Rowedota motoshimaensis sp. nov.

(Figs 1C, 2C, 3C, 4C, 5C, 8)

Material examined. Holotype: WMNH-2015-INV-1767 (MOT, 29 September 2015) (length 9.6 mm, width 0.8 mm). Paratypes: WMNH-2015-INV-710 (MOT, 29 August 2015) (length 5.6 mm, width 0.7 mm); WMNH-2015-INV-711 (MOT, 29 August 2015) (length 11.7 mm, width 1.2 mm); WMNH-2015-INV-715 (MOT, 29 August 2015) (length 9.5 mm, width 0.8 mm); WMNH-2015-INV-1400 (MOT, 31 July 2015) (length 7.6 mm, width 1.0 mm).

a WMNH-INV: Invertebrate Collection of the Wakayama Prefectural Museum of Natural History, BIK-EC: Echinodermata Collection of the Biological Institute of Kuroshio. b MOT: Motoshima Islet, Mera, Tanabe, Wakayama, NIS: Nishidomari, Otsuki, Kochi, ARA: Araha Beach, Chatan, Okinawa.

Other material: 3 specimens, WMNH-2015-INV-712–714 (MOT, 29 August 2015); 2 specimens, WMNH-2015-INV- 1397, 1398 (MOT, 31 July 2015); 1 specimen, WMNH-2015- INV-1404 (MOT, 21 May 2015); 1 specimen, WMNH-2015- INV-1455 (MOT, 17 June 2015); 1 specimen, WMNH-2015- INV-1766 (MOT, September 2015); 2 specimens, WMNH- 2016-INV-1, 2 (MOT, 22 January 2016).

Description. Body small, anaesthetized length of largest type specimen (WMNH-2015-INV-711) 11.7 mm (aπer preservation), cylindrical, slightly tapered toward posterior end (Fig. 1C, Table 1). Body colour (in preserved and living specimens) white to light brown, not very transparent. Mouth anterior; anus posterior. Oral disc inclined toward ventral side.

Ten tentacles non-retractile, slender, with bumpy skin. Each tentacle with two pairs of digits; sensory cups absent; distal two digits longer than subdistal two digits (Fig. 2C). Subdistal digits sometimes absent from individual tentacles. Colour of tentacles white (in preserved specimens). Calcareous ring composed of five radial and five inter-radial plates, these all thin and weak, with distally flattened ginglymus ends (Fig. 3C). Polian vesicle single (Table 1). Stone canal undiscovered. Male and female systems observed in different specimens, both ovary and testis consisting of two long, unbranched 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 4C, 5C, 8, Table 2). Wheel ossicles rounded hexagonal with six spokes. Inner margin of rim not parallel to outer margin, and rim narrower at corners. Teeth small and blunt, mostly semi-circular, 8–21 per radiant (means of 11–19 in different specimens), discontinuous at concavities of rim at corners. Spokes not broad, their breadth amounting to 14–23% of diameter (means 16–22%) (Fig. 8). Wheel ossicles scattered sparsely in ventral inter-radial body-wall. Wheel ossicle aggregations (wheel papillae) absent. Wheel ossicle diameter varying widely even in same tissue sample: 61–133 µm (means 84–103 µm) in anterior dorsal side, 56–110 µm (means 70–95µm) in anterior ventral side, 56– 107 µm (means 81–85 µm) in posterior dorsal side, and 53– 103 µm (means 70–75 µm) in posterior ventral side. Wheel ossicle diameter in anterior dorsal side significantly different among five specimens (Kruskal–Wallis test, P <0.05), but not in other parts among three fully checked specimens (Table 2). In two of latter specimens (WMNH-2015-INV-711, -715), wheel ossicle diameter significantly different among four body wall parts (Kruskal–Wallis test, P s<0.05), but not in third specimen (WMNH-2015-INV-710).

Sigmoid-hook ossicles thick and large, arranged in 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. One to three additional ambiguous rows of sigmoid-hook ossicles scattered in inter-radii, facing different ways. Outer edge of bend without minute teeth (Fig. 8). Shank broadest on proximal side of bend, and gradually thinning through bend. Shank length 70–104 µm (means of 78–95 µm in different specimens) in anterior dorsal side, 73–96 µm (means 75–91 µm) in anterior ventral side, 61–93 µm (means 74–85 µm) in posterior dorsal side, and 61–94 µm (means 71–82 µm) in posterior ventral side. Shank length significantly different among five specimens in anterior dorsal part and among three specimens in other parts (Kruskal–Wallis test, P s<0.05) (Table 2). Shank length in each of latter three specimens significantly different among four body wall parts (Kruskal–Wallis test, P s<0.05).

Tentacles containing spinous rod ossicles (Figs 5C, 8, Table 3), most being branched distally and some with central branches. Length 37–88 µm, means 58–70 µm, significantly different among five specimens (Kruskal–Wallis test, P <0.01) (Table 3).

Distribution. In sandy-gravel sediment of lower intertidal zone in calm inlet along Pacific coast of Wakayama Prefecture, Japan.

Etymology. The specific name motoshimaensis is derived from the name of the type locality islet, Motoshima.

DNA barcode sequence. A 650bp sequence of the mitochondrial CO1 gene was obtained from a paratype specimen (WMNH-2015-INV-1400, GenBank accession number LC152968). Base frequency was A=16.8%, C=13.8%, G=24.8%, T=44.6%.