Satsumaocnus kaiyomarui sp. nov. Yamana, Thandar, & Setiamarga

[New Japanese name: Satsuma-kinko]

(Figs 2, 3, 4, and 5; Table 1 and 2)

Etymology. The specific name kaiyomarui is the genitive case of the R/V Kaiyo Maru of Fisheries Agency of Japan.

Diagnosis. As for the genus.

Material examined. Holotype, WMNH-2018-INV-3 (contracted length 62 mm, width 17 mm). Paratypes, 10 individuals: WMNH-2018-INV-4—only contracted measurements (length x breadth) recorded (69 mm x 24 mm); WMNH-2018-INV-5 (58 mm x 20 mm); WMNH-2018-INV-6 (42 mm x 12 mm); WMNH-2018-INV-10 (28 mm x 9 mm); WMNH-2018-INV-11 (23 mm x 8 mm); WMNH-2018-INV-12 (14 mm x 16 mm); WMNH-2018-INV-13 (23 mm x 6 mm). Three observed paratypes were also used for molecular analysis reported previously (Yamana et al., 2019): WMNH-2018-INV-7 (27 mm x 11 mm; Genbank accession numbers LC 425500 for COI, LC 425503 for H3); WMNH-2018-INV-8 (21 mm x 9 mm; Genbank accession numbers LC 425501 for COI, LC 425504 for H3); WMNH-2018-INV-9 (19 mm x 7 mm; Genbank accession numbers LC 425502 for COI, LC 425505 for H3)

Holotype description. Body form straight, posteriorly tapering. Live coloration yellow. dorsal podia short, papillae-like, restricted to radii in two longitudinal rows; ventral podia (pedicels), retractile, restricted to radii in three apparently zig-zag rows, absent anteriorly. In preserved state, anterior end expanded and posterior conspicuously contracted. Body wall soft, thin in anterior (<0.5 mm), thick in posterior (>1.0 mm); preserved coloration pure white (Fig. 2A, B); contracted length 62 mm, width 17 mm (see Table 1). Tentacles 10, dendritic, equal length, all tentacles arranged in a single circle (Fig. 2F), each with two tufts, subdivided basally (Fig. 2 F’). Body form cylindrical, with five oral valves (Fig. 2C) and five radially arranged anal teeth, each accompanied by a pair of anal papillae lying in a circle outside of the anal teeth (Fig.2D). Tentacles and introvert are of same coloration as the adjoining body wall.

Calcareous ring low, stout, radial and interradial elements compact, not forming a mosaic and lacking posterior prolongations (Fig. 3). All plates sagittate, each with a posterior depression and anterior projection. The mid-ventral element R 1 and adjoining two interradial elements IR1 and IR2 not fused, both IR1 and IR2 narrower than RI, both with sharper anterior projection (Fig. 3) but mid-dorsal interradial plate (IR5) with deepest posterior depression. Retractor muscles originate from respective longitudinal muscle at 20–30% of body length from anterior end, more anterior dorsally, and insert on anterior projection (Fig. 3) of corresponding radial plate.

Polian vesicle single, mid-ventral in position, in line with RI, fusiform in shape, narrowing distally (distal end 1/6–1/3 of vesicle length); stone canal also single, mid-dorsal in position, in line with IR5; madreporite brain-/coralshaped, on the left side of anterior portion of the dorsal mesentery (Table 1, Fig. 2G, H). Gonadal stolon attached anteriorly, gonad in two tufts, one on each side of dorsal mesentery, most tubules short, branched. Respiratory trees paired, larger tree situated on right side of body cavity, reaching about half body length. Smaller tree situated on left side, length approximately 1/5–1/4 of the right one. Small intestine extends from mid-dorsal region in line with IR5, turns towards the right dorsal interradius in line with IR4, passes along IR4 to about 3/4 the body length and then extends anteriad to middle of body along interradius (IR5) before opening into the cloaca.

Body wall ossicles as an external layer of small (22–35 µm) x-framed hourglass ossicles (Fig. 4F, G, Table 2) and an inner layer of sparsely distributed, large (123–154 µm) dendriform rods (Fig. 4 F, G, Table 2), sometimes forming delicate plates. Buttons, thick plates, x-shaped ossicles and/or rosettes absent. Tentacle ossicles mostly large, thick rods (55–643 µm), distally branched or perforated by few small holes (Fig. 4A, Table 2). Peri-oral ossicles, sparse, occurring as small (39–49 µm) typical, simple cups and large (169–369 µm) spinous rods (Fig. 4B, Table 2). Ossicles of pharyngeal villi as large (78–251 µm) rods, with few distal perforations and/or branches, sometimes also with a medial third branch/arm (Fig. 4C, Table 2). Introvert ossicles also as small (26–31 µm), x-framed hourglasses (Fig. 4D, Table 2). Oral valve ossicles as mostly small (24–34 µm) hourglasses and medium to large (101–358 µm), often distorted or rod-derived supporting plates at tip of valve (Fig. 4E, Table 2). Ossicles of dorsal podia mostly small (24–34 µm) hourglasses and small to large (85–175 µm), irregular supporting plates at tip (Fig. 4H, Table 2). Ossicles of ventral podia, mostly small (24–39 µm) hourglasses, medium-sized (57–149 µm) terminal supporting plates and large (451–514 µm) endplate (Fig. 4I, Table 2). Peri-anal ossicles as small (16–32 µm) hourglasses. Anal papillae with medium-sized (85–175 µm), irregular supporting plates (Fig. 4J, Table 2). Ossicles were also detected in the basal part of ovary as medium-sized (78–163 µm), distally branched spinous or dendriform rods (Fig. 4K, Table 2).

Variations in paratypes. Body form in life straight or slightly U-shaped, straight in preserved state. Contracted dimensions of the largest paratype (WMNH-INV-2018-4) 69 mm x 24 mm (Table 1). All paratypes with 10 equal tentacles in a single circle (Fig. 2 F), each tentacle basally divided into two large tufts. Except for five small specimens (WMNH-INV- 2018-9–13), the posterior end of all specimens is strongly contracted. Polian vesicle always single, in RI, stone canal also single in IR5. calcareous ring low, stout, without posterior prolongations; oral valves present. All specimens possess abundant x-framed hourglasses in body-wall, but simple cups and scarce spinous rods in perioral skin lacking in some individuals (Table 2). Hourglasses absent in the introvert of several specimens (Table 2). In all specimens dendriform rods sparser in ventral body wall. Gonadal tubules (Table 1) absent in some specimens (perhaps due to immaturity). Other ossicles of the paratypes consistent with those of the holotype.

Distribution. Known only from type locality.

Ecology/behaviour. All individuals were observed basally attached with their posterior end, or this end was embedded in the sediment (pebbles and coral debris) (Fig. 6A, B). Most specimens were observed with their tentacles extended into the water column while feeding. (Fig. 6C). Sometimes animals were half-embedded in the sandy gravel bottom. Other movements were not observed (Fig. 6D).

a Registration number, WMNH-INV: Invertebrate Collection (INV) of the Wakayama Prefectural Museum of Natural History (WMNH).

b Localities for the expedition of Fisheries Agency of Japan in August, 2017, shown in Fig. 1

c Positions classified by Ludwig’s [1889 –1892 (1889), p156]: RI, medioventral; IR5, mediodorsal.

Rod: a Length, a total length along with longitudinal line; b Breadth, a stem breadth at central region; c Width, a total width crossing to longitudinal line.

Cup: a Length, a long side of lectangle rim; b Breadth, a short side of lectangle rim; c High, a distance between upper edge of upside rim and belower edge of downside rim. In all three directions, spinneret and/or process were omitted from the measurement. Among the three directions, one vertical direction was omitted from the measurement.