Scutopus hamatanii, Saito & Salvini-Plawen, 2014

Saito, Hiroshi & Salvini-Plawen, Luitfried v., 2014, Four new species of the aplacophoran class Caudofoveata (Mollusca) from the southern Sea of Japan, Journal of Natural History 48 (45 - 48), pp. 2965-2983 : 2970-2973

Scutopus hamatanii sp. nov.

( Figures 4–5 )

Scutopus sp. Hamatani (2000)

Type locality

Western Wakasa Bay , between Kanmuri-jima Island and Kyogasaki point, Tango Peninsula, southern Sea of Japan, 35°45.02 ′ N, 135°20.10 ′ E, 94–95 m.

Type depository

Department of Zoology, National Museum of Nature and Science, Tsukuba.

Etymology

This species is named in honour of Dr. Iwao Hamatani, who first recognized Scutopus species in the North Pacific region.

Material examined

Holotype. NSMT-Mo 78601, ethanol preserved specimen, a part of sclerites and radula are mounted on slide glasses, body length 8.7 mm, 35°45.02 ′ N, 135°20.10 ′ E, 94–95 m, 25 October 2012. Paratypes. #1–3: NSMT-Mo 78602–78604, body length 5.2–6.7 mm, from type locality; #4: NSMT-Mo 78605, body length 8.5 mm, 35°34.59 ′ N, 135°20.32 ′ E, 51–52 m, 10 July 2012; #5–6: NSMT-Mo 78606–78607, body length 2.8–5.8 mm, 35°45.68 ′ N, 135°20.77 ′ E, 100–105 m, 21 June 2013.

Description of holotype

Animal small, 8.7 mm long, slender, almost uniform in diameter along body, 0.6 mm in foregut region, 0.5 mm in midgut region, and 0.4 mm in prepallial region ( Figure 4A ). Boundary of foregut region and midgut region demarcated by groove. Midgut and midgut sac regions with distinct groove of midventral suture line. Pedal shield postoral, but flanking mouth opening laterally ( Figure 4C ).

Dominant sclerites covering dorsal, lateral, and large part of ventral surfaces of midgut region, midgut sac region, and prepallial region elongate drop-shaped, sharply pointed at tip, thin, flat, broadly keeled on midline near tip, often with longitudinal groove on each side of median keel, up to 170 µm long × 35 µm wide in midbody ( Figure 5I, J ), gradually increasing in size posteriorly, up to 190 µm long × 45 µm wide in prepallial region ( Figure 5K ). Sclerites of peribuccal region minute, oval, flat, c. 20 µm long × 10 µm wide ( Figure 5A, B ), more elongated and narrowing distally in posterior scales. Sclerites in foregut region of two types: one narrow, oar-blade shaped with weak waist, up to 60 µm long × 20 µm wide ( Figure 5C, E ), and the other triangular, without waists, up to 50 µm long × 27 µm wide ( Figure 5D, F ), both types with median keel. Sclerites along midventral suture slender in distal half, widening in proximal half, pointed at tip, keeled on midline near tip, up to 80 µm long × 20 µm wide ( Figure 5G ). Similar but wider sclerites flanking those narrow sclerites ( Figure 5H ). Posterior margin of pallial region with long needles, slightly flared in the proximal half, up to 250 µm long × 28 µm wide ( Figure 5M ). Inside of posterior margin of pallial region with narrow spatula-shaped sclerites that are slender in distal half, pointed at tip, wide, thin, flat at proximal half, up to 145 µm long × 21 µm wide ( Figure 5L ).

Radula large, c. 350 µm long, distichous, arranged in 12 transverse rows, slightly sclerotized except for posteriormost two pairs; each tooth sickle-shaped, c. 100 µm long, alated in medial as well as lateral edges, differentiated into 8–9 triangular denticles at medial edge ( Figure 4D–G ).

Additional description from paratypes

Colour of living animals pale pink or cherry blossom in foregut and midgut regions, light brown with dark brownish dots in midgut sac region.

Remarks

Scutopus hamatanii sp. nov. closely resembles Scutopus ventrolineatus Salvini- Plawen, 1968 in possessing a midventral suture (groove) and less sclerotized, wide radular teeth. S. hamatanii differs from S. ventrolineatus in geographical provenance and by having more slender sclerites along the ventral suture, wider, elongate triangular radular teeth with less bending, and triangular radula denticles. Hamatani (2000) gave a brief account and an illustration of an undetermined Scutopus species from the Sea of Japan, Konoura, Akita Prefecture, northern Japan at the depth of 10 m. Dr. Iwao Hamatani kindly provided us with the Scutopus specimens collected from Konoura as well as colour slide films of specimens for our disposition. He recognized two species among nine specimens which he kept in separate glass tubes, including one specimen which lacks the anterior body. Unfortunately, all the specimens are decalcified, and only one mount of the radula exists. We regard this species introduced as Scutopus sp. by Hamatani (2000) to be identical with the present species, because of the morphology of the radula including the numerous features described by Hamatani, namely, 12–15 transverse rows, and 6–10 denticles in each tooth; the brownish colouration of the body matches that of S. hamatanii .

Figures

Figure 4. Scutopus hamatanii sp. nov. (A) holotype, NSMT-Mo 78601, preserved condition; (B) paratype #5, NSMT-Mo 78606, preserved condition; (C) pedal shield of holotype; (D–G) radula of holotype: (D) lateral view with anteriorly protruding end of radula membrane; (E) dorsal view; (F, G) close up of teeth. Scale bars: 0.5 mm for A, B; 200 µm for C–E; 50 µm for F, G.

Figure 5. Scutopus hamatanii sp. nov. Sclerites of holotype. (A, B) peribuccal region; (C–F) in foregut region; (G–H) at ventral suture; (I) in anterior midgut sac region; (J) in posterior midgut sac region; (K) in prepallial region; (L) inside of posterior margin of pallial region; (M) at posterior margin of pallial region. Long scale bar (left): 100 µm for A–L; Short scale bar (right): 100 µm for M.