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How they found it out

Hydra (a freshwater polyp) is a model member of the Cnidaria phylum. It is positioned at the base of the tree of life of multicellular organisms. That was an outstanding naturalist of the 17th century - Antonie van Leeuwenhoek - who discovered hydra using his wonderful magnifying lens. Although that time this unique animal didn't raise the interest of scientists.

No one knows how long hydra would have been unknown if a 30-years-old tutor from Switzerland whose name was Abraham Trembley, hadn't found that marvelous animal. In order to study it more thoroughly, the inquisitive tutor cut it in two parts. At one of the pieces, which Trembley called "a head", grew the new body and at another piece grew the new "head". By the 14th day two parts formed two new living organisms.

This discovery advanced the profound and serious research by Trembley. The investigator set out his results in a book "Memoirs Concerning the Natural History of a Type of Freshwater Polyp with Arms Shaped Like Horns (Memoires, pour servir a l'histoire d'un genre de polypes d'eau douce, a bras en forme de cornes, 1744).

However the plain observations over the behavior and reproduction (budding) of the animal couldn't satisfy the naturalist, of course. In order to test his assumptions he began to experiment on hydra.

One of the renowned Trembley's experiments is when he turned the hydra inside out by means of a pig bristle hair. After this procedure the animal continued to live like nothing had happened. But that wasn't not due to the ability of the inner side to fulfill the functions of the outer side (and vice versa), but because the cells of the layer, that had moved inside during the reversing, penetrated through the new outer layer outside and occupied their former place.

In his other experiments Trembley cut the hydra into smaller and smaller pieces, but every time it regenerated and there was no limit to this. Today scientists know that hydra is able to totally regenerate out of 1/200 part of its body. That time this fact was real astonishing even for the venerable stars of science and impelled them to take the biology problem of regeneration in earnest.

About 250 years have past since Trembley experimented on hydra. Hundreds of articles and books have been written about this animal. Nonetheless it is still of great interest for the researches.

What's today known?

Everyone knows that animals have no adaptations to radioactive radiation. When subjected to it they may get a fatal dose and die. Experiments with green hydra (Chlorohydra viridissima) revealed that it can in some way feel the lethal hazard and rushes away from the radiation source.

Too large dose of X-rays causes the hydra's death. Reduced doses leave it alive but suppress its reproduction. Suddenly it was found out that small doses act in an unexpected way: they intensify the budding process in green hydra and increase the regeneration ability.

Surprising were the results of experiments when the walls of the tank were colored with all colors of spectrum. It was shown that hydra, having no any organs of vision, are able to distinguish colors and even show preference for some of them. Thus green hydra like blue-violet hue, brown hydra (Hydra oligactis, or Pelmatohydra oligactis - called so by some taxonomists) prefers cyan.

What is hydra inside?

By its appearance it resembles a glove, set vertically, with the fingers turned up, though, in fact, it has 5 to 12 such "fingers". With most of species there is a slight narrowing under the tentacles that separates the "head" from the rest of the trunk. At the head end there is a mouth that is opened into the gastric cavity (gastric paunch).

The walls of hydra's body consist of two layers, as with all cnidarians. The outer layer is formed of several types of cells: 1) containing muscular fibers, which allow hydra to move; 2) nerve cells that provide the animal's feeling of a touch, a change of temperature, a presence of mixture in water and other stimuli; 3) interstitial cells - the most active cells in the process of regeneration and repair of damaged or lost parts of body; 4) stinging cells - are used to catch food and to defend from predators and that are situated mainly on the tentacles.

Cnidarians is the singular group of animals that possess such a weapon as stinging cells. Any living cell contains cytoplasm, most of them contain nucleus. But only the stinging cells include a bubble-like capsule (cyst) with a coiled stinging thread inside.

Hydra anchors itself to some substrate with a foot (basal disk) and spreads its tentacles that move constantly. When the prey is found nearby, several stinging cells shoot towards the potential food. The stinging threads are hollow inside and as they pierce the prey's body with their sharp ends, the poison from the stinging cyst gets along the canal into the prey and kills it. A stinging cyst can be used only once. The discharged cyst is sloughed off and is then changed into a new one that develops from the special germ cells.

The cells that form the inner layer are responsible for a digestion of food. They release digestive juice into the gastric cavity. The juice acts on food so that the prey tissues soften and break down into small particles. The tip of the inner layer cell directed into the gastric cavity is equipped with a several long filaments. These move incessantly and rake the food particles towards the cells. The inner layer cells are able to spread amoeba-like pseudopods (false feet) and engulf the food particles. The further digestion takes place inside cells in food vacuoles like with protists.

Hydra is a true predator and feeds on animals solely, though (and this is not the norm) in some cases it can eat dead organic matter. Anyway the detailed research revealed that hydra can assimilate fats, proteins and carbohydrates that are of the animal origin.

Two methods of reproduction

Hydra reproduce in two ways - asexually and sexually. The first way is called budding. Young hydra arise on the parent organism in the form of buds. As they grow bigger they separate from the parent body and begin their own independent life. If the food is in plenty the process of budding goes very intensively. One hydra can give 30 generations 25 individuals in each.

In the bad conditions (autumn cold, drought, swamping, large amounts of carbon dioxide) hydra starts to reproduce sexually. Most hydra species are unisexual. But there are some with which the parent body produces both testes and ovaries (male and female reproductive organs).

Ovaries form in the outer layer of cells. They are spherical and contain one egg each that resembles an amoeba. The egg engulfs the interstitial cells that surround it, grows fast and reaches 1.5 mm across the diameter. Then it becomes rounded and divides in two unequal parts. As a result, the number of chromosomes in the cell's nucleus twice reduces. Now the mature egg goes out of the ovary into the water through the gap in the ovary's wall, though it remains attached to the parent organism by a thin pedicle. At the same period with other hydra, which carry testes, the sperm cells develop that look like unicellular organisms - flagellates. After they leave the testis they move by means of long flagella and at last one of the sperm cells finds the egg, penetrates inside and fertilizes it. Right after that the fertilized egg starts division and the development of a new organism begins.

Hydra's embryo is packed in two covers. The outer one is quite thick and contains chitin molecules. This is a good protection for a new hydra to bear severe environment. When the conditions improve (warming in spring, raining season, etc.) a young hydra breaks the protective cover and begins normal life.

A hunter in the tank

Hydra is a mysterious animal and is an attractive object for biologists. But the aquarium hobbyists can say something less nice about it. Hydra can do much harm when in the fish tank.

If you want to observe hydra, settle it into the separate tank, otherwise it will eat tiny animals, which serve as food for the fish, kill larvae and the fry. In the spawning or rearing tank hydra will quickly reproduce by budding and will do the young fish in.

In natural habitat hydra have few enemies. One of the fish parasites - a ciliate infusoria Trichodina pediculus can attack it. Hydramoeba hydroxena and some species of water fleas (Anchistropus sp.) can live on its body. A free living flatworm Planaria feeds on hydra. Nonetheless, do not use these animals to fight hydra in the tank: e.g. trichodinas and planarias are the same foes for fish as for hydra. There is one more hydra's enemy - a great pond snail. But it is no good too, since transmits some fish infections and will not fail to feed up on delicate aquarium plants.

Some aquarists put hungry young gouramy in the tank with hydra. Others fight it using the knowledge of its behavior: they know that hydra prefer well-alight spots. They shade all but one sides of the tank and place a glass from inside of this wall. Within 2-3 days nearly all hydra will gather there. The glass is taken out and cleaned.

These little animals are very susceptible for the presence of copper ions in water. Therefore another way to fight hydra is to take a copper wire, remove insulating cover and fasten a bundle above the air pump. When all hydras are dead, remove the wire out.

Chemical ways of fighting can also be successful, e.g.:
- ammonium sulphate - 5g per 100 l (must be used once);
- ammonium nitrate - 6 g per 100 l (three times in the intervals of 3 days);
- 3% hydrogen peroxide solution (for the tank with no plants and a good aeration) - dilute the needed amount of substance (2 teaspoons per each 10 l) in a glass of water and then slowly pour it in the tank above the nozzle of the working air pump.

Fighting hydra is most effective when you apply two or three different methods at once.

By S. Sharaburin
Translated by Tatiana Karpova (Moscow)
(MSU, Biology faculture, Dep. zoology and ecology).