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Skeletons and fission in Anthrozoan cnidarians: Inferences from molecular phylogenies

Jonathan Geller, Ph.D.
Moss Landing Marine Laboratories

Wednesday, March 8, 2000
3:00 p.m.—Pacific Forum

Scleractinian corals have traditionally been thought to be derived from ancestors that resemble modern sea anemones which lack hard skeletons. It is generally assumed, therefore, that the lack of skeletons in modern actiniarian sea anemones reflects an ancestral character state. Cadet Hand (1966) suggested that paired mesenteries in actiniaria reflect the presence of skeletal septa in ancestors, as in modern hard corals. He thus hypothesized that Actiniaria evolved from scleractinia, and the lack of a skeleton is a derived characteristic. Here, I present support for Hand’s hypothesis from a molecular phylogeny of corals and sea anemones using mitochondrial 16S rRNA sequences.

Some species of sea anemones in the genus Anthopleura grow as solitary individuals while others undergo fission to form clones. The evolutionary origins of clonal growth in 13 species of Anthopleura were inferred from a molecular phylogeny derived from 395 bp of the mictochondrial 16S rRNA gene and 410 bp of the mitochondrial cytochrome oxidase subunit III gene. Sequences from these genes were combined and analyzed by minimum parsimony, maximum likelihood, and neighbor-joining methods. Trees from each method were similar, with strong support for eastern and western Pacific clades. Growth modes have evolved repeatedly—best trees from each method predicted a minimum of four gains or losses of clonality. Alternative trees in which clonal species were constrained to be monophyletic were significantly worse than best trees. Although clade composition was mostly consistent with geographic groupings, A. artemisia from California was included in the western Pacific clade. Likewise, A. midori from Japan was not placed in a clade containing other Asian congeners. The history of Anthopleura includes repeated shifts between clonality and solitariness, as well as intercontinental dispersal.

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