Phylogenetic position of the enigmatic genus Atherospio and description of Atherospio aestuarii sp. nov. (Annelida: Spionidae) from Japan

BACKGROUND: There are currently two species within the small enigmatic genus Atherospio Mackie & Duff, 1986, which belongs to the Pygospiopsis-Atherospio group in the family Spionidae Grube, 1850. The taxonomic relationship of the genus Atherospio with other spionid or spioniform genera is currently not well understood due to its unusual morphological characteristics. METHODS: Here, we describe a new Atherospio species, Atherospio aestuarii sp. nov., based on materials collected from three localities in Japan: Hirota Bay (Iwate Prefecture), Ago Bay (Mie Prefecture), and Yakushima Island (Kagoshima Prefecture). We have also evaluated the possible systematic position of this new species by conducting molecular phylogenetic analyses using the nuclear 18S, 28S, and mitochondrial 16S rRNA gene sequences. RESULTS: The morphology of A. aestuarii sp. nov. resembles that of A. disticha Mackie & Duff, 1986 and A. guillei (Laubier & Ramos, 1974) in having branchiae fused to the notopodial lamellae on a restricted number of segments from chaetiger 7, modified neurochaetae on chaetiger 5, and at least some bidentate neuropodial hooks with the secondary tooth below the main fang. The form and arrangement of the modified aristate neurochaetae in double vertical rows closely resemble those found on chaetigers 4 and 5 of A. disticha. The new species lacks the occipital antenna present in A. disticha. In this respect it resembles A. guillei, however, that species differs in having robust neuropodial spines on chaetiger 5 and peristomial papillae, and a preponderance of unidentate neurochaetae. Both A. guillei and the new species have slender needle-like notochaetae in their posteriormost chaetigers. Atherospio aestuarii sp. nov. is distinguished from both congeneric species by its branchial and neuropodial hook distributions. The new species is also unique in that it was recorded at relatively shallow depths, which included intertidal zones. The results of our molecular phylogenetic analysis indicate that the new species was included in a clade that included the genera of the Polydora complex, Pygospio Claparède, 1863, Glandulospio Meißner, Bick, Guggolz, Götting, 2014, Spio Fabricius, 1785, Microspio Mesnil, 1896, Marenzelleria Mesnil, 1896, Rhynchospio Hartman, 1936, Scolelepis Blainville, 1828, Dispio Hartman, 1951, and Malacoceros Quatrefages, 1843 with robust statistical support. The new species formed a clade with Dispio and Scolelepis, however, statistical support for the node was not significant.