Overview

Because they have no skeleton and do not participate in the building of reefs, sea anemones and their kin (Class Anthozoa; Orders Actiniaria, Zoanthidea, Corallimor-pharia Ceriantharia and Antipatharia) are not as intensely studied as their relatives, the hard corals (Order Sclerac-tinia in the same class). Nevertheless, they are prominent, and in some cases well-known, members of coral reef communities. The largest sea anemones, reaching almost a metre in diameter, are hosts to anemone fish, crustaceans, and various invertebrates that live among the anemones' tentacles in fascinating symbiotic relationships. Other anemones play guest rather than host and gain mobility by attaching to molluscs, crabs or shells housing hermit crabs. Zoanthids and corallimorphs can be abundant occupiers of space, sometimes when this is made available by bleaching or other damage to corals, and they too may have symbiotic associations with many other reef organisms, thus securing space in the reef column, protection from predators and/or access to food and energy sources.

Like hard and soft corals, the members of these groups have life cycles involving a short larval stage and a long polyp phase. The adult occurs as a single polyp in

Actiniaria (sea anemones) and Ceriantharia (tube anemones) and usually, but not invariably, as a colony or clone of interconnected polyps in Corallimorpharia (jewel anemones) and Zoanthidea (zoanthids). Antipatharia (black corals) are exclusively colonial. The major external characteristics of the polyp are a column, an oral disc bearing the mouth, siphonoglyph(s) (flagellated furrows that direct water currents into the gastrovascular cavity) and tentacles, and (in solitary forms) the pedal disc or bulb allowing attachment or anchorage. In colonial forms polyps are joined by basal stolons or common coenen-chyme. Main internal characters include the actinophar-ynx leading from the mouth and siphonoglyphs, and the paired mesenteries that may be complete or incomplete (reaching or not reaching the actinopharynx at their upper limit) and on which the gonads develop. (See Fig. 19.1 for examples of these characters in various orders.)

Animals from these orders contribute greatly to the taxonomic and ecological diversity of the GBR and to the dominant role of the Anthozoa in the coral reef environment. They are mostly carnivorous, feeding on tiny planktonic or benthic animals or particulate matter, and sometimes utilising symbiotic dinoflagellates in a

Figure 19.1 Anatomical features. A-D, Actiniaria: A, diagrammatic sea anemone internal features; B, external features (Anthopleura handi); C, external features, Edwardsiidae; D, diagramatic TS through actinopharynx region. E-F, Cerian-tharia: E, external features; F, TS through actinopharynx region. G-H, Zoanthidea: G, external features; H, TS through actinopharynx region. A, acontia; Act, actinopharynx; Cap, capitulum; CM, complete mesentery; DD, dorsal directives; Ect, ectoderm; IM, incomplete mesentery; St, stomata; MF, mesenterial filament; Mes, mesoglea; OD, oral disc; PD, pedal disc; P, physa; RM, retractor muscle; S, scapus; Sca, scapulus; Sip, siphonoglyph; Sph, sphincter; T, tentacles; VD, ventral directive. (Images redrawn from various sources by W. Napier.)

Figure 19.1 Anatomical features. A-D, Actiniaria: A, diagrammatic sea anemone internal features; B, external features (Anthopleura handi); C, external features, Edwardsiidae; D, diagramatic TS through actinopharynx region. E-F, Cerian-tharia: E, external features; F, TS through actinopharynx region. G-H, Zoanthidea: G, external features; H, TS through actinopharynx region. A, acontia; Act, actinopharynx; Cap, capitulum; CM, complete mesentery; DD, dorsal directives; Ect, ectoderm; IM, incomplete mesentery; St, stomata; MF, mesenterial filament; Mes, mesoglea; OD, oral disc; PD, pedal disc; P, physa; RM, retractor muscle; S, scapus; Sca, scapulus; Sip, siphonoglyph; Sph, sphincter; T, tentacles; VD, ventral directive. (Images redrawn from various sources by W. Napier.)

similar manner to the hard corals. Their attractive radially symmetrical form and dramatic colouration, combined with relatively simple environmental requirements, at least in some cases, make this group very popular with the aquarium trade. Because of this, some are well known to hobbyists and their food and habitat requirements and even breeding and dividing patterns are recorded on numerous web pages, although scarcely documented in the scientific literature.

Whereas the classification and identification of Scle-ractinia and Alcyonaria are primarily based on skeletal elements (solid skeleton in hard corals and elaborate spinules in soft corals), other features of the polyp, both macro- and microscopic, must be used for these predominantly skeleton-free orders. Valid identification of smaller specimens often requires verifying details of external and internal anatomy under a dissecting microscope as well as histological preparation of thin sections and tissue squashes for high power microscopic study. Fortunately for the field observer, many coral reef species have large size, bright colouration, unique habitat preferences or typical associates that allow for field identification with reasonable certainty.

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