Evolution & Systematics

The Cnidarian phyla is widely dispersed around the world with an estimated 10 000 species (Franzo et al 2012). The systematics of this phyla are still under much discussion with phylogenetic trees based on combinations of morphological and life history traits and molecular data with some research focusing on genes encoding toxins. At a basal level two classes are largely confirmed; Anthozoa and Medusozoa (Franzo et al 2012). Anthozoa life cycles involving a fertile polyp stage preceding any medusoid stage opposite to the medusozoa and containing circular mitochondrial DNA as opposed to straight for the medusozoa (Franzo et al 2012). Actiniaria, an order within Anthozoa and containing family Actiniidae with E. quadricolor is not well resolved molecularly.

Park et al (2012) used sequences for 13 protein-coding genes with five fossil calibration points to estimate divergence time within Cnidarian. The node for last common ancestor for Actiniaria is thought, with 95% confidence, to have diverged approximately 550 mya at the start of the Cambrian period (Park et al 2012). However, this node was based on only two species used to represent Actiniaria order. A more comprehensive schematic of Actiniaria was produced earlier in Daly et al (2003). This paper found that members of the family Actiniidae, situated within an unresolved clade within Anthozoa, could be differentiated by both morphological characters and substitutions in the commonly sequenced 16S gene in mitochondrial rDNA. More work will be needed in future to finalize the schematics and evolution history of this species.