Abstract
Statistical phylogenetic analyses of one-hundred-and-eleven 5.8S and partial-28S ribosomal DNA sequences (total aligned length = 434 nucleotides) including jellyfishes representing approximately 14% of known scyphozoan morphospecies (21% genera, 62% families, and 100% orders) are presented. These analyses indicate stauromedusae constitute a fifth cnidarian class (Staurozoa) basal to a monophyletic Medusozoa (= Cubozoa, Hydrozoa, and Scyphozoa). Phylogenetic relationships among the medusozoans are generally poorly resolved, but support is found for reciprocal monophyly of the Cubozoa, Hydrozoa, Coronatae, and Discomedusae (= Semaeostomeae + Rhizostomeae). In addition, a survey of pairwise sequence differences in Internal Transcribed Spacer One within morphospecies indicates that scyphozoan species diversity may be approximately twice recent estimates based upon morphological analyses. These results highlight difficulties with traditional morphological treatments including terminology that obfuscates homologies. By integrating molecular phylogenetic analyses with old and new morphological, behavioural, developmental, physiological, and other data, a much richer understanding of the biodiversity and evolution of jellyfishes is achievable.