Phylogeny of Medusozoa and the evolution of cnidarian life cycles

The considerable morphological disparity present within Cnidaria is evidenced by delicate siphonophores, massive medusae and corals, feathery hydroids, interstitial polyps and box jellies possessing complex eyes. Despite this diversity, these relatively simple metazoans are united in possessing nematocysts, most probably as a result of common ancestry. Corroborating evidence that cnidarians are monophyletic within Metazoa comes from molecular sequence data of the small subunit (SSU) of the ribosome (Collins, 1998; Kim et al., 1999; Collins, 2000), as well as the large subunit (LSU) of the ribosome (Medina et al., 2001). Most phylogenetic analyses of Cnidaria have focused on determining the relationships among the four main taxa that compose it – Anthozoa, Cubozoa, Hydrozoa, and Scyphozoa (Werner, 1973; Salvini-Plawen, 1978; Brusca & Brusca, 1990; Meglitsch & Schram, 1991; Bridge et al., 1992, 1995; Schuchert, 1993). From this work, a consensus has emerged that Anthozoa is the sister group of the remaining cnidarians, which are collectively referred to as Medusozoa (Petersen, 1979), or less often as Tesserazoa (SalviniPlawen, 1978). Particularly convincing evidence for the monophyly of medusozoans is their shared possession of linear mitochondrial genomes (Bridge et al., 1992) and medusae. However, relationships among the major medusozoan groups remain contentious (Collins, 2000). There is a general correspondence between major taxonomic divisions and life-cycle differences within Cnidaria. Anthozoan ontogeny is most straight forward, involving a planula, settlement, and growth into a sessile polyp, which is the adult stage. Cubozoa, the most recently defined class of the phylum Cnidaria (Werner, 1975), is comprised of species that have planulae, which settle and develop into sessile polyps. The cubopolyp subsequently metamorphoses entirely into a single sexual medusa (Werner et al., 1971; Arneson & Cutress, 1976). Many hydrozoans possess planulae, polyps and medusae. Usually, the hydrozoan medusa develops from a tissue mass termed the entocodon and is budded laterally from polyps. Of all the cnidarian groups, however, hydrozoans have the greatest variation in life cycles and the polyp or medusa stages are entirely lacking for some groups (as for example in the Trachymedusae