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Graphical abstract Highlights • 17 mtDNA genomes spanning Ophiuroidea used to explore phylogeny and gene order evolution. • mtDNA data support monophyletic Ophintegrida and Euryophiurida. • Only 3 mtDNA gene order arrangements observed and all sampled Euryalida with same order. • Euryalid gene order likely ancestral for Ophiuroidea. Abstract Brittle stars are conspicuous members of benthic ecosystems, fill many ecological niches and are the most speciose of all classes of echinoderms. With high levels of biodiversity, elucidating the evolutionary history of this group is important. Understanding of higher-level relationships within Ophiuroidea has been aided by multilocus nuclear data and DNA barcoding. However, the degree of consistency between mitochondrial and nuclear data within ophiuroids remains unclear and deserves further assessment. In this study, 17 mitochondrial genomes spanning the taxonomic breadth of Ophiuroidea were utilized to explore evolutionary relationships through maximum likelihood analyses, Bayesian inference and comparative assessment of gene order. Our phylogenetic analyses, based on both nucleotide and amino acid residues, support recent findings based on multilocus nuclear data and morphology, in that the brittle star clades Ophintegrida and Euryophiurida were recovered as monophyletic with the latter comprising Euyalida, Ophiuridae and Ophiopyrgidae. Only three different arrangements of the 13 protein coding and 2 ribosomal RNA genes were observed. As expected, tRNA genes were more likely to have undergone rearrangement but the order of all 37 genes was found to be conserved in all sampled Euryalida and Ophiuridae. Both Euryalida and the clade comprised of Ophiuridae and Ophiopyrgidae, each had their own conserved rearrangement of protein coding genes and ribosomal genes, after divergence from their last common ancestor. Euryalida has a rearrangement of the two ribosomal RNA genes, rrnS and rrnL , in contrast to Ophiuridae and Ophiopyrgidae, which had an inversion of the genes nad1 , nad2 , and cob relative to Ophintegrida. Further, our data support the gene order found in all sampled Euryalida as the most likely ancestral order for all Ophiuroidea. [ABSTRACT FROM AUTHOR]

Mitochondrial genomes provide an additional data source for nuclear phylogenomics. Recently, the taxonomic classification of Ophiuroidea was changed dramatically based on a molecular phylogenetic analysis that utilized a huge nuclear dataset of transcriptome and target-capture approaches. However, the mitochondrial genome analysis of Ophiuroidea was not conducted sufficiently in depth to allow comparison with current phylogenetic relationships, especially for the Ophintegrida. In this study, eight mitogenomes were newly reported in Amphilepidida and Ophiacanthida. Phylogenetic analyses were undertaken based on the nucleotide sequences of 13 protein coding genes (PCGs), 22 tRNA, and two rRNA, and the amino acid translated sequences of 13 PCGs. The results of the phylogenetic analysis suggested that the amino acid translated sequences were more useful than the nucleotide sequences for the analysis of higher phylogenetic relationships. The mitochondrial gene order of 13 PCGs and two rRNA in Amphilepidida and Ophiacanthida was relatively conserved. However, more complex gene orders occurred in the phylogeny of Amphilepidida and Ophiacanthida, including 22 tRNA. The results of our study suggest that Ophiuroidea has undergone more complex gene rearrangements than other classes of echinoderms. [ABSTRACT FROM AUTHOR]

The fiords of southwestern New Zealand may promote the existence of genetically divergent populations as a result of geological and ecological isolation. The ophiuroidAstrobrachion constrictumlives on black coral in these fiords, and is thought to have a relatively short pelagic larval duration that may limit dispersal among populations. Samples ofA. constrictumwere collected from seven sites (four fiords) in Fiordland. Analysis of the mitochondrial COI gene using single-stranded conformational polymorphism revealed eight haplotypes, representatives of which were sequenced. These included nine variable sites, none of which were phylogenetically informative.G-test and AMOVA revealed little structuring of samples within or among fiords, but there was evidence for reduced gene flow among the most geographically separated fiords. We conclude that fiords do not house relict populations from earlier glaciations, and that little differential sorting of haplotypes has occurred in the last 10,000 years. [ABSTRACT FROM PUBLISHER]