Hydra (genus)

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 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 milimeters 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).

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 a ring of five 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. Some species of hydra exist in a mutual relationship with various types of green algae. The hydra offers the algae protection from predators and in return, the algae uses photosynthesis to give the hydra a food source.

Morphology
Hydras 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 simple compared to mammalian nervous systems. The hydra does not have a recognisable 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. This experiment has not been repeated successfully in the 20th or 21st centuries; the remains of the hydra do not reform.

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. Due to the simplicity of the nerve net, hydras generally react in the same way, regardless of the direction of the stimulus.

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 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 "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
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 a cold winter, sexual reproduction occurs in some hydras, producing unfertilized eggs. These eggs are then fertilized by sperm from testes which form on the external surface of the stalk. 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 considered to be hermaphrodites.

Feeding
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 gastrovascular 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 undigestible 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.

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

Senescence
It has often been assumed that hydrae 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).