INVERTEBRATE ZOOLOGY - H. J. Shcherbak - 2008

KINGDOM METAZOA

SUBKINGDOM EUMETAZOA

DIVISION DIPLOBLASTICA, OR RADIATA

PHYLUM CNIDARIA, OR COELENTERATA

SUBPHYLUM ANTHOZOA

CLASS HEXACORALLIA

Most commonly colonial corals, the majority of which possess an external calcareous Skeleton, and more rarely solitary, skeletonless forms, such as sea anemones. The number of tentacles and septa in the gastral cavity of the polyps is typically a multiple of six. The tentacles are unbranched, and the mesoglea is thin.

The order Actiniaria, or sea anemones, comprises skeletonless, predominantly solitary corals. They are often brightly colored, resembling fantastical flowers, which is why they are called sea anemones. Their body shape is cylindrical, the oral disc is well-developed, and it is surrounded by concentric circles of 16 to 1,000 tentacles. In most sea anemones, the number of tentacles and septa is a multiple of six, and more rarely eight or nine (Fig. 97). The Pharynx is flattened and bears two siphonoglyphs; the beating of their cilia generates two Water currents: one directed into the gastral cavity and the other from the gastral cavity outward. The mesoglea in sea anemones is well-developed and quite dense; in some species, it acquires a Cartilage-like consistency due to the presence of fibrous material, thereby performing a supportive function.

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Fig. 97. Sea anemones (from Avsinitsev):

a — general view; b, c — transverse sections at the level of the pharynx and the gastral cavity, respectively:

1 — tentacles; 2 — pedal disc; 3 — body Column; 4 — pharynx; 5 — septa;

6 — muscular ridges; 7 — epidermis; 8 — gastrodermis; 9 — mesoglea; 10 — Stomach

Among sea anemones, there are predators that feed on crustaceans, Mollusks, and small fish, as well as suspension feeders that consume microscopic planktonic organisms (Bacteria, Algae, Protozoa). Some sea anemones lead a sessile lifestyle, while others are capable of active locomotion through the contraction of the pedal disc Muscles. Representatives of the family Myniadidae possess a gas-filled bladder, which enables them to float in the water column with their tentacles pointing downwards.

Sea anemones inhabit all seas and oceans, but they reach their highest diversity in the tropics. Four species of sea anemones inhabit the Black Sea, including Actinothoe clavata and Actinia equina (the latter species is also found in the Sea of Azov).

Certain species of sea anemones enter into symbiotic relationships with other animals. For instance, anemones of the genus Adamsia settle on the mollusk shell inhabited by a hermit crab. The anemone obtains food scraps and a means of transportation, whereas the hermit crab receives reliable protection from its tentacles. Giant anemones of the genus Stoichactis (with an oral disc diameter of 1.5 m), which inhabit the shallow waters of the western coast of Australia, live in Symbiosis with amphiprion fish (clownfish) that find shelter among the anemone's tentacles. In turn, the anemones receive food scraps from the fish, and the continuous movement of the fish's fins facilitates the anemones' gas exchange. Symbiotic algae inhabit Anemonia sulcata, making these anemones more active during daylight hours.

The order Scleractinia, or stony corals (Fig. 98), is the most numerous group of corals. They are predominantly colonial, and more rarely solitary, corals possessing an external calcareous skeleton formed by epidermal Cells.

Fig. 98. Stony corals (Scleractinia) (from Naumov et al.):

a — Acropora; b — Leptoria

In a scleractinian colony, a distinction is made between individual polyps, termed zooids, and the coenosarc, which is the common tissue of the colony. The skeleton of an individual zooid is called a corallite. The body of a zooid consists of a lower part, immersed in the skeletal cup of the corallite, and an upper part bearing the Mouth and tentacles, which projects above the corallite (Fig. 99). When disturbed, the upper part of the polyp retracts into the interior of the lower part. Skeletal basal plates (tabulae) form on the pedal disc of the colony's zooids, separating the skeletons of living polyps from those of preceding generations lying beneath them. The epidermis of the pedal disc contains cells known as scleroblasts, which secrete fine chitinous filaments. Interweaving with one another, they form a delicate, dense network upon which crystals of calcium carbonate are deposited. Fusing into a continuous skeleton, they form a basal plate from which skeletal partitions, known as sclerosepta, grow inward into the soft Tissues of the polyp. The arrangement of the sclerosepta corresponds to the tissue septa of the polyp. At the periphery, a low cup, or theca, rises from the basal plate, surrounding the lower part of the polyp. In many colonial forms, the skeletal cups of individual polyps fuse into a unified colony skeleton. Living polyps are confined exclusively to the surface layer of the colony, whereas the entire bulk of the common skeleton (corallum) consists of the skeletons of past zooid generations. In terms of volume and mass, the skeleton vastly exceeds the living tissues, which cover The surface of the massive corallum as a thin layer. The colony shapes are diverse; moreover, within the same species, a colony may assume a spherical form in calmer waters, a branching form in flowing water, and so forth.

Fig. 99. Diagram of scleractinian Structure (from Marfenin): a — section of a colony; b — fragment of a transverse section through an individual polyp:

1 — expanded polyp; 2 — mouth; 3 — tentacles; 4 — soft septum; 5 — coenosarc; 6 — retracted polyp; 7 — scleroseptum; 8 — layer of living polyps; 9 — dead polyps (bare skeleton); 10 — basal plate (tabula); 11 — corallite; 12 — coenosarc cavity; 13 — acontia;

14 - mesenterial filaments; 15 - pharynx; 16 - pores in sclerosepta; 17 - sclerotheca; 18 - mesoglea; 19 - gastrodermis; 20 — epidermis

Scleractinians feed on what is brought to them by ocean currents: small planktonic organisms and organic particles; in addition, they absorb Amino Acids and Peptides directly from the water. Scleractinian corals also obtain nutrients from symbionts—zooxanthellae (single-celled flagellates *Symbiodinium microadriaticum*). Zooxanthellae receive carbon dioxide as well as nitrogen and phosphorus compounds from the corals, which are necessary for Photosynthesis, while in return, the corals receive organic matter and oxygen from them. It has been proven that symbionts are essential for The formation of the coral skeleton.

Madreporarian corals inhabit shallow depths (up to 50 m), where symbionts are provided with intense illumination. This is where coral reefs are formed—massive settlements of marine organisms possessing a hard calcareous skeleton. Madreporarian corals form the foundation of such a reef, though soft corals, hydrocorals, sedentary polychaetes, mollusks, bryozoans, and calcareous algae also settle here. Reef-building corals require a sufficiently high and relatively stable water Temperature (no lower than +20° C), normal oceanic salinity (35 ‰), and bright light; therefore, coral reefs are distributed exclusively in the tropics.

A distinction is made between fringing reefs, which border the coastline; barrier reefs, which extend parallel to the shore at some distance from it; and atolls, which are ring-shaped coral islands. There are also coral banks that form

on shallows, alongside dense coral communities in lagoons and bays. In the Indian and Pacific Oceans, there are hundreds of atolls ranging from 1 to 160 km or more in diameter. Coral polyps have formed reefs, atolls, and islands with a total area of about 27 million km². The Great Barrier Reef, which stretches for 2,000 km along the eastern coast of Australia, reliably protects it from erosion by Pacific Ocean waves. Most Pacific island nations are located on coral islands.

A coral reef is a unique ecosystem comprising, alongside corals and other reef-building organisms, a vast array of diverse animals and plants. This gives rise to highly productive, multi-species underwater communities upon which the overall productivity of the World Ocean largely depends. However, coral reefs are rather vulnerable biocenoses capable of surviving only within a fairly narrow range of environmental conditions. Today, the existence of coral reefs is threatened by rising near-bottom water temperatures, water pollution, or outbreaks of coral-eating animals. For instance, the rapid population growth of the crown-of-thorns starfish (Acanthaster planci) could have caused the destruction of the Great Barrier Reef had it not been for the timely measures taken by the Australian government.

Limestone, which makes up coral reefs, serves as a building material for the populations of tropical coastal countries. In addition, the skeletons of madreporarian corals are used to pack water filters, as well as for polishing and grinding wood and metal products. Fossilized coral skeletons are used in geology as index fossils to determine the age of sedimentary deposits.