Overview of sea anemones

Abstract

To romantics they are Gosse’s ‘blossomed beauties’ and Roughley’s ‘flowers of the reef’, but despite their botanical common name, sea anemones are voracious animals — Dalyell’s ‘fell devourers of whatever they can overpower’. They belong to the Anthozoa, one of four extant classes (Figure 1.1) within the phylum Cnidaria (tentacle-bearing Radiata having intrinsic nema-tocysts [cnidae]: Hyman, 1940), the simplest animals among the Eumetazoa. All classes of Cnidaria are evolutionarily derived from a sessile polyp. The strictly marine Anthozoa (including ‘anemones’, and hard and soft corals) are distinguished from other Cnidaria by their lack of a medusoid stage, being exclusively polypoid in body form. Also, the anthozoan polyp’s coelen-teron (body cavity) is partially divided into chambers by radially arranged mesenteries (not septa) projecting inward from the column wall, and a laterally compressed, tubular actinopharynx extends into the centre of the coelen-teron (Figure 1.2). The Anthozoa are viewed by Werner (1973) as having diverged from the other classes and their tetramerous stem form quite early in the evolution of the Cnidaria. Alternatively, Grasshoff (1984) argues that the Anthozoa are the stem of the Cnidaria, the tetraradial construction of the other classes being a secondary simplification related to the production of medusae.