Sea anemones

Scientific name : Hexacorallia

With the general name of sea anemones, we define all the soft bodied exacorals, without skeleton. The phylum is Cnidaria, including animals with two very basic possible growth stages, polyp or jellyfish. See below a description of the typical polyp anatomy. Cnidaria are divided into 4 classes (Hydrozoa, Sciphozoa, Cubozoa and Anthozoa). Anthozoans include the 2 subclasses Hexacorallia and Octocorallia (distinguished by the number of tentacles). Based on the name, exacorallia should have 6 tentacles, actually they have usually a larger number (multiple of 6). They includes Scleractinia or hard corals, Antipatharia or black corals, and the 4 soft bodied orders of Actiniaria, Zoanthidea, Corallimorpharia, Ceriantharia, dealt below. An anemone is basically the typical polyp: a small sac, attached to the bottom by an adhaesive foot, with a column shaped body (1) ending in an oral disc (2). The mouth is in the middle of the oral disc, surrounded by the tentacles (3). The internal anatomy 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. Tentacles are provided with a terrible weapon: the cnidoblasts, stinging cells. Each cnidoblast contains a small vescicle filled with toxins, 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 (active on the nervous system). Our immediate reaction is usually pain and hitching, small size preys are paralyzed. Anemones are found in every marine environment. The large sea anemones (ord. Actiniaria) and the Corallimorpharia are often found in lagoon, back reef, reef front, because they live in symbiosis with zooxanthellae. Zoantidea and other Actiniaria are more abundant along the external reef and deep reef. Ceriantaria normally prefer lagoon and bay environments. All those animals feed mostly on plankton and particulated matter, even if they can capture larger preys. The species with zooxanthellae take advantage from the photosintetic production of the algae. The closest group is the Scleractinia or hard corals.The more obvious difference is the hard carbonate skeleton of the latter. Only in some scleractinians the skeleton is concealed by living tissues and expanded tentacles also during the day: these particular species resemble sea anemones.