Etymology
Actinia + -aria
Kingdom: Animalia
Phylum: Cnidaria
Class: Anthozoa
Subclass: Hexacorallia
Order: Actiniaria
Norwegian: anemoner
Characteristics:
The order of sea anemones, Actiniaria, includes many colorful species, some of them resembling the anemone, a terrestrial flower. Although there are a few pelagic species, the majority of sea anemones are polyps attached to some surface by an adhesive foot. The cylinder-shaped body ends in an oral disk, surrounded by tentacles. The tentacles are armed with stinging cells used to paralyze their prey. Like other members of the subclass Hexacorallia, they have a sixfold symmetry. The number of tentacles, for instance, is usually dividable by six.
Unlike many other cnidarians, the sea anemones lack the medusa phase in their life cycle. Eggs are fertilized by sperm and produce larvae. These attach to some surface to develop a new polyp. Additionally, many anemones are capable of asexual reproduction by budding - small pieces of the foot develop to new polyp, genetically identical with the offspring. Anemones belong to the group cnidarian animals and are closely related to corals and jellyfish. Anemones are typically a polyp with stinging cells in their tentacles. The stinging cells paralyze and capture prey, which is then moved with the aid of tentacles to the mouth and melted in a central body cavity. Anemones have a "base plate" that it uses to attach itself to rocks or similar surfaces. The foot is not permanently attached to the surface, so the anemone can move, albeit very slowly. Some species attach themselves to seaweed and some are free swimming. The larval stage is always swimming.
There are about 30 known species on the Swedish west coast, including several in shallow water. In the whole world, there are about 1200 species. Some anemones also live in symbiosis with crabs and anemone fishes. Those that live in symbiosis with crabs attach themselves to their shell and is there as a protection for the crab because it is poisonous. And in turn, the anemones can make use of leftovers as the crab unravels. The symbiosis with clown fish works well because the fish have a special mucous layer that protects them from the anemone's poisonous tentacles the anemone can get the food scraps from the fish, which is advantageous.
The mouth opening of higher sea anemones is oval or slit-like. The pharynx is strongly laterally compressed and has two siphonoglyphs. Only the described primitive species have only one underdeveloped siphonoglyph or it is absent altogether. The beating of the cilia of the siphonoglyph creates two currents of water: one directed inside the gastric cavity and bringing oxygen (in some sea anemones, food particles), and the other moving in the opposite direction and taking out carbon dioxide and excretion products.
The muscular system of sea anemones reaches a high level of development for coelenterates. The ectodermal system consists of longitudinal fibers lying in the tentacles and radial fibers around the mouth opening. The endodermal system consists of the annular musculature of the tentacles, oral disc, pharynx, body walls, and foot disc. On the gastric septa lie longitudinal muscle rollers.
The nervous system of sea anemones consists of an ectodermal network of nerve cells present in all parts of the body and an underdeveloped endodermal network covering only the gastric septa. Especially many nerve cells are concentrated at the bases of the tentacles and on the oral disc. However, this does not lead to the formation of a perioral nerve ring, since the nerve cells are very loosely located here. Another cluster of nerve cells is located near the sole. It is interesting to note that different parts of the body are particularly sensitive to certain stimuli. The sole, for example, is sensitive to mechanical irritations and does not perceive chemical ones. The oral disc, on the contrary, is very sensitive to chemical irritations and almost does not react to mechanical ones. Perhaps only the walls of the body and the tentacles react to mechanical, chemical and electrical stimuli, but the tentacles are much more sensitive to them than the walls of the body.
The common reaction of an anemone to irritation is to contract the body. At the same time, the oral disk and tentacles retract, and the body walls close over them, compressed by a special muscular ring. Anemones that lead a burrowing lifestyle, as Edwardsia described above, are quickly buried in the ground. With prolonged exposure to an irritant, sea anemones tend to crawl as far as possible from it.
Anemones do not form a skeleton, although the ectoderm of some species secretes a chitinoid cuticle covering the lateral surface of the body and the sole. Perhaps only in deep-sea sea anemones from the family Galatheanthemidae, leading an immobile, attached way of life, a strong cuticular sheath, which encloses the long worm-like body of anemones, takes on the character of a protective skeleton, similar to the ectodermal skeleton of most hydroid polyps. The dark brown protective sheaths of galatpeanthemids rise to a height of 2–3 to 150 mm. Above their mouth, about 1 cm in diameter, protrudes the upper part of the anemone's body with a corolla of numerous thin tentacles. Galateanthemids are one of the deepest coelenterates. They were first discovered several years ago, when a period of systematic exploration of the maximum depths of the ocean began. These anemones most often live on the bottom and slopes of deep oceanic depressions - the Kurile-Kamchatka, Philippine, Japanese and others - at a depth of 6-10 thousand meters. Their lifestyle has not yet been completely studied.
The body of anemones is sometimes very strong, although they are devoid of a skeleton. The fact is that the mesoglea of sea anemones usually reaches a significant development and often acquires the density of cartilage due to the appearance in it of a dense fibrous connective substance.
Sea anemones reproduce both asexually and sexually. However, asexual reproduction plays a much smaller role in them. Cases of budding in Actiniaria are generally very rare. More often there is a division of one individual into 2 and even into 3-6 unequal parts. Transverse division is noted only in primitive anemones Gonactinia. In G. prolifera, for example, it proceeds as follows: at a certain height, a corolla of tentacles first grows from the walls of the body, then the upper part laces off and separates from the lower. At the top, the sole is restored, and at the bottom, the oral disk and pharynx, as well as the second circle of tentacles, are formed. The second division of gonactinia sometimes begins before the first has ended.
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