Systematic relationships of the urochordates based on partial 18S rDNA sequences

Abstract

Systematic relationship of 13 species representing seven orders of anthozoans, and one hydrozoan as an outgroup, were investigated. Distance and parsimony methods were used to analyze nucleotide sequence data obtained from the 18S rDNA. The inferred phylogeny indicates that two orders, Ceriantharia and Antipatharia within subclass Ceriantipatharia, are not closely related to each other. Instead, the former appears to be the most ancestral group among anthozoans but the latter is more closely related to order Actiniaria within subclass Zoantharia. The result also indicates that the actiniarian may not be a monophyletic group, because Epiactis japonica is related more to order Scleractinia. Subclass Octocorallia appears to be a monophyletic group, within which order Pennatulacea diversed at first, and then orders Alcyonacea and Gorgonacea diversed later.