Fig. 1.

Phylogenetic positions and morphology of cnidarians used in the present study. a Molecular phylogeny of corals and sea anemones inferred using a maximum likelihood method based on the general nucleotide substitution model. The scale bar indicates 0.1 substitutions per site. b Goniastrea aspera (R): b1 A small isolated colony, which appears brown due to the zooxanthellae which it contains. Often this species occurs as larger colonies covering several meters in shallow water habitats that may sometimes be quite turbid. b2 Closeup of polyps showing cream-coloured lobes of the oral disc. b3 Closeup of the skeleton showing the complex skeletal structure underlying each polyp. c Fungia fungites (R): c1 The colony consists of a single polyp which is usually withdrawn during the day and expanded at night. c2 Closeup of the mouth area, covered by sometimes multicoloured living tissue. c3 The skeleton consists of closely spaced septa which support the living tissue. d Galaxea fascicularis (C): d1 A large encrusting colony. d2 Closeup of the polyps, which can be of diverse colours. Galaxea is unusual amongst corals in that the polyps are often extended during the day. d3 The Galaxea skeleton differs considerably from those of many other massive corals in that only thin layers of coenostium link the individual polyps. e Portion of a colony of Acropora millepora (C). f Colony of Acropora digitifera (C). g Colony of Porites lutea (C). h The sea anemone, Aiptasia pallida. i The starlet sea anemone, Nematostella vectensis. Photo credits are given in the Acknowledgements