All coral polyps have a skeleton. Are corals an animal or a plant? Where are corals found in nature? The structure of coral polyps

Coral structures

CORAL REEFS – THE MAIN ECOLOGISTS OF THE PLANET

Lifestyle

Most coral polyps inhabit warm tropical seas, where the water temperature does not drop below +20 °C, and at depths of no more than 20 meters, in conditions of abundant plankton, which they feed on. Usually during the day the polyps shrink, and at night they stretch out and straighten their tentacles, with the help of which they catch various small animals. Large single polyps are capable of catching relatively large animals: fish, shrimp. Some species of coral polyps live due to symbiosis with autotrophic protozoa (single-celled algae), which live in their mesoglea.

Structure

Coral polyps have muscle cells that form longitudinal and transverse muscles. Available nervous system, forming a dense plexus on the oral disc.

Corals are usually called only the skeleton of the colony, which remains after the death of many small polyps. Many coral polyps are reef builders. The skeleton can be external, formed by the ectoderm, or internal, formed in the mesoglea. As a rule, polyps occupy cup-shaped depressions on the coral, visible on its surface. The shape of these polyps is columnar, in most cases with a disc at the top, from which the corollas of the tentacles extend. The polyps are motionlessly attached to a skeleton common to the entire colony and are connected to each other by a living membrane covering it, and sometimes by tubes piercing the limestone.

The skeleton is secreted by the outer epithelium of the polyps, and mainly by their base (foot), so living individuals remain on the surface of the coral structure, and the whole thing continuously grows. The number of polyps involved in its formation is also constantly increasing through their asexual reproduction (

Coral polyps are a class of marine invertebrate animals belonging to the phylum Coelenterata. These are colonial or, less commonly, solitary polyps that live in the seas. Currently, two subclasses are distinguished: eight-rayed and six-rayed coral polyps. The first group includes colonial organisms with 8 feathery tentacles (red coral, sea pen, organ). Polyps of the second group, mostly solitary, have a number of tentacles that is a multiple of six (anemones, madrepore corals). In total, approximately six thousand species of coral polyps are known, distributed in seas with high salinity in different climatic zones. Most coral polyps live in warm tropical seas with a water temperature of at least 20 degrees, at a depth of up to 20 m, provided there is an abundance of food - plankton.

The most primitive firms were found in strata dating back to the Cambrian period. Since the beginning of the Mesozoic era, subclasses of eight-rayed and six-rayed coral polyps have appeared on Earth.

Hydroid coral polyps differ from other representatives of the class in their more complex structure, the presence of a pharyngeal tube located inside the body and ending in the oral opening, the division of the body by radial partitions into separate cavities and a more perfect structure of the body as a whole. The vast majority of coral polyps form colonies; representatives of rare species are solitary.

These organisms have a typical cylindrical body shape. At the posterior end of the body there is a flat, round surface, the so-called “sole,” with which the polyps attach to objects under water. At the front edge there is a mouth opening with one or several rows of tentacles around the circumference. The mouth opening opens into a wide pharyngeal tube - a kind of stomach. The structure of the stomach wall is three-layered, similar in structure to the wall of the animal’s body. Undigested food remains are removed into the external environment through the mouth. The gastric cavity is multi-chambered due to its division into parts by radial septa. During the day, the polyps compress their tentacles, and at night they spread out and catch prey - various small plankton animals. Single polyps of large sizes can also catch larger animals - mollusks and fish.

Some types of coral polyps have stinging organs in the form of threads that perform a protective function, which, in case of danger, are thrown out through the mouth or through holes in the wall of the polyp's body. In the structure of the body wall of a coral polyp, three layers are distinguished - ectoderm, mesoderm and endoderm. The ectoderm covers the outside of the entire body of the individual and is the inner lining of the pharyngeal tube. The ectoderm consists of three layers: epithelial, nervous and muscular. Many representatives of this group of animals have a calcareous skeleton of ectodermal origin, which gives rise to horny and calcareous deposits that form the polypnyak. Mesoderm is a gelatinous substance that fills the cavities of coral polyps and includes spindle- or stellate-shaped cells and fibers. Calcareous bodies may also be present in the mesoderm, which sometimes join together to form a dense calcareous mass. The endoderm is divided into the same three layers as the ectoderm, but the structure of the ectoderm and endoderm layers is different. These organisms do not have special sense organs.

Reproduction of coral polyps occurs both asexually and sexually. Asexual reproduction occurs by budding. The development of germ cells occurs in the endoderm of the mesentery. Young organisms leave their mother at the planula stage, floating in the water column for a certain time, and then attaching to the bottom during transformation into an adult.

Extensive colonies of coral polyps are formed when budding does not reach completion. Corals in the seas are motionless and outwardly look like plant branches. But each coral branch is nothing more than accumulations of small animals - coral polyps. These clusters are called colonies. The coral “grows” with the birth of young polyps. If conditions environment favorable, coral growth is about 1 cm per year. When colonies of coral polyps grow in the tropics, barrier reefs and atolls (coral islands) are formed. The vast majority of coral polyps in reefs are six-rayed corals. Due to their calcareous skeleton, they are widely used as building material. The Mediterranean Sea is home to red and black eight-rayed corals, the skeletons of which are highly prized as jewelry.

Coral polyps are a class of marine invertebrate animals belonging to the phylum Coelenterata. These are colonial or, less commonly, solitary polyps that live in the seas. Currently, two subclasses are distinguished: eight-rayed and six-rayed coral polyps. The first group includes colonial organisms with 8 feathery tentacles (red coral, sea pen, organ). Polyps of the second group, mostly solitary, have a number of tentacles that is a multiple of six (anemones, madrepore corals). In total, approximately six thousand species of coral polyps are known, distributed in seas with high salinity in different climatic zones. Most coral polyps live in warm tropical seas with a water temperature of at least 20 degrees, at a depth of up to 20 m, provided there is an abundance of food - plankton.

The most primitive firms of coral polyps were found in strata dating back to the Cambrian period. Since the beginning of the Mesozoic era, subclasses of eight-rayed and six-rayed coral polyps have appeared on Earth.

Hydroid coral polyps differ from other representatives of the class in their more complex structure, the presence of a pharyngeal tube located inside the body and ending in the oral opening, the division of the body by radial partitions into separate cavities and a more perfect structure of the body as a whole. The vast majority of coral polyps form colonies; representatives of rare species are solitary.

These organisms have a typical cylindrical body shape. At the posterior end of the body there is a flat, round surface, the so-called “sole,” with which the polyps attach to objects under water. At the front edge there is a mouth opening with one or several rows of tentacles around the circumference. The mouth opening opens into a wide pharyngeal tube - a kind of stomach. The structure of the stomach wall is three-layered, similar in structure to the wall of the animal’s body. Undigested food remains are removed into the external environment through the mouth. The gastric cavity is multi-chambered due to its division into parts by radial septa. During the day, the polyps compress their tentacles, and at night they spread out and catch prey - various small plankton animals. Single polyps of large sizes can also catch larger animals - mollusks and fish.

Some types of coral polyps have stinging organs in the form of threads that perform a protective function, which, in case of danger, are thrown out through the mouth or through holes in the wall of the polyp's body. In the structure of the body wall of a coral polyp there are three layers - ectoderm y, mesoderm y and endoderm y. Ectoderm a covers the outside of the entire body of the individual and is the inner lining of the pharyngeal tube. Ectoderm A consists of three layers: epithelial, nervous and muscular. Many representatives of this group of animals have a calcareous skeleton of ectodermal origin, which gives rise to horny and calcareous deposits that form the polypnyak. Mesoderm A is a gelatinous substance that fills the cavities of coral polyps and includes spindle- or stellate-shaped cells and fibers. Calcareous bodies may also be present in the mesoderm, which sometimes join together to form a dense calcareous mass. Endoderm A divided into the same three layers as the ectoderm A , but the structure of the ectoderm layers s and endoderm differently. These organisms do not have special sense organs.

Reproduction of coral polyps occurs both asexually and sexually. Asexual reproduction occurs by budding. The development of germ cells occurs in the endoderm of the mesentery. Young organisms leave their mother at the planula stage, floating in the water column for a certain time, and then attaching to the bottom during transformation into an adult.

The body of a coral polyp is usually cylindrical in shape and is not divided into a trunk and a leg. In colonial forms of coral polyps, the base is immersed in the common body of the colony - the coenosarc, and in solitary forms it turns into an attachment sole.
The tentacles of these organisms are always hollow, arranged in one or several closely spaced corollas.

There are two large groups of coral polyps - eight-rayed (Octocorallia) and six-rayed (Hexacorallia).
The first group always has eight tentacles and they are equipped with small outgrowths at the edges - pinnules; in the second group the number of tentacles is larger and, as a rule, a multiple of six.

The tentacles of six-rayed corals are almost always smooth and without quilts. The upper part of the polyp, between the tentacles, is called the oral disc. In its middle there is a slit-like mouth opening. Internal structure coral polyps are much more complex than those of hydroid and scyphoid polyps. The mouth leads into a laterally compressed pharynx lined with ectoderm. Typically, a groove runs along one of the edges of the pharynx, carrying cells with very long eyelashes- siphonoglyph. Sometimes there are two siphonoglyphs, in such cases they are located on opposite narrow edges of the pharyngeal tube. The cilia continuously move and drive water into the intestinal cavity. The latter is divided into chambers by longitudinal partitions (septa).

In the upper part of the body of a coral polyp (in the pharynx area), the septa are complete (adhering with one edge to the body wall, the other to the pharynx) or incomplete (not reaching the pharynx). The septa have openings through which all the chambers communicate with each other.
In the lower part of the coral polyp (below the pharynx), the septa grow only to the body wall. As a result, the central part of the gastric cavity - the stomach - remains undivided. The free edges of the septa are thickened and are called mesenteric filaments. They play an important role in the digestion of food, as they contain many glandular cells that secrete digestive enzymes.

In corals with a single siphonoglyph, the two mesenteric filaments, located on a pair of opposing septa, are not thickened and bear cells with long, strong cilia. Being in constant motion, the cilia drive water out of the gastric cavity of the coral polyp. Collaboration two mesenteric filaments and a siphonoglyph (or two opposing siphonoglyphs, as in sea anemones), ensures a constant change of water in the gastric cavity. As a result, fresh, oxygen-rich water constantly flows there, and along with it bacteria, planktonic organisms, and detritus particles that coral polyps feed on. With the reverse flow of water, they are carried out carbon dioxide, metabolic products and undigested food remains.

The number of septa and chambers in coral polyps always coincides with the number of tentacles, the cavity of which is a continuation of the corresponding chambers of the gastric cavity. Thus, eight-rayed corals always have eight septa and chambers, while six-rayed corals always have six.
Septa are laid gradually and always in pairs.

Like all coelenterates, corals are radially symmetrical. However, their internal organization also contains features of bilateral symmetry (laterally compressed pharynx and siphonoglyphs). Only one plane of symmetry can be drawn through the longitudinal axis of the pharynx, which divides the body of the coral polyp into two mirror halves. The chambers lying against the narrow edges of the pharyngeal tube differ from the others in the location of the muscular ridges. These chambers and the septa that form them are called guide chambers, by which the “dorsal” and “ventral” sides of the body of a coral polyp are conventionally determined.
The muscle cells of coral polyps separate from the ecto- and endoderm and pass into the mesoglea, forming a layer of longitudinal and transverse muscles in the walls of the body. In addition, in the mesoglea of ​​each septum, on one side there is a thin layer of transverse muscles, and on the other, a powerful ridge of longitudinal muscles.

The mesoglea is represented in most six-rayed corals by a thin supporting plate. But in eight-rayed corals it reaches significant development, especially in the trunk and branches of the colony. The gelatinous substance of the mesoglea is strengthened by collagen elements and filled with a huge number of skeletal calcareous needles - spicules, or sclerites.
Thus, the mesoglea forms a strong support for the coral colony. At the same time, it is involved in the transport of nutrients, as it is penetrated by a dense network of endodermal canals connecting the individual intestinal cavities of coral polyps into one cavity. These same channels play an important role in the rhythmic change between the active and passive states of a colony of coral polyps.

The skeleton reaches significant development in many coral polyps. In eight-rayed corals, this is the internal, mesogleal skeleton, consisting of sclerites, which develop in special cells - scleroblasts. Sometimes the sclerites merge with each other or are united by an organic horn-like substance, forming the skeleton of a coral colony. It may also consist of pure horny substance.

Among the six-rayed corals there are non-skeletal forms (anemones and periantharia). More often, however, there is a skeleton, and it can be either internal (in the form of a rod of horn-like substance) or external (calcareous), but always of ectodermal origin.
But neither the calcareous nor the organic skeleton is able to maintain the constancy of the body shape of coral polyps. This is achieved in a different way. All polyps have a kind of hydroskeleton, which reaches the greatest perfection in coral polyps.

Thanks to DC water created by siphonoglyphs, increased pressure occurs in the gastric cavity, without which the coral polyp would have the shape of an empty two-layer sac. The polyp expands under the pressure of the fluid filling the gastric cavity. It can remain in this state for a very long time and with almost no energy expenditure. Meanwhile, in other animals such a tense state cannot last long, since the muscles get tired, the animal must change the position of the body or move in space.
But one should not think that once expanded, the coral polyp will retain its shape indefinitely. Periodically, it is disrupted by the contraction of any muscle group. Compression of the circular muscles, for example, lengthens the body of the polyp and makes it thinner, contraction of the longitudinal muscles of the tentacles leads to their bending, etc.
In case of danger, all muscles contract at once, water from the gastric cavity is squeezed out and the polyp is compressed or drawn into the colony.

Colonies of coral polyps, as a rule, are not polymorphic, but some eight-rayed corals exhibit dimorphism - two types of polyp structure.
All corals are characterized only by the polypoid state. They do not form jellyfish. The gonads develop in the endoderm of the septa of coral polyps.