SEA ANEMONES
CLASSIFICATION
Sea anemones are invertebrates, or animals lacking backbones, in contrast with fishes, which are vertebrates. Over 95% of all the kinds of animals in the world are invertebrates, most of them insects.
Primitive animals, anemones belong to the phylum variously known as Cnidaria (with a silent "c") or Coelenterata ("se-len-ter-a'-ta"). The former name alludes to the cnidae, or nematocysts, that are manufactured by all members of this phylum, and only by them. The latter means "hollow gut," referring to the single body cavity that serves as stomach, lung, intestine, circulatory system, and everything else. There is but one opening (the mouth) into this cavity, through which all water, food, and gametes pass in and out. It is surrounded by few or many tentacles, which are finger-like or filamentous projections, typically studded with nematocysts. They are active in capturing food and transferring it to the mouth, and may be used defensively, too.
Members of Class Anthozoa (which also includes hard and soft corals), sea anemones live attached to firm objects, generally the sea floor, or embedded in its sediments. An anemone's mouth points generally away from the substratum, and is surrounded by relatively short tentacles. Unlike most other anthozoans, sea anemones lack skeletons of any sort and are solitary. Anthozoans such as corals commonly exist as colonies, with many anemone-like individuals attached to one another. Each cylindrical individual is called a polyp. Members of the other three cnidarian classes may exist as polyps, but additionally (or exclusively) as medusae (singular is medusa). A medusa is little more than an upside- down polyp lacking a skeleton, free to swim in the open sea, with somewhat lengthened tentacles -- in short, a jellyfish.
By contrast with their fish symbionts, the 10 host anemones are not all closely related to one another. Belonging to anthozoan Order Actiniaria (hence the term "actinian"), they are members of three different families. The Actiniidae, of which Entacmaea and Macrodactyla are members, is the largest family of sea anemones, and that to which most common, temperate, shore species belong. The exclusively tropical Stichodactylidae, with genera Heteractis and Stichodactyla, is the main host family. Also tropical, Thalassianthidae contains three genera, including Cryptodendrum. Unlike the fish, in which all members and only members of damselfish subfamily Amphiprioninae are symbiotic, most members of families Actiniidae and Thalassianthidae do not participate in symbioses with fishes, and there are also some non-symbiotic stichodactylids.
IDENTIFYING SEA ANEMONES
Nearly all publications on anemone identification are technical. They deal with features such as nature of the animal's muscles, size and distribution of nematocysts, and arrangement of tentacles in relation to internal anatomy. Such characters, which are retained in preserved specimens, require dissection and histological examination to study. They are used partly because most species from the tropics (especially prior to the 20th century) and from deep seas (until the recent advent of submersibles) were originally known only from preserved specimens. We believe actinians can be identified in the field, based on appearance and habitat, although some experts consider nematocyst analysis essential.
A sea anemone is an extremely simple animal. It may be thought of as a cylinder that is closed at both ends. The lower, or pedal, end may be pointed for digging into soft sediments. In anemones of most families, like the host actinians, it is adapted as a pedal disc, which attaches firmly to a solid object like a coral branch or rock (often buried in sediment). In the center of the oral disc, at the opposite, unattached end, is the mouth. Hollow tentacles, arising from the oral disc, surround it. They may be few or many, and arrayed in radial rows or in circlets. Their form is highly diverse -- short or long, thin or thick, pointed or blunt, globular or tree-like. Tentacle number, form, and arrangement are very important in distinguishing genera and species. The cylindrical column (body) of anthozoans is not completely hollow, the name Coelenterata notwithstanding. In sea anemones, vertical partitions (mesenteries) extend from the column wall across the central space part or all of the way to the throat (actinopharynx). Viewed in cross-section, the column therefore resembles a wheel with spokes. Mesenteries also attach on the underside of the oral disc (the radiating lines of attachment may be visible in an animal that is well expanded, has few tentacles, and/or has a thin oral disc), and tentacles arise between them.
In animals with few tentacles, much of the oral disc, the mouth, and sometimes even the upper end of the throat, into which the mouth opens, are visible. The disc can be radially or circularly patterned; the mouth, which can be circular or elongate, may be elevated on a conical projection and may differ in colour from the oral disc.
The column is appropriately tapered to accommodate a pedal and/or oral disc of smaller or greater diameter than itself. In most species of host actinian, the oral disc is much broader than the column. The column, which may be patterned (commonly splotches of colour or longitudinal stripes), can also bear specialized structures along part or all of its length. For example, some tropical anemones (but none that hosts clownfishes) have branched projections from the lower column. Most host actinians have, in the upper part, longitudinal rows of small warts (verrucae; singular is verruca) to which particles of gravel may adhere; commonly these are pigmented differently from the rest of the column.
Sea anemone colour pattern can be important for field identification, but colour itself, being highly variable in most actinians, is of little diagnostic value. Symbiotic algae may affect anemone (as well as coral) colour, either by imparting their own golden brown colour, or by stimulating the animal to produce a pigment that protects the algae against excessive sunlight. Therefore, anemones often blend in with corals and with sand, explaining how such large animals may be so difficult to detect in nature.
Presence or absence of verrucae is a character defining genera. Thus, all species in a particular genus do (e.g. Stichodactyla) or do not (e.g. Entacmaea) have verrucae. Arrangement of tentacles is also important in defining genera. There may be one tentacle per space between mesenteries (so that number of tentacles equals number of mesenteries attaching to the oral disc) or there may be more than one tentacle between each two mesenteries. Members of the family Actiniidae have one tentacle per space. Anemones of families Stichodactylidae and Thalassianthidae can have so many tentacles because up to several, radially arrayed rows of tentacles arise from alternate spaces (the endocoels), whereas only one tentacle arises from the other spaces (exocoels). The single tentacle is positioned at the very edge of the oral disc (margin). This arrangement may be obvious when the animals are well extended.
Keys and descriptions may not work well with captive animals. As explained in chapter 6, aquarium-kept anemones can lose their colour after a remarkably short time, probably because their algae do not thrive under artificial conditions, and tentacle shape may also change. Fish symbionts should not be used to identify anemones in captivity, as they can be in nature, because many fishes can acclimate to most host anemones.