MATUS, D.S.*; HUBER, J.L.; HALANYCH, K.M.; MARTINDALE, M.Q.; University of Hawaii; Exelixis, Inc.; Woods Hole Oceanographic Institution; University of Hawaii: The Phylogenetic Position of the Chaetognaths: A Molecular Approach Using Developmental Regulatory Genes

Chaetognaths, or arrow worms, are marine vermiform animals that are ubiquitous and abundant members of oceanic zooplankton communities.�They are active predators possessing a torpedo-like, coelomate body with lateral and caudal fins and are characterized as hermaphroditic undergoing direct development, lacking either a trochophore-like (i.e. lophotrochozoan) or dipleurula-like (i.e. deuterostome) larval stage. Historically, several features have allied them with deuterostomes, including a tripartite adult body plan, post-anal tail, and developmental features (e.g., radial cleavage, enterocoely, and posterior fate of the blastopore).�Molecular rDNA and �total evidence� studies have failed to support the inclusion of the chaetognaths within Deuterostomia.� These and other studies have not been able to discriminate whether chaetognaths are basal protostomes, ecdysozoans or lophotrochozoans.�Here we show that the complement of Hox/ParaHox genes from the pelagic chaetognath, Flaccisagitta enflata points towards placing these animals within the Ecdysozoa, the most diverse and best-studied group of metazoans.� This conclusion is supported by identification of diagnostic amino acid motifs as well as multiple methods of phylogenetic inference.�Although Ecdysozoan lineages (e.g., arthropods and nematodes) typically molt, chaetognaths do not. The inclusion of chaetognaths in this group alters our perception of the evolutionary plasticity of metazoan body plans and will help clarify developmental, morphological, and genomic evolution in this highly successful clade of animals.