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 Hands 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
modes have evolved repeatedlybest 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
blooms- Are populations increasing globally in response to changing ocean conditions?
Last updated: December 19, 2000