The phylogenetic affinities of the chaetognaths: a molecular analysis

The chaetognaths, or arrowworms, constitute a small and enigmatic phylum ofmarine invertebrates whose phylogenetic affinities have long beenuncertain. A popular hypothesis is that the chaetognaths are the sistergroup of the major deuterostome phyla: chordates, hemichordates, andechinoderms. Here we attempt to determine the affinities of thechaetognaths by using molecular sequence data. We describe the isolationand nucleotide sequence determination of 18S ribosomal DNA from one speciesof chaetognath and one acanthocephalan. Extensive phylogenetic analysesemploying a suite of phylogenetic reconstruction methods (maximumparsimony, maximum likelihood, evolutionary parsimony, and two distancemethods) suggest that the hypothesized relationship between chaetognathsand the deuterostomes is incorrect. In contrast, we propose that thelineage leading to the chaetognaths arose prior to the advent of thecoelomate metazoa.