Sea anemone
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{{Taxobox | color = pink | name = Sea anemones | image = Giant Green Anemone.gif | image_width = 250px | image_caption = Giant green anemone, Southern California | regnum = Animalia | phylum = Cnidaria | classis = Anthozoa | subclassis = Hexacorallia | ordo = Actiniaria | subdivision_ranks = Families | subdivision =
- Suborder Endocoelantheae
- Suborder Nyantheae
- Suborder Protantheae
- Suborder Ptychodacteae
Named after a terrestrial flower, the anemone, the sea anemone is a group of water-dwelling, filter feeding animals of the order Actinaria. As a cnidarian, it is closely related to coral and jellyfish. An anemone is basically the typical polyp: a small sac, attached to the bottom by an adhaesive foot, with a column shaped body ending in an oral disc. The mouth is in the middle of the oral disc, surrounded by the tentacles with stinging cells (cnidocytes) or cnidoblasts. Each cnidoblast contains a small vesicle filled with toxins - actinoporins, an inner filament and an external sensitive hair. When the hair is touched, it mechanically triggers the cell explosion, the extrusion of the filaments that injects a dose of poison in the flesh of the aggressor or prey. The poison is actually a mix of toxins, including neurotoxins, which serve to paralyze and capture the prey, which is then moved by the tentacles to the mouth for digestion inside a central cavity. Actinoporins have been reported as highly toxic to fish and crustaceans, which may be the natural prey of sea anemones. In addition to their role in preying, it has been suggested that actinoporins could act, when released in water, as efficient repellents against potential predators.
Image:Common clownfish.jpg The internal anatomy of anemones is very simple. There is a stomach, no anus: what is undigested is then rejected through the mouth. A primitive nervous system, without centralization and a true brain, allows mechanical reactions to any stimulations. Anemones range in size from less than 1¼ cm (½ in) to nearly 2 m (6 ft) in diameter. They can have a range of 10 tentacles to hundreds.
Unlike other cnidarians, anemones (and other anthozoans) entirely lack the medusa stage of the life cycle. The polyp produces eggs and sperm, and the fertilized egg develops into a planula that develops directly into another polyp.
Few anemones are parasitic to marine organisms. Anemones tend to stay in the same spot unless they are unhappy with that location, or a predator is attacking them. In the case of an attack, anemones can uproot themselves and swim away to a new location.
The sexes in sea anemones are separate. Both sexual and asexual reproduction may occur. In sexual reproduction males release sperm which stimulates females to release eggs, and fertilization occurs. The eggs or sperm are ejected through the mouth. The fertilized egg develops into a planula, which finally settles down somewhere and grows into a single anemone. They can also reproduce asexually by budding, binary fission, and pedal laceration. Asexual reproduction involves pulling apart into two halves, or, in some species, small pieces of the pedal disc break off and regenerate into a small anemone. Laceration is a process of fragmentation of the basal disk, or by pulling itself into two parts.
Image:Anemone.bristol.750pix.jpg Image:Ocean reef.jpg
Image:Haeckel Actiniae.jpgOther close relations to the sea anemone are the solitary, tube-dwelling anemones and the hydras.
The sea anemone has a foot which in most species attaches itself to rocks or anchors in the sand. Some species attach to kelp and others are free-swimming. Although not plants and therefore incapable of photosynthesis themselves, many sea anemones form an important symbiosis with certain single-celled green algae species which reside in the animals' gastrodermal cells. These algae may be either zooxanthellae, zoochlorellae or both. The sea anemone benefits from the products of the algae's photosynthesis, namely oxygen and food in the form of glycerol, glucose and alanine; the algae in turn are assured a reliable exposure to sunlight, which the anemones actively maintain. The preponderance of species inhabit tropical reefs, although there are species adapted to relatively cold waters, intertidal reefs, and sand/kelp environments.
Some sea anemones form symbiotic relationships with crabs and anemone fish, also known as clownfish. In the former situation, anemones will either attach or be attached to the shell of a hermit crab (by the crab's own volition), providing additional protection for the crab and allowing the anemone to eat scraps when the crab feeds. A similar relationship can be formed between a sea anemone and a clownfish. The clownfish presses itself into the anemone, living comfortably within the stinging tentacles: This is possible because of a protective mucus that covers the clownfish. The clownfish benefits from this symbiotic relationship because it is protected by the anemone. The anemone benefits because the anemone gets food scraps from the clownfish.
Most Actinaria do not form hard parts that can be recognized as fossils but a few fossils do exist; Mackenzia, from the Middle Cambrian Burgess Shale of Canada, is the oldest fossil identified as a sea anemone.
External links
es:Anémona fr:Anémone de mer io:Aktinio he:שושנת ים nl:Zeeanemoon ja:イソギンチャク pl:Ukwiał pt:Anêmona sk:Sasanky fi:Merivuokot sv:Anemon zh:海葵