Redescription of Trilobodrilus nipponicus (Annelida: Dinophilidae). Trilobodrilus nipponicus

The marine interstitial archiannelid Trilobodrilus nipponicus Uchida and Okuda, 1943 is redescribed based on specimens collected interidally at sandy beach in Ishikari Bay, Hokkaido, northern Japan. The ciliation pattern was observed by scanning electron microscopy, which showed some discrepancies between what were illustrated in the original description based on light microscopy; e.g., the lateral ciliary tufts regularly arranged on each body segment were not observed and the third anterior ciliary band was dorsally not continuous in our material. The single, mid-dorsal ciliary tuft on the head, so far recorded only for T. nipponicus among congeners and also present in our material, may be species-specific for this taxon, but may have been simply overlooked in other species, due to the difficulty in confirmation. The epidermal inclusions, previously not known for this species, are ellipse in shape, with one end of the long axis concavely waned inward; spindle glands were not found. We determined sequences of the nuclear 18S and 28S rRNA genes, as well as the mitochondrial cytochrome c oxidase subunit I gene from T. nipponicus for facilitating the future DNA taxonomy of this group of microscopic animals. A preliminary molecular phylogenetic inference placed T. nipponicus in a clade formed by other dinophilids included in the analysis, showing the two European congeners T. axi and T. heideri are more closely related to each other than to T. nipponicus.