Stinging cells in hydra. The structure of freshwater hydra

06.11.2019 -

In ancient Greek myth, Hydra was a multi-headed monster that grew two heads instead of being severed. As it turns out, the real animal, named after this mythical beast, has biological immortality.

Freshwater hydras have remarkable regenerative abilities. Instead of repairing damaged cells, they are constantly replaced by stem cell division and partial differentiation.

Within five days, the hydra is almost completely renewed, which completely eliminates the aging process. The ability to replace even nerve cells is still considered unique in the animal world.

More one feature freshwater hydra is that a new individual can grow from separate parts. That is, if a hydra is divided into parts, then 1/200 of the mass of an adult hydra is enough for a new individual to grow from it.

What is hydra

Freshwater hydra (Hydra) is a genus of small freshwater animals of the phylum Cnidaria and class Hydrozoa. This is essentially a solitary, sedentary freshwater polyp, which lives in temperate and tropical regions.

There are at least 5 species of the genus in Europe, including:

Hydra vulgaris (common freshwater species).
Hydra viridissima (also called Chlorohydra viridissima or green hydra, the green coloring comes from chlorella algae).

Hydra structure

Hydra has a tubular, radially symmetrical body up to 10 mm long, elongated, sticky leg at one end, called the basal disc. Omental cells in the basal disc secrete a sticky fluid, which explains its adhesive properties.

At the other end is a mouth opening surrounded by one to twelve thin mobile tentacles. Every tentacle dressed in highly specialized stinging cells. Upon contact with prey, these cells release neurotoxins that paralyze the prey.

The body of a freshwater hydra consists of three layers:

“outer shell” (ectodermal epidermis);
« inner shell"(endodermal gastroderma);
gelatinous support matrix called mesogloe, which is separated from nerve cells.

The ectoderm and endoderm contain nerve cells. In the ectoderm, there are sensory or receptor cells that receive stimuli from environment, such as water movement or chemical irritants.

There are also ectodermal nettle capsules that are expelled, releasing paralyzing poison and, Thus, serve to capture prey. These capsules do not regenerate, so they can only be discarded once. Each tentacle contains from 2500 to 3500 nettle capsules.

Epithelial muscle cells form longitudinal muscle layers along the polypoid. By stimulating these cells, polyp may shrink quickly. The endoderm also contains muscle cells, they are called so because of their function, the absorption of nutrients. Unlike ectoderm muscle cells, they are arranged in a ring. This causes the polyp to stretch as the endodermal muscle cells contract.

The endodermal gastrodermis surrounds the so-called gastrodermis intestinal cavity. Since this cavity contains How digestive tract, so vascular system, it is called the gastrovascular system. For this purpose, in addition to muscle cells in the endoderm, there are specialized gland cells that secrete digestive secretions.

In addition, the ectoderm also contains replacement cells, as well as endoderm, which can be transformed into other cells or produce, for example, sperm and eggs (most polyps are hermaphrodites).

Nervous system

Hydra has nerve network, like all hollow animals (coelenterates), but it does not have coordination centers such as ganglia or a brain. Nevertheless there is an accumulation sensory and nerve cells and their extension on the mouths and stem. These animals respond to chemical, mechanical and electrical stimuli, as well as light and temperature.

The nervous system of hydra is structurally simple compared to the more developed nervous systems of animals. Nerve networks connect sensory photoreceptors and touch-sensitive nerve cells located on the body wall and tentacles.

Respiration and excretion occur by diffusion throughout the epidermis.

Feeding

Hydras primarily feed on aquatic invertebrates. When feeding, they extend their body to its maximum length and then slowly extend their tentacles. Despite their simple structure, tentacles unusually expand and can be five times the length of the body. Once fully extended, the tentacles slowly maneuver in anticipation of contact with a suitable prey animal. Upon contact, the stinging cells on the tentacle sting the victim (the ejection process takes only about 3 microseconds), and the tentacles themselves wrap around the prey.

Within a few minutes, the victim is drawn into the body cavity, after which digestion begins. Polyp can stretch significantly its body wall to digest prey more than twice the size of the hydra. After two or three days, the indigestible remains of the victim are removed by contraction through the opening of the mouth.

The food of freshwater hydra consists of small crustaceans, water fleas, insect larvae, water moths, plankton and other small aquatic animals.

Movement

The hydra moves from place to place, stretching its body and clinging to an object alternately with one or the other end of the body. Polyps migrate about 2 cm per day. By forming a gas bubble on its leg, which provides buoyancy, the hydra can also move towards the surface.

Reproduction and lifespan.

Hydra can reproduce both asexually and in the form of germination of new polyps on the stalk of the mother polyp, by longitudinal and transverse division and under certain circumstances. These circumstances are still have not been fully studied, but lack of food plays a role important role. These animals can be male, female or even hermaphrodite. Sexual reproduction is initiated by the formation of germ cells in the wall of the animal.

Conclusion

The unlimited lifespan of the hydra attracts the attention of natural scientists. Hydra stem cells have the ability to perpetual self-renewal. The transcription factor has been identified as a critical factor for continuous self-renewal.

However, it appears that the researchers still have a long way to go before they can understand how their findings could be applied to reducing or eliminating human aging.

Application of these animals for needs person is limited by the fact that freshwater hydra cannot live in dirty water, so they are used as indicators of water pollution.

The first person to see and describe the hydra was the inventor of the microscope and the greatest naturalist of the 17th-18th centuries, A. Levenguk.

Looking at aquatic plants under his primitive microscope, he saw a strange creature with “hands in the form of horns.” Leeuwenhoek even managed to observe the budding of a hydra and see its stinging cells.

The structure of freshwater hydra

Hydra is a typical representative of coelenterates. The shape of its body is tube-shaped, at the anterior end there is a mouth opening surrounded by a corolla of 5-12 tentacles. Immediately below the tentacles, the hydra has a small narrowing - the neck, separating the head from the body. The posterior end of the hydra is narrowed into a more or less long stalk, or stalk, with a sole at the end. A well-fed hydra has a length of no more than 5-8 millimeters, a hungry one is much longer.

The body of the hydra, like that of all coelenterates, consists of two layers of cells. In the outer layer, the cells are diverse: some of them act as organs that kill prey (stinging cells), others secrete mucus, and others have contractility. Nerve cells are also scattered in the outer layer, the processes of which form a network covering the entire body of the hydra.

Hydra is one of the few representatives of freshwater coelenterates, the bulk of which are inhabitants of the sea. In nature, hydras are found in various bodies of water: in ponds and lakes among aquatic plants, on the roots of duckweed, with a green carpet covering ditches and pits with water, small ponds and river backwaters. In reservoirs with clean water hydras can be found on bare rocks near the shore, where they sometimes form a velvety carpet. Hydras are light-loving, so they usually stay in shallow places near the shores. They are able to discern the direction of light flow and move towards its source. When kept in an aquarium, they always move to a lighted wall.

If you put more aquatic plants into a vessel with water, you can observe hydras crawling along the walls of the vessel and the leaves of the plants. The sole of the hydra secretes a sticky substance, due to which it is firmly attached to stones, plants or the walls of the aquarium, and it is not easy to separate it. Occasionally, the hydra moves in search of food. In the aquarium, you can mark the place of its attachment daily with a dot on the glass. This experience shows that in a few days the movement of the hydra does not exceed 2-3 centimeters. To change place, the hydra temporarily sticks to the glass with its tentacles, separates the sole and pulls it towards the front end. Having attached itself with its sole, the hydra straightens and again leans its tentacles one step forward. This method of movement is similar to how the caterpillar of moth butterflies, colloquially called a “surveyor,” walks. Only the caterpillar pulls the rear end towards the front, and then moves the head end forward again. When walking this way, the hydra constantly turns over its head and thus moves relatively quickly. There is another, much slower way of moving - sliding on the sole. With the force of the muscles of the sole, the hydra barely noticeably moves from its place. Hydras can swim in water for some time: having detached themselves from the substrate, spreading their tentacles, they slowly fall to the bottom. A gas bubble may form on the sole, which carries the animal upward.

How do freshwater hydras feed?

Hydra is a predator; its food is ciliates, small crustaceans - daphnia, cyclops and others; sometimes it comes across larger prey in the form of a mosquito larva or a small worm. Hydras can even cause harm to fish ponds by eating fish fry that hatch from the eggs.

Hydra hunting is easy to observe in an aquarium. Having spread its tentacles wide so that they form a trapping net, the hydra hangs with its tentacles down. If you watch a sitting hydra for a long time, you can see that its body is slowly swaying all the time, describing a circle with its front end. A cyclops swimming past touches the tentacles and begins to fight to free itself, but soon, struck by stinging cells, it calms down. The paralyzed prey is pulled up to the mouth by the tentacle and devoured. During a successful hunt, the small predator swells with swallowed crustaceans, whose dark eyes shine through the walls of the body. Hydra can swallow prey larger than itself. At the same time, the predator’s mouth opens wide, and the walls of the body stretch. Sometimes part of the out-of-place prey sticks out of the hydra's mouth.

Reproduction of freshwater hydra

At good nutrition the hydra quickly begins to budding. The growth of a bud from a small tubercle to a fully formed hydra, but still sitting on the body of the mother, takes several days. Often, while the young hydra has not yet separated from the old individual, the second and third buds are already formed on the body of the latter. This is how asexual reproduction occurs sexual reproduction observed more often in autumn when the water temperature drops. Swellings appear on the hydra's body - gonads, some of which contain egg cells, and others - male reproductive cells, which, floating freely in the water, penetrate the body cavity of other hydras and fertilize the immobile eggs.

After the eggs are formed, the old hydra usually dies, and from the eggs, when favorable conditions young hydras emerge.

Regeneration in freshwater hydra

Hydras have an extraordinary ability to regenerate. A hydra cut into two parts very quickly grows tentacles on the lower part and a sole on the upper part. In the history of zoology, remarkable experiments with hydra, carried out in the middle of the 17th century, are famous. Dutch teacher Tremblay. He not only managed to obtain whole hydras from small pieces, but even fused halves of different hydras with each other, turned their bodies inside out, and obtained a seven-headed polyp, similar to the Lernaean hydra from myths Ancient Greece. Since then, this polyp began to be called hydra.

In the reservoirs of our country there are 4 types of hydras, which differ little from each other. One of the species is characterized by a bright green color, which is due to the presence in the body of hydra of symbiotic algae - zoochlorella. Of our hydras, the best known are the stalked, or brown, hydra (Hydra oligactis) and the stalkless, or common hydra(N. vulgaris).

Movement. Hydra can move from place to place. This movement occurs in different ways: either the hydra, bending in an arc, sticks with the tentacles and partly the glandular cells surrounding the mouth to the substrate and then pulls up the sole, or the hydra seems to “tumble”, attaching itself alternately with the sole and with the tentacles.

Nutrition. The stinging capsules entangle the prey with their threads and paralyze it. The prey processed in this way is captured by the tentacles and directed into the mouth opening. Hydras can “overpower” very large prey that exceeds them in size, for example evenfish fry. The extensibility of their mouth and entire body is great. They are very voracious - one hydra can swallow up to half a dozen daphnia in a short period of time. Swallowed food enters the gastric cavity. Digestion in hydras is apparently combined - intra- and extracellular. Food particles are drawn in by endoderm cells with the help of pseudodopodium inside and are digested there. As a result of digestion, nutrients accumulate in the cells of the endoderm, and grains of excretory products appear there, which are released from time to time in small portions into the gastric cavity. Excretion products, as well as undigested parts of food, are thrown out through the mouth

I - individual with male gonads; II—individual with female gonads

Reproduction. Hydras reproduce asexually and sexually. Pr; Through asexual reproduction, buds are formed on the hydra, which gradually detach from the mother’s body. Budding of hydras under favorable nutritional conditions can occur very intensively; observations show that in 12 days the number of hydras can increase 8 times. During the summer period, hydras usually reproduce by budding, but with the onset of autumn, sexual reproduction begins, and hydras can be both hermaphroditic and dioecious (stalked hydra).

Reproductive products are formed in the ectoderm from interstitial cells. In these places, the ectoderm swells in the form of tubercles, in which either numerous spermatozoa or one amoeboid egg are formed. After fertilization, which occurs on the body of the hydra, the egg cell is covered with a membrane. Such a shell-covered egg overwinters, and in the spring a young hydra emerges from it. There is no larval stage of hydras.

External structure of the hydra

The hydra's body size is about 1 cm, excluding the length of the tentacles. The body has cylindrical shape. On one side there is mouth opening surrounded by tentacles. On the other side - sole, they attach the animal to objects.

The number of tentacles can vary (from 4 to 12).

Hydra has a single life form polyp(i.e., it does not form colonies, since during asexual reproduction the daughter individuals are completely separated from the mother; hydra also does not form jellyfish). Asexual reproduction occurs budding. At the same time, a new small hydra grows in the lower half of the hydra’s body.

Hydra is capable of changing its body shape within certain limits. It can bend, bend, shorten and lengthen, and extend its tentacles.

Internal structure of the hydra

Like all coelenterates internal structure The body of the hydra is a two-layer sac, forming a closed (there is only a mouth opening) intestinal cavity. The outer layer of cells is called ectoderm, internal - endoderm. Between them there is a gelatinous substance mesoglea, mainly performing a supporting function. The ectoderm and endoderm contain several types of cells.

Mostly in the ectoderm epithelial muscle cells. At the base of these cells (closer to the mesoglea) there are muscle fibers, the contraction and relaxation of which ensures the movement of the hydra.

Hydra has several varieties stinging cells. Most of them are on the tentacles, where they are located in groups (batteries). The stinging cell contains a capsule with a coiled thread. On the surface of the cell, a sensitive hair “looks” out. When the hydra's victims swim by and touch the hairs, a stinging thread shoots out of the cage. In some stinging cells, the threads pierce the arthropod's cover, in others they inject poison inside, in others they stick to the victim.

Among the ectoderm cells, Hydra has nerve cells. Each cell has many processes. Connecting with their help, nerve cells form the hydra nervous system. Such nervous system called diffuse. Signals from one cell are transmitted across the network to others. Some processes of nerve cells contact epithelial muscle cells and cause them to contract when necessary.

Hydras have intermediate cells. They give rise to other types of cells, except epithelial-muscular and digestive-muscular. All these cells provide the hydra with a high ability to regenerate, that is, restore lost parts of the body.

In the body of the hydra in the fall they are formed germ cells. Either sperm or eggs develop in the tubercles on her body.

The endoderm consists of digestive muscle and glandular cells.

U digestive muscle cell on the side facing the mesoglea there is a muscle fiber, like epithelial muscle cells. On the other side, facing the intestinal cavity, the cell has flagella (like euglena) and forms pseudopods (like amoeba). Digestive cell rakes up food particles with flagella and captures them with pseudopods. After this, a digestive vacuole. The nutrients obtained after digestion are used not only by the cell itself, but are also transported to other types of cells through special tubules.

Glandular cells secrete a digestive secretion into the intestinal cavity, which ensures the breakdown of prey and its partial digestion. In coelenterates, cavity and intracellular digestion are combined.