Sea anemone

Sea anemone
	; Sea anemone at the Monterey Bay Aquarium
Scientific classification
Kingdom:	Animalia;
Phylum:	Cnidaria;
Class:	Anthozoa;
Subclass:	Hexacorallia;
Order:	Actiniaria;
Diversity
	46 families
Suborders
	Endocoelantheae; Nyantheae; Protantheae; Ptychodacteae

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The 49th plate from Ernst Haeckel's Kunstformen der Natur, 1904, showing various sea anemones classified as Actiniae

Sea anemones are a group of water dwelling, predatory animals of the order Actiniaria; they are named after the anemone, a terrestrial flower. As cnidarians, sea anemones are closely related to corals, jellyfish, tube-dwelling anemones, and Hydra.

[edit] Anatomy

A sea anemone is a polyp, attached at the bottom to the surface beneath it by an adhesive foot, called a pedal disk, with a column shaped body ending in an oral disk. The mouth is in the middle of the oral disk, surrounded by tentacles armed with many cnidocytes, which are cells that function as a defense and as a means to capture prey. Cnidocytes contain nematocyst, capsule-like organelles capable of everting, giving phylum Cnidaria its name.^[1] The cnidae that sting are called nematocysts. Each nematocyst contains a small vesicle filled with toxins—actinoporins—an inner filament and an external sensory hair. When the hair is touched, it mechanically triggers the cell explosion, a harpoon-like structure which attaches to organisms that trigger it, and injects a dose of poison in the flesh of the aggressor or prey. This gives the anemone its characteristic sticky feeling.

The poison is a mix of toxins, including neurotoxins, which paralyze the prey, which is then moved by the tentacles to the mouth/anus for digestion inside the gastrovascular 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 predation, it has been suggested that actinoporins could act, when released in water, as repellents against potential predators.^{[citation needed]} Anemonefish, small banded fish in various colors, are not affected by their host anemone's sting and shelter from predators within its tentacles.^[2]

The internal anatomy of anemones is simple. There is a gastrovascular cavity (which functions as a stomach) with a single opening to the outside which functions as both a mouth and an anus: waste and undigested matter is excreted through the mouth/anus, which can be described as an incomplete gut. A primitive nervous system, without centralization, coordinates the processes involved in maintaining homeostasis as well as biochemical and physical responses to various stimuli. Anemones range in size from less than 1¼ cm (½ in) to nearly 2 m (6 ft) in diameter.^[3] They can have a range of ten tentacles to hundreds.

The muscles and nerves in anemones are much simpler than those of other animals. Cells in the outer layer (epidermis) and the inner layer (gastrodermis) have microfilaments that group into contractile fibers. These fibers are not true muscles because they are not freely suspended in the body cavity as they are in more developed animals. Since the anemone lacks a skeleton, the contractile cells pull against the gastrovascular cavity, which acts as a hydrostatic skeleton. The anemone stabilizes itself by shutting its mouth, which keeps the gastrovascular cavity at a constant volume, making it more rigid.

[edit] Life cycle

Asexual reproduction of sea anemone via budding.

Unlike other cnidarians, anemones (and other anthozoans) entirely lack the free-swimming 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.

Anemones tend to stay in the same spot until conditions become unsuitable (prolonged dryness, for example), or a predator attacks them. In that case anemones can release themselves from the substrate and use flexing motions to swim to a new location.

The sexes in sea anemones are separate for some species while some are hermaphroditic. Both sexual and asexual reproduction may occur. In sexual reproduction males release sperm to stimulate females to release eggs, and fertilization occurs. Anemones eject eggs and sperm through the mouth. The fertilized egg develops into a planula, which settles and grows into a single polyp. Anemones can also reproduce asexually, by budding, binary fission (the polyp separates into two halves), and pedal laceration, in which small pieces of the pedal disc break off and regenerate into small anemones.

[edit] See also

Sea anemone dermatitis

[edit] Gallery

Sea anemone at Cape Arago, Oregon	Sea anemone in process of cloning (longitudinal fission)	Colonial zebra anemone from East Timor.	Actinia equina
Sea anemones	Sea anemones, Anthopleura sola engaged in war for territory.	Deep–sea anemone	Sea anemones in a "mini-reef" marine aquarium
Venus flytrap sea anemone	A shrimp living with a Beaded sea anemone	An Anemonefish and an Anemone
			A porcelain crab living with an anemone, probably Entamacea quadricolor
Colonial anemones attached to black coral from East Timor.	Anthopleura sola is in process of consuming a "by the wind sailor", Velella velella	A nudibranch, species Aeolidia papillosa starts to eat a sea anemone.	Hundreds of anemones at low tide - they all are clones.
Close-up of anemone.	This anemone and hermit crab have developed a symbiotic relationship.	A cluster of several small Sea Anenomes growing together.