Taeniogyrus mijim Yamana & Tanaka & Nakachi 2017, sp. nov.

Taeniogyrus mijim sp. nov.

(Figs 1A, 2A, 3A, 4A, 5A, 6)

Material examined. Holotype: WMNH-2015-INV-1457 (MOT, 17 June 2015) (length 11.2 mm, width 1.0 mm). Paratypes: WMNH-2015-INV-1402 (MOT, 31 July 2015) (length 7.2 mm, width 0.7 mm); WMNH-2015-INV-1456 (MOT, 17 June 2015) (length 9.4 mm, width 1.0 mm); BIK-EC-T0200 (NIS, 29 October 2014) (length 6.0 mm, width 0.4 mm); BIK-EC-T0201 (NIS, 29 October 2014) (length 8.5 mm, width 0.9 mm).

Other material: 5 specimens, WMNH-2015-INV-139–143 (MOT, 21 May 2015); 30 specimens, WMNH-2015-INV- 155–184 (MOT, 17 June 2015); 3 specimens, WMNH-2015- INV-483–485 (NIS, 29 October 2014); 224 specimens, WMNH-2015-INV-486–709 (MOT, 29 August 2015); 529 specimens, WMNH-2015-INV-868–1396 (MOT, 31 July 2015); 1 specimen, WMNH-2015-INV-1401 (MOT, 31 July 2015); 1 specimen, WMNH-2015-INV-1403 (MOT, 21 May 2015); 285 specimens, WMNH-2015-INV-1481–1765 (MOT, 29 September 2015); 1 specimen, WMNH-2016-INV-3 (MOT, 22 January 2016).

Description. Body small, anaesthetized length of largest specimen (WMNH-2016-INV-3) 13.0 mm (aπer preservation), cylindrical, slightly tapered toward posterior end (Fig. 1A, Table 1). Body colour (of preserved specimens) translucent, white to pink, sometimes greyish. Living specimens translucent orange. Mouth anterior; anus posterior. Oral disc inclined toward ventral side.

Ten tentacles non-retractile, slender, with smooth or, rarely, bumpy skin. Each tentacle with one pair of digits; sensory cups absent (Fig. 2A). Colour of tentacles white to whitish-pink (preserved specimens), sometimes greyish. Calcareous ring composed of five radial and five inter-radial plates, these all being thin and weak, with distally flattened ginglymus ends (Fig. 3A). Polian vesicle single (Table 1). Stone canal undiscovered. Two long and unbranched tubules of ovaries, and one or two long, unbranched tubules of testis present. Testis tubules shorter than ovary tubules. One ambiguous row of ciliated funnels situated in inter-radius of middle to posterior body cavity, near right side of mid-ventral longitudinal muscle.

Body wall containing wheel and sigmoid-hook ossicles (Figs 4A, 5A, 6, Table 2). Wheel ossicles rounded-hexagonal with six spokes. Inner margin of rim not parallel to outer margin, and rim narrower at corners. Teeth sharp but short, 7–16 per radiant (means of 10–13 in different specimens); oπen reduced in number and size at corners, but with no discontinuities in tooth series there. Spokes broad, their breadth amounting to 18–29% of wheel ossicle diameter (means 22–26%) (Fig. 6). Wheel ossicles scattered sparsely in ventral inter-radial body wall, dense in anterior and posterior inter-radial body wall. Wheel ossicle aggregations (wheel papillae) absent. Wheel ossicle diameter 52–81 µm (means 63–71 µm) in anterior dorsal side, 59–79 µm (means 62–70µm) in anterior ventral side, 50–78 µm (means 55–70 µm) in posterior dorsal side, and 55–70 µm (means 61–64 µm) in posterior ventral side, differing significantly among specimens (Kruskal-Wallis test, P s<0.05) in anterior dorsal and ventral sides and posterior dorsal side but not in posterior ventral side. Wheel ossicle diameter in two fully checked specimens (WMNH-2015-INV-1456, BIK-EC-TC200) significantly different among these four body parts (Kruskal-Wallis test, P s<0.05), but not in third specimen (BIK-EC-201).

Sigmoid-hook ossicles in body wall arranged in two single rows in inter-radii, along both sides of longitudinal muscles, with pointed end facing toward inter-radius and blunt end facing toward longitudinal muscles (Fig. 4A). Outer edge of bend without minute teeth (Fig. 6). Shank broadest at bend, its length 65–84 µm (means of 74–82 µm in different specimens) in anterior dorsal side, 75–85 µm (means 76–81 µm) in anterior ventral side, 61–75 µm (means 64–70 µm) in posterior dorsal side, and 54–71 µm (means 64–69 µm) in posterior ventral side. Shank length of sigmoid-hooks significantly different among specimens (Kruskal-Wallis test, P s<0.05) in anterior dorsal side and posterior dorsal and ventral sides but not in anterior ventral region (Table 2). Shank length in all three fully checked specimens significantly different among four body parts (Kruskal-Wallis test, P s<0.05).

Tentacles containing rod ossicles (Figs 5A, 6, Table 3), these spinous, most branching distally but lacking branches elsewhere. Rod ossicle length 37–74 µm, means of 55–64 µm in different specimens, significantly different among them (Kruskal-Wallis test, P <0.01) (Table 3).

Distribution. In sandy-gravel sediment in calm inlets along the Pacific coast of Wakayama and Kochi Prefectures, Japan.

Etymology. The specific name mijim, a noun in apposition, is derived from the Japanese word meaning “particle”.

DNA barcode sequence. A 650bp sequence of the mitochondrial CO1 gene was obtained from a paratype specimen (WMNH-2015-INV-1402, GenBank accession number LC152965). Base frequency was A=19.1%, C=14.5%, G=22.8%, T=43.7%.