Chaetoderma marisjaponicum, 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 : 2977-2981

Chaetoderma marisjaponicum sp. nov.

( Figures 8–9 )

Type locality

Western Wakasa Bay , between Kanmuri-jima Island and Kyogasaki point, Tango Peninsula, southern Sea of Japan, 35°45.26 ′ N, 135°20.48 ′ E, 98–99 m.

Type depository

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

Etymology

This species is named after the type locality, the Sea of Japan .

Material examined

Holotype. NSMT-Mo 78621, ethanol preserved specimen, a part of sclerites and radula are mounted on slide glasses, body length 17.5 mm, 35°45.26 ′ N, 135°20.48 ′ E, 98–99 m, 25 April 2013. Paratypes. #1–3: NSMT-Mo 78622–78624, body length 1.75–21.5 mm, 35°45.68 ′ N, 135°20.77 ′ E, 100–105 m, 21 June 2013; #4: NSMT-Mo 78625, body length 14.3 mm, 35°45.11 ′ N, 135°20.43 ′ E, 96 m, 2 September 2010; #5– 6: NSMT-Mo 78626–78627, body length 10.2–16.1 mm, 35°45.82 ′ N, 135°20.05 ′ E, 101–102 m, 27 March 2012; #7: NSMT-Mo 78628, body length 3.6 mm, 35°45.03 ′ N, 135°20.21 ′ E, 95 m, 13 August 2012: #8–10: NSMT-Mo 78629–78631, body length 6.2–17.0 mm, 35°45.20 ′ N, 135°20.20 ′ E, 96–98 m, 25 October 2012.

Description of holotype

Animal 17.5 mm long, slender, almost uniform in diameter along body, 0.8 mm in foregut region, 0.95 mm in midgut sac region ( Figure 8A ). Boundary of foregut region and midgut region demarcated by groove and presence of erect sclerites in midgut region. Mouth surrounded by pedal shield located near centre ( Figure 8B ).

Dominant sclerites covering surface of midgut, midgut sac and prepallial regions, lanceolate with fine median keel in entire length, accompanied by three to five ribs similar to keel but shorter, on each side of keel ( Figure 9F–J ); length up to 290 µm long × 50 µm wide in posterior midgut region ( Figure 9H ), 317 µm long × 62 µm wide in prepallial region ( Figure 9J ). Sclerites of peribuccal region minute, oval, flat, 16–20 µm long × 8–9 µm wide ( Figure 9A, B ). Sclerites in foregut region small, lanceolate-oblong, flared at base, blunt at top, with weak waist, sculptured like dominant sclerites, up to 90 µm long, varying from 19 µm to 37 µm wide ( Figure 9C–E ). Posterior margin of pallial region with linear lanceolate wider in proximal half, up to 490 µm long × 36 µm wide ( Figure 9L ). Sclerites inside margin fine, lanceolate, strongly keeled, up to 105 µm long × 20 µm wide ( Figure 9K ).

Radula of single pair of sclerotized sickle-shaped teeth, narrowing in basal third, c. 36 µm long, without basal connection to basal plate. Basal plate wedge-shaped, 196 µm long, 65 µm in frontal width, 50 µm in lateral width, sclerotized for about 85% of plate length. Cuticular lateral supports about less than half the length of entire radula apparatus, structured into three lobes on each side ( Figure 8F ).

Additional description from paratypes

Colour of living animals light brown with yellowish dots in internal organs seen through the translucent body wall. Foregut region pale pink ( Figure 8C ). Yellowish colour of internal organs in living animal also seen in small specimens ( Figure 8D ). Much smaller specimens with reddish colouration in life ( Figure 8E ), which appears to fade out in larger specimens.

In smaller specimen, size of radula apparatus small but teeth almost same size as in larger specimen. In paratype #1 with body length 11 mm, length of teeth c. 36 µm, that of basal plate c. 135 µm, sclerotized at about 53% of entire plate length ( Figure 8G ).

Remarks

From the Northwest Pacific region, five species of the genus Chaetoderma are known to date ( Figure 10 ): Ch. japonicum Heath, 1911 , Ch. akkesiense Okuda, 1943 , Ch. scheltemae ( Ivanov 1984) , Ch. kafanovi ( Ivanov 1984) and Ch. callosum ( Ivanov 1984) . The first two species were described from the Pacific coast of Japan, whereas the remaining three species were from the Sea of Japan. Ch. marisjaponicum differs from Ch. japonicum by a more slender body: the body diameter of the 17.5 mm long Ch. marisjaponicum holotype is 0.8 mm in the foregut region, and 0.95 mm in the midgut sac region, whereas that in the holotype of Ch. japonicum with a body length of 17 mm is 1.3 mm in the midgut area (metathorax) and 1.5 mm in midgut sac area (preabdomen) ( Heath 1911). Another difference between the present species and Ch. japonicum is the size of erected sclerites in the midgut region. Heath (1911) described the sclerites as ‘the spines are of the usual type,’ but the difference of alignment of sclerites between midgut and midgut sac regions are clearly shown in Heath’ s illustration (plate 3, figure 7). Shigeno et al. (2007) employed this feature as one of the characters to identify their specimens as Ch. japonicum , which were collected from near the type locality (off Oi-gawa River, Shizuoka Prefecture, Albatross Station 3721, 370– 450 m). The specimens of Shigeno et al. have ‘the trunk covered with vertically aligned spicules in anterior half and posteriorly directed spicules in the posterior half’ ( Shigeno et al. 2007, p. 123), although the difference is not obvious in the figure of whole animal ( Shigeno et al. 2007, p. 125 figure 2A; Figure 10B herein). In this aspect, their specimens match Ch. japonicum , in contrast to Ch. marisjaponicum , which has obviously long, erect sclerites in the midgut region. Whether their ~ 7 mm specimens, however, really belong to Ch. japonicum may be doubted, as the mature Ch. japonicum , apart from the size, is clearly more slender ( Figure 10A ); in addition, as far as the organization is known for comparison, the gonopericardioduct is paired ( Heath 1911, pp. 67–68), not fused.

In Chaetoderma akkesiense , sclerites are uniformly aligned throughout the midgut and midgut sac regions ( Figure 10C ). Ch. marisjaponicum differs from Ch. akkesiense also by the narrower keel of the dominant sclerites, and by wider, larger sclerites in the foregut region. Ch. scheltemae has a much larger body, which attains 35 mm ( Figure 10E ), but the sclerites are much smaller than those of Ch. marisjaponicum . Chaetoderma kafanovi also has a larger body, which attains 40 mm ( Figure 10F ). Other than the difference of the body size, the present species differs from Ch. kafanovi by the pedal shield that totally surrounds the mouth opening, and by much narrower sclerites in the foregut region. Chaetoderma marisjaponicum resembles Ch. callosum ( Figure 10D ) in the morphology of the dominant sclerites, in the pedal shield surrounding the mouth, and in the shape of the radula teeth, but differs in having a more slender body and lacking triangular sclerites in the foregut region.

Figures

Figure 8. Chaetoderma marisjaponicum sp. nov. (A) holotype, NSMT-Mo 78608, preserved condition; (B) pedal shield of holotype; (C) paratype #8, NSMT-Mo 78629, specimen in life; (D) paratype #7, NSMT-Mo 78628, specimen just after fixation in formalin; (E) paratype #3, NSMT-Mo 78624, specimen in life; (F) radula of holotype; (G) radula of paratype # 1, NSMT-Mo 78622. Scale bars: 5 mm for A, C; 200 µm for B; 0.5 mm for D, E; 100 µm for F, G.

Figure 9. Chaetoderma marisjaponicum sp. nov. Sclerites of holotype. (A, B) peribuccal region; (C–E) in foregut region; (F, G) in anterior midgut sac region; (H, I) in posterior midgut sac region; (J) in prepallial region; (K) inside of posterior margin of pallial region; (L) at posterior margin of pallial region. Shortest scale bar: 10 µm for A, B; long scale bar (left): 100 µm for C– K; long scale bar (right): 100 µm for L.

Figure 10. Chaetoderma species from the NW Pacific region. (A) Ch. japonicum Heath; (B) Chaetoderma sp. of Shigeno et al.; (C) Ch. akkesiense Okuda; (D) Ch. callosum (Ivanov); (E), Ch. scheltemae (Ivanov); (F) Ch. kafanovi (Ivanov). Scale bar: 5 mm. A after Heath (1911); B after Shigeno et al. (2007); C after Okuda (1943); D–F after Ivanov (1984).