sweeping a collecting net through weedy areas. They are multicellular organisms which are usually a few millimetres long and are best studied with a microscope. Biologists are especially interested in hydras due to their regenerative ability; and that they appear not to age or die of old age. However, presently, there is no scientific consensus on whether Hydra undergo senescence, as discussed hereinafter.

Morphology

Schematic drawing of a discharging nematocyst

Hydra has a tubular body up to 10mm long when extended, 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 called a cnidocil. Upon contact with prey, the contents of the nematocyst are explosively discharged, firing a dart-like thread containing neurotoxins into whatever triggered the release which can paralyse the prey, especially if many hundreds of nematocysts are fired.

Hydra has 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. Hydramacin is a bactericide recently discovered in Hydra; it protects the outer layer against infection. This would make them "diploblastic" (contains two cell layers.)

The nervous system of 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 and excretion occurs by diffusion through the epidermis.

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.

Hydra showing sessile behaviour

Hydras are generally sedentary or sessile, but do occasionally move quite readily, especially when hunting. They do this by bending over and attaching themselves to the substrate with the mouth and tentacles and then release the foot, which provides the usual attachment, this process is called looping. 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.

Reproduction and life cycle

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 fertilize 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 nymph hydras. Hydras are hermaphrodites and may produce both testes and an ovary at the same time.

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

Feeding

Hydras mainly feed on small aquatic invertebrates such as Daphnia and Cyclops.

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 manoeuvred 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 behaviour of the hydra demonstrates the sophistication of what appears to be a simple nervous system.

Some species of Hydra exist in a mutual relationship with various types of unicellular algae. The algae are protected from predators by the Hydra and, in return, photosynthetic products from the algae are beneficial as a food source to the Hydra.

Morphallaxis

Nineteenth-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.^{[citation needed]}

The hydra undergoes morphallaxis (tissue regeneration) when injured or severed.

Senescence

It has often been assumed that hydras are one of the few animals that do not undergo senescence (aging), and so are biologically immortal. Daniel Martinez appeared to provide evidence for this assumption in a 1998 article in Experimental Gerontology.^[4] This publication has been widely cited as evidence that hydra do not senesce and that they are proof of the existence of non-senescing organisms generally. However, in 2010 Preston Estep published (also in Experimental Gerontology) a letter to the editor arguing that the Martinez data support rather than refute the hypothesis that hydra senesce.^[5]

Genomics

A draft of the genome of Hydra magnipapillata was reported in 2010.^[6]

See also

Wikimedia Commons has media related to: Hydra (genus)

• Turritopsis nutricula, another cnidarian that scientists believe to be immortal
• Lernaean Hydra, a Greek mythological aquatic creature

References