Coral polyps features. Class coral polyps - general characteristics

The class of coral polyps includes exclusively marine animals that have only a polypoid shape.

The vast majority of coral polyps are sedentary and live in colonies that can be as large as desired. The symmetry of the body of coral polyps is either eight-ray, or the number of rays is greater and a multiple of six. However, there are also cases of a more specialized, so-called two-beam symmetry, approaching the bilateral symmetry of higher animals.

The body of coral polyps has the shape of a cylindrical bag, with one end (sole) of which the animal is attached to the substrate, and at the opposite end a mouth opening is formed, which has the shape of a slit with a wide groove (siphonoglyph). Like hydroid polyps, their oral opening is surrounded by a corolla of hollow tentacles located near the transition of the oral fissure to the body wall. The tentacles can be simple or feathery. The cavity of the tentacles, combined with the gastric mouth.

The number of tentacles according to the symmetrical structure in some forms is equal to eight, and in others it is a multiple of six.
Coral polyps differ from hydroid polyps in a much higher organization. The wall of their body has a significantly composition structure than that of hydrozoans. Here we meet with a clear independent longitudinal and annular muscle cells lying under the ectoderm and endoderm. The nerve plexus is also more developed. It is deeper in the body wall and includes sensory and ganglion cells. More developed and mesoglea, having sheets or fibrous appearance with cells immersed in it, which come from the ectoderm and endoderm.

The gastric system is also much more complicated here. The mouth opening leads into a tube (pharynx) hanging down into the gastric cavity. The pharynx is lined with ectoderm, which wraps inward along the edges of the mouth opening. The gastric emptying is divided by partitions or septims, which extend from the inner surface of the body wall and divide the gastric cavity into chambers. At the upper end, the septa fuse with their edges with the pharynx, and below they protrude into the gastric cavity with free edges. The free edges of the septa are thickened and have the appearance of a tortuous formation, which is called the mesenteric cord. The epithelium of the mesenteric cords contains the digestive glands involved in digestion. At the lower end of the cord there are a large number of stinging cells, and this end in some forms is quite long and can be thrown out through the mouth or special openings. The cord is used for defense and attack. Such filamentous formations are called acontions.

Coral polyps, with the exception of some rare forms (for example, sea anemones), have a skeleton that most often consists of lime, less often of horny substance, and in some forms, of both. The skeleton can be either external or internal.
Reproduction in coral polyps is sexual and asexual. In colonial forms, asexual reproduction occurs by budding, and in rare forms, longitudinal or transverse division is observed. Sex cells mature in the endoderm. Fertilization and development of the egg goes into the gastric cavity until the formation of a planula, which comes out through the mouth, swims freely for some time, and then settles to the bottom, is fixed with its front end to it and turns into a polyp. Bark ¬ catching polyps dioecious. The spermatozoa exit through the mouth, breaking through the wall of the body, and through the mouth enter the individual of the opposite sex. Coral polyps are divided into two subclasses: eight-ray and six-ray corals.

From the colonial forms, stony or stony corals should be distinguished. Their clones consist of innumerable polyps with a calcareous skeleton. With the growth of such skeletons, coral reefs and islands are formed. The east coast of Australia has a huge barrier reef 1400 km long. Coral islands, atolls, have the form of a ring; inside which is a lagoon. Coral skeletons have different color shades of pale pink or red. These types of corals are used by humans to make jewelry.

The class Coral polyps belongs to the intestinal cavities and includes about 6 thousand species. There is no medusa stage in their life cycle. Coral polyps, depending on the species, can be either solitary or colonial. The size of single forms can reach a meter or more in diameter, and individual specimens of colonies can be less than a centimeter in size.

Coral polyps mainly live in tropical seas at shallow depths.

A characteristic feature of colonial coral polyps is the presence of a calcareous or horny skeleton. Polyps with a calcareous skeleton form coral reefs. Single coral polyps do not have such a skeleton; they can move along the bottom, burrow into the benthos, and even swim a little while bending.

Corals are called the skeleton of colonial forms. Ancient corals formed huge deposits of limestone, which are now used in construction.

The skeletal structures of the coral polyp form in the lower parts of either the ectoderm or the mesoglea. As a result, it turns out that individual individuals of the colony sit in recesses on a common skeleton. Communication between polyps is carried out due to a layer of living tissue on the surface of the coral.

There are incomplete radial septa in the intestinal cavity (eight, or a multiple of six). The cavity has bilateral symmetry, not radial. The mouth opening is surrounded by numerous tentacles. Colonial forms feed on plankton (crustaceans and other arthropods). Solitary coral polyps, such as sea anemones, feed on larger animals (fish, crustaceans).

Coral polyps have muscle cells and a muscular system.

Near the mouth opening there is a denser plexus of nerve cells.

Coral polyps reproduce asexually and sexually. Asexual reproduction is carried out by budding. In some single polyps, in addition to budding, a longitudinal division of the individual into two parts is possible. During sexual reproduction, germ cells are formed in the endoderm, usually on the partitions of the intestinal cavity. The spermatozoa leave the male and swim into the intestinal cavity of the female, where fertilization takes place. A floating larva (planula) develops from the zygote, which swims out and after some time settles in a new place, giving rise to a new polyp.

Anemones are a detachment of coral polyps, mostly solitary. They are distinguished by a bag-shaped body, the absence of a mineral skeleton, numerous tentacles, and a variety of bright colors. Some sea anemones enter into symbiosis with hermit crabs living in shells left over from molluscs. In this symbiosis, cancer uses anemone as a means of defense against predators (stinging cells of the coelenterates). Anemone moves with the help of cancer, which allows it to capture more food.

Coral polyps are sensitive to water pollution. So the decrease in oxygen in the water leads to their death.

There are over 5000 different coral species. The length of the coral polyp does not exceed 1 cm, but in all other respects it is very similar to the sea anemone. In fact, sea anemones are coral polyps, but solitary and without a skeleton. Most coral species live in tropical seas. Some polyps are solitary, but most species form large colonies. Some build a solid skeleton around themselves. It is from these skeletons that coral reefs are gradually formed. The skeleton of horn corals resembles a branching tree or deer antlers. The skeleton of soft corals is like a spongy rubber jelly. corals- these are intestinal. Like everyone else, they get food with tentacles. But unlike other classes, coral polyps do not have a jellyfish stage in their life cycle, they live all their lives in the form of polyps. In a coral colony, each polyp is associated with adjacent living tissue, most commonly in the sole area. Thus, the colony acts as one giant super organism. Each polyp catches prey on its own, but then the food is distributed among neighboring members of the colony. This is important because in some corals certain polyps cannot feed themselves, their function is to protect or support the colony by building up the outer skeleton.

In reef-building corals, each polyp forms a cup-shaped skeleton of calcium salts dissolved in water under itself and on the sides of the body. When a danger arises, such as an attack or a starfish eating corals, they are drawn into a protective bowl. Later, when the danger has passed, the animal leans out.
Feeding polyps spread small tentacles and catch food particles brought by waves and currents. Trapping tentacles capture small animals and protozoa and direct food into the mouth, like their large relatives -. Many polyps come out to hunt only at night. It is at this time that marine plankton rises closer to the surface. Polyps with fluttering, shiny tentacles make coral reefs look like a glittering carpet of many colors.
Under favorable conditions, usually on a full moon night, all corals in a given area simultaneously release eggs and sperm into the water. Clouds of reproductive products float near the surface. Each egg is fertilized by a spermatozoon, and from it a tiny larva develops, which swims in the sea for some time. Later, it sinks to the seabed, attaches itself to rocks, and develops into a coral polyp equipped with a corolla of tentacles. If this single individual survives safely, after a few weeks it buds several new polyps - a small colony is formed. As the number of individuals increases, the colony grows.

coral polyp resembles a miniature sea anemone. His body is entirely occupied by the digestive cavity. The folds of the inner layer of cells (mesenterium), growing inside the cavity, increase the absorption surface of nutrients.
Some corals form brightly branched outgrowths that are tough and leathery to the touch.
The reefs formed by the corals have many cavities, caves and overhangs - perfect hiding places for other creatures!
During the breeding season, corals release clouds of eggs and sperm into the seawater.

coral groups:
- Stony corals - with a stony skeleton, the main reef-formers.
- Alcyonaria (soft corals)
- Horn corals (gorgonians) - sea fans
- sea feathers

Only single coral polyps, and even then not all, are devoid of skeletal formations. On the contrary, colonial corals have a skeleton, most often consisting of carbonic lime, less often (in some Octocorallia corals) of a horn-like substance. In octagonal corals, the calcareous skeleton lies within the mesoglea and, in the simplest case, consists of scattered microscopic calcareous needles (Fig. 112). The latter, like sponges, are formed inside special cells - scleroblasts. The noble coral has so many calcareous needles (spicules) that most of them merge into a dense mass, forming a solid skeleton. In six-ray corals (Fig. 113), the calcareous skeleton is arranged differently. In a young individual, the plantar plate is first distinguished outward by the cells of the ectoderm, and then the skeletal cup, or theca, around the body of the polyp. Further from the theca, skeletal septa (scleroseptae) grow into the body, pushing the wall of the polyp in front of them deep into its gastric cavity.

In colonies consisting of a huge number of individuals, the cups of neighboring polyps often merge. So, at conn. Octocorallia internal skeleton (located inside the mesoglea), in conn. Hexacorallia is external in origin, since it lies outside the ectoderm, representing the product of its vital activity.

Scleroseptae and real soft septa have a well-defined number and arrangement, which serve as an important systematic feature.

Eight-rayed corals Octocorallia (Fig. 111) have 8 septa dividing the gastric cavity into 8 peripheral chambers; two of these chambers, falling against the narrow edges of the pharyngeal tube, are called directional. Muscular rollers are located on septa in a strictly defined order. As a result, two muscle rollers face into the cavity of one of the guiding chambers (conventionally called the ventral cavity). The muscle ridges do not protrude into the other guiding chamber (dorsal).

In six-rayed corals, the arrangement of septa is more complicated (Fig. 111). The number of partitions arranged in pairs is a multiple of six, but there are at least 12 of them. Partitions do not appear all at once. First, six pairs of first-order septa are formed, which divide the gastric cavity into 12 chambers. Chambers lying between two partitions of one pair are called internal, while those located between partitions of different pairs are called intermediate. Further partitions occur in pairs, forming within the intermediate chambers.

Class Coral polyps (Anthozoa)

Coral polyps are marine colonial, rarely solitary polyps that develop without generational change. They mainly live in warm tropical seas, where the water temperature is not lower than 20 ° C, and at depths of no more than 20 m, in conditions of abundant plankton, which they feed on. In total, about 6 thousand species of coral polyps are known. Many of them have a calcareous skeleton and are reef-forming.

Coral polyps, despite the general similarity of the structure with hydroids, differ from the latter in the following features:

The sizes of coral polyps are larger and they have a highly developed mesoglea,

Most species are well developed skeleton(horny or calcareous). The skeleton may be external, formed by the ectoderm, or internal, formed in the mesoglea;

- touring cavity divided by septa into chambers. There is an ectodermal pharynx with flagellar grooves-siphonoglyphs that provide water flow in the gastric cavity;

- gonads formed in the endoderm. Reproduction is asexual and sexual. development with metamorphosis. Larva - planula. There is no alternation of generations;

Available muscle cells, forming longitudinal and transverse muscles;

- nervous system forms a dense plexus on the oral disc;

The ray symmetry is broken and there is a transition to two-beam, or bilateral, symmetry.

Rice. 96. The structure of a six-rayed coral polyp (according to Pfurgsheller): 1 - tentacles, 2 - mouth, 3 - pharynx, 4 - septa, 5 - plantar plate, 6 - calyx, 7 - scleroseptae, 8 - polyp tissues

Rice. 97. Formation of the internal skeleton in eight-ray polyps (according to Hadorn): 1 - tentacles, 2 - skeletal needles at the base of the tentacles, 3 - stomach with septa, where eggs ripen, 4 - skeletal cords, 5 - mesoglea, gastric canal in the trunk of the colony, surrounded by a skeleton, 6 - the trunk of the colony

There are two subclasses of modern coral polyps: Octocorallia and Hexacorallia, between which there are significant differences in organization. Therefore, in characterizing the morphology and physiology of coral polyps, it is more convenient to give a comparative outline of the organization of Octocorallia and Hexacorallia.

Comparative morphophysiological characteristics of 6- and 8-ray coral polyps. The body of polyps is cylindrical. Single polyps are attached to the substrate with their soles, and colonial colonies to the body are attached to the coenosarca. On the oral pole of the polyp there is a mouth, always surrounded by hollow tentacles (Fig. 96). By the number of tentacles, it is easy to distinguish subclasses of coral polyps: 8-rayed ones always have eight tentacles and they are pinnate, with lateral outgrowths, while 6-rayed tentacles are smooth and their number is a multiple of six (Fig. 96, 97).

The gastric cavity is complex. The mouth leads into a unidirectionally flattened pharynx with a folded ectodermal lining. Octocorallia at one end of the pharyngeal fissure has siphonoglyph- groove lined with ciliated epithelium. Hexacorallia has two siphonoglyphs - in both corners of the pharyngeal fissure. Siphonoglyphs ensure the flow of water through the gastric cavity. The slit-like pharynx and the presence of 1-2 siphonoglyphs violate the radial symmetry of the polyps, and therefore, in 8-ray polyps, only one, and in 6-ray polyps, only two planes of symmetry can be drawn. The pharynx leads to the gastric cavity, which is subdivided

Rice. 98. Transverse sections through the eight-beam and six-beam polyps (A - according to Hickson, B - according to Hyman): 1 - pharynx, 2 - pharyngeal cavity, 3 - siphonoglyph, 4 - ventral guiding chamber, 5 - septum, 6 - muscular roller of the septum, 7 - dorsal guiding chamber, 8 - internal chambers between septa of the first order, 9 - internal chambers between secondary septa, 10 - intermediate chambers, 11 - ectoderm, 12 - endoderm, mesogley blackened

radial partitions - septa. Septa are lateral folds of the endoderm, each fold respectively consisting of two layers of endoderm, between which there is a mesoglea with muscle cells. The septa adhere to the pharynx with a free edge, and do not close below the pharynx, forming the stomach. The edges of the septa are thickened, corrugated, seated with stinging and digestive cells, forming mesenteric filaments. Their free ends are called acontions. The prey that enters the stomach of the polyp is tightly wrapped with mesenteric filaments, killed and gradually digested under the influence of digestive enzymes. The presence of septa increases the digestive surface in polyps. The number of septa and their location are different in the two subclasses (Fig. 98).

Octocorallia has eight septa with muscular ridges. Pairs of septa extending from two corners of the flattened pharynx are called guiding chambers. The guiding chamber opposite the single siphonoglyph differs in that the muscular ridges in its septa are turned inward. This chamber is conditionally called "ventral". On the septa of the opposite "dorsal" chamber, the ridges face outwards from the chamber. Thus, the location of the muscular ridges in the septa of Octocorallia also breaks the radial symmetry.

Hexacoralha has many septa, at least 12, and their number is a multiple of six. The muscle ridges in the guiding chambers are turned outward and do not violate the two-beam symmetry determined by the shape of the pharynx and two

siphonoglyphs. Septa in 6-beam polyps form gradually. Initially, there are six pairs of first-order septa that adhere to the pharynx. Between the septa of each pair, the main chambers are formed, and between them - intermediate ones, in which additional pairs of second-order septa are formed, etc. (Fig. 98).

The nutrition of coral polyps is varied. Many feed on plankton or catch small animals with their tentacles. Large single polyps - Anemones (Actinia) are able to catch large animals: fish, shrimp. Well, recently it turned out that some of the species of coral polyps live due to symbiosis with unicellular algae that live in their mesoglea.

For coral polyps, leading mainly an attached lifestyle, the presence of a skeleton is characteristic, which is formed differently in different subclasses.

In 8-ray polyps, the skeleton is internal and is formed in the mesoglea, it can be horny or calcareous. Skeletal elements (Fig. 99) are formed in scleroblast cells. The skeletal needles may fuse with each other or be joined by the horny substance to form the skeleton of the colony. For example, in the noble coral (Corallium rubrum), the skeletal trunk of the colony is calcareous, purple in color. From above, the branch of the colony is covered with ectoderm. The internal skeleton is permeated with a network of endodermal channels connecting all members of the colony (Fig. 97).

In 6-beam polyps, the skeleton is external, secreted by the ectoderm, less often internal or absent. The growth of the outer skeleton around a young polyp comes from the area of ​​the sole, where the plantar plate first appears, and calcareous septa - sclerosepts - form on it, and then a calyx is formed - the theca, which protects the entire polyp to the level of the tentacles. The skeleton is often overgrown with folds of skin from above and gives the impression of being internal.

There are polyps without a skeleton, such as sea anemones. In many 8-ray polyps, the skeleton is poorly developed and is replaced by a hydroskeleton - the turgor of the colony, provided by the filling of the gastric cavity with water.

Reproduction and development. Polyps can reproduce asexually: by budding, division in the transverse and longitudinal directions.

Before sexual reproduction, gonads mature on septa in the endoderm. Polyps are usually dioecious. Spermatozoa through ruptures in the wall of the gonads exit into the gastric cavity, and then out and penetrate through the mouth into the cavity of the female. Fertilized eggs develop for some time in the mesoglea of ​​the septum. Planula larvae usually leave the parent polyp, and then settle on a solid substrate and turn into polyps (Fig. 100, 5). In many coral polyps, development proceeds without metamorphosis and the planula larva does not form.

Overview of subclasses and orders of coral polyps. In total, five subclasses of coral polyps are known, of which three subclasses are known only in the fossil state (Tabulata, Rugosa, Heliolitoidea). Two subclasses are represented by modern forms (Octocorallia and Nexocoralla) (Fig. 101, 102).

Subclass Eight-pointed corals (Octocorallia)

Eight-pointed corals have eight tentacles, eight septa, and an internal skeleton. There is a violation of radial symmetry to bilateral due to the presence of one siphonoglyph and the location of muscular ridges in the septa (Fig. 98 A).

Detachment Alcyonaria (Alcyonaria)- the most numerous, including about 1300 species of marine polyps. Most of them are soft corals, without a developed skeleton, with separate spicules scattered in the mesoglea. They form colonies of various shapes: branched, lobed, spherical. Alcyonaria colonies - "hands" (Fig. 103) can serve as an example of soft corals. Only some species of the genus Tubipora, an organ, have a developed calcareous skeleton, which forms tubules in the mesoglea, welded together by transverse plates. Their skeleton is vaguely shaped like an organ, hence their name. Organs form large spherical colonies and participate in reef formation. Corals of the genus Versemia fruticosa are common in the White Sea. Alcyonaria often form dense thickets on rocky soils.

Order Horn corals (Gorgonacea) make up polyps with an internal horny skeleton. It is also a species-rich order (1200 species), found mainly in tropical areas, but some of them have adapted to living in polar regions. Fan-shaped colonies form polyps of the genus Gorgonia, called the fan of Venus.

Rice. 101. Eight-pointed corals (according to Dogel): A - Alcyonaria Gersemia, B - Pennatula sea pen, C - Leptogorgia horn coral

Among the gorgonians are commercial red corals (Corallium rubrum) and species close to it, mined in the Mediterranean, Red and other seas. Their organic skeleton is impregnated with lime and has various shades of red. Valuable jewelry is made from red coral.

Order Sea feathers (Pennatulacea). Sea feathers form colonies of a feather-like form: with a thick trunk, on which polyps are located on the sides in regular rows. The number of species is small (300). Some species are common in the Arctic Ocean, and among them there are the largest colonies up to 2.5 m high (Umbrella encrinus). Pennatula colonies are capable of glowing. Sea feathers, unlike other coral polyps, do not adhere to the substrate. They anchor in the ground, and sometimes swim from place to place.

Subclass Six-pointed corals (Hexacorallia)

Six-pointed corals have many smooth tentacles, the number of which is a multiple of six. The gastrovascular cavity is divided by a complex system of septa, the number of which is also a multiple of six. Six-beam symmetry is broken to two-beam due to two siphonoglyphs and the slit-like shape of the pharynx. Often the skeleton is external, calcareous, rarely absent. There are five orders of six-ray corals.

Squad of anemones (Actinaria) includes mainly large forms of single polyps, devoid of a skeleton. Anemones are able to move slowly on the sole. These are active predators, sometimes even eating small fish. Often they are brightly colored, and they are called sea anemones. Some anemones are in symbiosis with hermit crabs, which serve them for movement, and anemones with stinging properties protect hermits from enemies (Fig. 104).

Order of Ceriantharia (Ceriantharia)- solitary burrowing polyps with strong muscles and no skeleton.

Detachment of Zoantaria (Zoantharia)- solitary and colonial polyps with underdeveloped muscle cells.

Detachment Antipatharia (Antipatharia) form pinnate colonies with an axial horny skeleton. This includes commercial black coral, from the skeleton of which various artistic products are made: pipes, cane handles, knives.

Order Madrepore corals (Madreporaria)- the most extensive and includes more than 2500 species. This includes both solitary and colonial polyps. All madreporaceae are characterized by the presence of a powerful calcareous skeleton. This group of corals are the main reef builders. These include medulla (Leptoria) in the form of hemispheres with bizarre furrows, mushroom corals (Fungia), etc.

Coral reefs and their origin. Mass settlements of coral polyps with a calcareous skeleton form reefs. The reef consists mainly of madrepore polyps, but six-ray corals are also partially involved, as well as other animals with a skeleton: sponges, bryozoans, mollusks, etc.

Coral reefs are unique ecosystems characterized by a special composition of autotrophic and heterotrophic organisms that are interconnected by food chains and other forms of interspecific relationships. The population of coral reefs is so large and diverse that they are called marine "oases". These are reserves of marine fauna and flora, they deserve human protection.

Reef-forming coral polyps are distributed only in the tropical regions of the World Ocean, as they need normal oceanic salinity (at least 35% ppm), high and constant water temperature (at least 20 ° C). In addition, corals are sensitive to light and water saturation with oxygen and, therefore, are found in shallow water and usually do not go to a depth of more than 50 m. The dependence of the distribution of corals on light is determined by their symbiosis with unicellular algae - symbiodiniums, or zooxanthellae, inhabiting the cells of the endoderm of polyps. The mutual benefit of their coexistence is as follows. Algae receive protection from corals and carbon dioxide (respiratory products) for photosynthesis, as well as some nitrogen and phosphorus compounds deficient in sea water from polyp dissimilation products. Coral polyps, in turn, receive oxygen from algae, which is necessary for respiration, as well as for activating the processes of skeleton formation. In addition, polyps partially feed on algae, but not in the way that was previously thought - by digesting them in the cytoplasm, but through the direct use of photosynthesis products coming directly from algae cells. Symbiosis is also based on the pace of life cycles of these species. Like all protozoa, zooxanthellae have a diurnal rhythm of reproduction, while corals exist for a long time. dying

algae are digested in the cytoplasm of the polyp. Thus, this system is based on a waste-free process. At the same time, the dependence of coral polyps on zooxanthellae is especially great, without which they die.

Reefs are coastal, barrier and atolls - ring-shaped coral islands. For the first time, the hypothesis about the origin of coral reefs was proposed by Charles Darwin (1836). He applied the method of historical geology about secular land fluctuations to explain the formation of coral islands. In his opinion, all types of reefs were formed as a result of land subsidence (Fig. 105). If the island, surrounded by a coastal reef, gradually sinks, its shores recede from the reef, which completes itself to the surface of the ocean and turns into a barrier reef. When the island is completely submerged, a ring remains from the former barrier reef, i.e., a coral island is formed - an atoll, which is then gradually populated by plants and animals. There are many other hypotheses about the origin of various types of reefs, but Charles Darwin's hypothesis remains the most reasoned and has stood the test of time. At present, this hypothesis is supplemented by new scientific data. It is assumed that the change in the level of the land depends not only on its subsidence, but also on changes in the level of the ocean during periods of glaciation or melting of ice caps near the poles. From the dying coral reefs, immersed in the ocean, sedimentary rocks arose - coral limestones. In the Paleozoic, these rocks were formed by subclasses of corals Rugosa and Tabulata, and starting from the Mesozoic, mainly by madrepore polyps.