title: Whole mitochondrial genome of the Ram's Horn Squid shines light on the phylogenetic position of the monotypic order Spirulida (Haeckel, 1896).
abstract: The phylogenetic position of the only known species within the order Spirulida, the Ram's Horn Squid, Spirula spirula, may be the key to resolving relationships within Decapodiformes (squids and cuttlefishes). Spirula spirula possesses several unique features including an internal calcareous chambered shell unlike the familiar cuttlebone of Sepiidae (cuttlefishes). The shell is reduced to a gladius or absent in other decapod clades. To resolve decapodiform phylogenetic relationships we sequenced the mitochondrial genome of S. spirula and Sepiadarium austrinum and analysed these along with other mitochondrial genomes. Sequence analyses found that S. spirula and Sepiidae, the only two extant phragmocone bearing groups, were not sister taxa. Rather, in most analyses S. spirula was placed within a clade containing Bathyteuthoidea and Oegopsida either as the sister taxon to Bathyteuthoidea+Oegopsida or the sister taxon to Bathyteuthoidea only, depending upon the analysis method. Sepiidae was the sister taxon to a clade containing all remaining decapods. Spirulid mitochondrial gene order was identical to that of Octopodiformes, which we recognize as close to that of ancestral molluscs. The phylogenetic position of Idiosepiidae differed among analysis methods of molecular sequence data. However, gene order analysis resolved a highly supported monophyletic relationship containing Idiosepiidae and Sepiolida.
authors.affiliation: Marine Biology & Aquaculture, James Cook University, Townsville, Queensland 4810, Australia; Department of Ecology, Environment and Evolution, School of Life Sciences, La Trobe University, Melbourne 3086, Victoria, Australia. Electronic address: jan.strugnell@jcu.edu.au.
authors.forename: Jan M
authors.initials: JM
authors.lastname: Strugnell
authors.affiliation: La Trobe Institute for Molecular Science, La Trobe University, Melbourne, Victoria, Australia; Life Sciences Computation Centre, Victorian Life Sciences Computation Initiative, Melbourne, Victoria, Australia.
authors.forename: Nathan E
authors.initials: NE
authors.lastname: Hall
authors.affiliation: NOAA National Systematics Lab., National Museum of Natural History, PO Box 37012, MRC 153, Washington, DC 20013-7012, USA.
authors.forename: Michael
authors.initials: M
authors.lastname: Vecchione
authors.affiliation: Richard-Wagner-Str. 29, 67655 Kaiserslautern, Germany.
authors.forename: Dirk
authors.initials: D
authors.lastname: Fuchs
authors.affiliation: Ryan Institute and School of Natural Sciences, National University of Ireland Galway, University Road, Galway, Ireland.
authors.forename: A Louise
authors.initials: AL
authors.lastname: Allcock
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D049832:Decapodiformes / Q000145:classification* / Q000235:genetics; D023061:Gene Order; D054629:Genome, Mitochondrial; D058889:Molecular Typing; D010802:Phylogeny; D017422:Sequence Analysis, DNA
keywords: Cephalopoda; Decapodiformes; Mitochondrial genome; Ram’s Horn Squid; Spirula spirula; Spirulida