Ordo Actiniaria

R. Hertwig, 1882

The order Actiniaria contains the true sea-anemones; these are solitary Hexacorallia which never possess a calcareous skeleton. Base rounded or forming an adherent disc, with or without basilar muscles. Column smooth or bearing various special organs or periderm, frequently differentiated into regions (see below), only rarely with longitudinal ectodermal muscle. Tentacles usually arranged in cycles, more than eight present in adults, often very numerous. In all British genera except Aureliania only one tentacle arises from each endocoel and exocoel. Mesenteries typically arranged in regular pairs and cycles but many variations occur; in most species at least some imperfect mesenteries are present. Ciliated tracts usually present on the filaments. Musculature variable in extent and development, often strong; sphincter, basilar and parietobasilar muscles may be present or absent, retractors always present. All categories of cnidae described may occur but rarely, if ever, in a single species.

The range of morphological variation within this order is wide; therefore a clear understanding of the characters peculiar to the group is necessary in order to obtain an accurate identification. Among the most important of these are the regions of the column and the occurrence of acontia, the latter being a feature that is not always easily determined in living specimens. Further, the structure of the patterns found on the discs and tentacles of many species is often characteristic, many species being instantly recognizable by this feature alone. Other characters, occurring within families rather than generally, will be described in the appropriate family introduction.

Divisions of the column: In many anemones the column is divided into regions of varying structure, externally appearing as differences in colour or texture. These regions are important systematic features, especially where external characters alone are used for identification.

Base: Typically this forms a flattened transverse disc, capable of more or less firm adhesion to a solid substratum. In the thenarian anemones this disc is sharply marked off from the column by a permanent corner, the limbus, which is produced by the combined action of the basilar and parietobasilar muscles. The limbus is distinct even when the anemone is not attached to a solid surface. In the Protantheae and other groups lacking these muscles a true, permanent limbus is absent, although the basal disc may be slightly differentiated from the column.
The athenarian anemones, which mostly burrow into soft substrata, lack basilar muscles and have a rounded base known as a physa, the structure of which varies significantly from genus to genus. The physa usually functions as a burrowing organ or as an anchor when the anemone is buried. It is very contractile but not retractile and is capable of being inflated into a bulbous structure wider than the column. The surface of the physa often bears numerous minute adhesive spots, rugae, which are often not easily seen unless attached to some solid object. Rugae may also occur on the columns of some anemones.

Scapus: This is the longest and most important division of the column, occurring directly above the base whatever its form. It is usually the thickest part of the body wall, having a firmer consistency than elsewhere. The scapus is usually contractile but less so than other regions, and only its uppermost part, if any, is retractile; regions above the scapus can usually be retracted into it. Many genera bear localized structures on the scapus, such as verrucae and acrorhagi, suckers, nemathybomes, cinclides, solid mesogloeal tubercles, or periderm.

Above the scapus various different regions occur. Often the upper edge of the scapus is folded inwards to form a permanent rim, the parapet, enclosing a circular groove or fosse, this structure is easily observed in the common beadlet anemone Actinia equina and occurs in many Actiniidae, Aurelianiidae, Metridiidae and Diadumenidae, a similar arrangement is found in the Zoantharia. If a sphincter muscle is present it is invariably located in the parapet region. Between the fosse and the edge of the oral disc a region of delicate texture - similar to the texture of the tentacles - is present; this is the capitulum, which is usually narrow and concealed by the parapet but in Metridium and most Diadumenidae is relatively long and conspicuous.

Alternatively the parapet, fosse and capitulum may be absent: instead a Smooth region of variable thickness, the scapulus, runs from the upper limit of the scapus to the edge of the disc. In British anemones a scapulus always occurs in genera which habitually possess periderm on the scapus. The scapulus is usually softer and more contractile than the scapus, often different in colour and texture, and never bears periderm. This region is present in most Hormathiidae and Edwardsiidae and in a few other genera. In the genus Edwardsia a very narrow capitulum occurs between the scapulus and the disc but this can be ignored for practical purposes. Similarly a slightly differentiated region between the scapus and disc in Nematostella and Halcampa is a capitulum rather than a scapulus but the distinction between these is immaterial.

In some genera no regionalization of the column, other than base and column occurs. Some of these may, in partial contraction, produce a parapet-like fold at the top of the column; this is not permanent and should not be confused with a true parapet and fosse. This is a frequent occurrence in species with strong musculature, such as Stomphia, Bolocera, Adamsia and Amphianthus.

Cinclides: Many anemones, particularly Acontiaria and Athenaria, possess tiny perforations or organized weak spots in the body wall, usually in the scapus or physa. These are cinclides which function as safety valves for the expulsion of water when the anemone contracts sharply, thus avoiding the risk of rupturing the body wall. In the Acontiaria cinclides also serve for the emission of acontia, probably a secondary but nevertheless useful function. Cinclides may be visible as tiny holes or dark spots, often raised on low mounds in some groups, e.g Hormathiidae and Diadumenidae, but in others such as Metridium they may be quite difficult to discern. The position of the cinclides is characteristic of some families, e.g. Aiptasiidae, where they are limited to the middle region of the column, but unfortunately this is not always an easy character to ascertain.

Acontia: In British anemones they are found only in the Acontiaria. Some species produce them readily when prodded or when being removed from the substratum but others are not so accommodating. In the latter case it may be necessary to perform a simple dissection to ascertain their presence. Care should be taken not to mistake mesenteric filaments for acontia; the former are invariably present but are always attached along one side to the edge of a mesentery. Most shore-dwelling acontiarians produce their acontia fairly readily and it is only certain offshore hormathiids which are likely to cause difficulties. With a little experience it is not difficult to recognize acontiarians, even if the acontia are not immediately evident.

Macrocnemes and microcnemes: In some genera the distinction between large perfect mesenteries and small, often tiny, imperfect ones is very sharp, with no mesenteries of intermediate size or development occurring. Such clearly defined large and small mesenteries are known as macrocnemes and microcnemeS respectively. Usually the macrocnemes bear strong retractor muscles, gonads and filaments, whereas microcnemes may lack all of these structures. Microcnemes may also be very short, restricted to the most distal part of the column. Typically this distinction into macro- and microcnemes occurs in elongated anemones, usually burrowers, such as Edwardsiidae and Halcampidae. The chambers between adjacent macrocnemes, irrespective of microcnemes, are known as macrocoeles.

Disc patterns: Many anemone species bear a complicated pattern on their discs and tentacles, which is often a useful guide to identification. These patterns are usually related to the mesenteric arrangement as they consist of a series of elements, either pigmented or dark areas caused by translucency, repeated on each radius of the disc. Generally the primary radii carry the basic pattern which is repeated on the secondaries but slightly further away from the mouth, again being further out on radii of succeeding cycles. Usually slight modifications of the pattern occur on different cycles of radii. Sagartia troglodytes provides a good example of the manner in which such patterns are built up. Commonly the typical pattern may be incomplete and, at first glance, atypical; a closer examination should reveal, however, that perhaps one or two elements of the pattern are missing, altering the overall effect dramatically, but that the remaining elements are still typical of the species pattern, although sometimes displaced or distorted. S. troglodytes is a particularly variable species, a common variation being a plain disc with only the B-shaped markings at the tentacle-feet present; or various elements may be differently emphasized in different specimens. In spite of this wherever a pattern, or part of one, occurs, careful analysis of its component parts should establish its relationship with the typical pattern of the species.

In addition to variations caused by loss of pattern components, other types occur. Frequently the directive radii and tentacles are coloured differently from the rest - plain white, blackish, etc. -interrupting the overall pattern as a prominent streak of contrasting colour along the directive axis. This is a very common occurrence but has no systematic relevance. Similarly some sagartiid species may exhibit a rayed appearance caused by alternate segments of the disc being light and dark in colour, this may be regular or irregular and may or may not affect the related tentacles. In many hormathiids, and sometimes members of other families, irregular streaks or splashes of opaque white or other colour may overlie the basic pattern but rarely obscure it completely. All such variations have no systematic relevance.

Illustrations are provided of all species which usually bear a constant, regular pattern; these figures have been built up from a study of many specimens and, hopefully, will provide a reasonably accurate guide. They must, however, be used with caution as aberrant specimens do occur; a successful identification relies on consideration of many characters, the disc pattern providing the final confirmation.

Anemones pose many problems in identification, not the least of which is the vast range of individual colour variation occurring within many species. However, in spite of this variation most species possess a distinctive and, to the experienced observer, instantly recognizable facies. This is a combination of many purely subjective characters which cannot be defined in concrete terms but is nevertheless a real and valuable feature. Once a species in all its varieties can be confidently identified without recourse to an anatomical examination the identification of other related and similar species becomes greatly simplified. Therefore, whenever a species is found in abundance, as commonly occurs on the shore, it is well worthwhile studying a large number of examples in the field in order to appreciate the range of variation, in both colour and form, that is possible within that species.