Falcidens ryokuyomaruae, 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 : 2973-2976

Falcidens ryokuyomaruae sp. nov.

( Figures 6–7 )

Type locality

Western Wakasa Bay , between Kanmuri-jima Island and Kyogasaki point, Tango Peninsula, southern Sea of Japan, 35°44.97 ′ N, 135°22.40 ′ E, 101–102 m.

Type depository

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

Etymology

This species is named after the research and training vessel Ryokuyo-maru of the Maizuru Fisheries Research Station, Kyoto University, which collected all the specimens examined in this paper.

Material examined

Holotype. NSMT-Mo 78608, ethanol preserved specimen, a part of sclerites and radula are mounted on slide glasses, body length 6.4 mm, 35°44.97 ′ N, 135°22.40 ′ E, 101–102 m, 2 September 2012. Paratypes. #1–3: NSMT-Mo 78609–78611, body length 4.6–7.0 mm, from type locality; #4–6: NSMT-Mo 78612–78614, body length 4.0– 4.5 mm, 35°39.20 ′ N, 135°21.47 ′ E, 69 m, 2 September 2010; #7: NSMT-Mo 78615, body length 7.1 mm, 35°34.59 ′ N, 135°20.32 ′ E, 51–52 m, 10 July 2012; #8: NSMT-Mo 78616, body length 4.8 mm, 35°45.03 ′ N, 135°20.21 ′ E, 95 m, 13 August

2012; #9–12: NSMT-Mo 78617–78620, body length 4.3–7.0 mm, 35°45.26 ′ N, 135° 20.48 ′ E, 98–99 m, 25 April 2013.

Description of holotype

Animal small, 6.4 mm long, anterior body stout, c. 0.9 mm in diameter in midgut region, posterior body slender, tail-like, c. 0.3 mm in prepallial region, terminating in tassel with long needle-like sclerites ( Figure 6A ). Boundary of foregut and midgut regions demarcated by groove. Pedal shield surrounding mouth located dorsally to centre.

Dominant sclerites covering whole surface of midgut and midgut sac regions lanceolate with flared base, up to 135 µm long × 40 µm wide, curved towards body, bluntly pointed at tip, broadly keeled on midline (seen as dark axial band in Figure 7E, F ), with one longitudinal groove on each side of median keel; grooves are bifurcated near the base ( Figure 7E, F ). Sclerites from foregut region to midgut sac region with or without basal notch. Sclerites of prepallial region similar to dominant sclerites, but less flared in basal portion ( Figure 7G, I ), occasionally parallel sided ( Figure 7H ). Sclerites of peribuccal region minute, elongate oval, flat, c. 40 µm long × 14 µm wide ( Figure 7A, B ). Sclerites in foregut region short, up to 120 µm long × 45 µm wide, slightly curved towards body, round at top, flared at base, with weak waist, keeled with one or two narrow grooves at both sides of keel ( Figure 7C, D ). Sclerites of posterior prepallial region rather broad, attaining 120 µm long × 35 µm wide, slightly concave on medial surface, pointed at tip, nearly parallel sided, with several fine grooves ( Figure 7J ). Posterior margin of pallial region with long needles slightly flared in the proximal half, up to 280 µm long × 16 µm wide ( Figure 7K, L ). Sclerites inside of posterior margin fine needles 80 µm long × 8 µm wide ( Figure 7M ).

Radula of single pair of sclerotized sickle-shaped teeth, c. 40 µm long, with small triangular thickening in proximal half, socketed by symphysis to apical notch of basal plate ( Figure 6D ). Basal plate wedge-shaped, 190 µm long, 65 µm in frontal width, 35 µm in lateral width, sclerotized at distal one-third. Cuticular lateral supports about half length of entire radula apparatus, substructured into three lobes at each side. Slightly sclerotized cuticular lining (transverse bar: as shown in Salvini-Plawen 1975, figure 7A, hatched portion between tips of lateral supports) of the distal radular pit ( Figure 6D , left).

Additional description from paratypes

Colour of living animals light brown with dark brownish maculation in anterior body, which is the colouration of the internal organs observable through the translucent body wall, dark brown in pedal shield ( Figure 6B, C ).

Remarks

This species shares with several other Falcidens species a slender, tail-like posterior body; these include F. gutturosus ( Kowalevsky, 1901) , F. loveni ( Nierstrasz, 1902) , F. caudatus ( Heath, 1918) , F. hartmani ( Schwabl, 1961) , F. crossotus Salvini- Plawen, 1968, F. procerus Salvini-Plawen, 1986 , F. targatus Salvini-Plawen, 1986 , F. acutargatus Salvini-Plawen, 1992 , and F. vasconiensis Salvini-Plawen, 1996 ( Schwabl 1961; Salvini-Plawen 1968, 1992, 1996). Only F. loveni from Indonesia, F. hartmani from off Southern California and F. procerus from the Peru– Chile trench inhabit Pacific waters, whereas the type species F. crossotus and the other representatives belong to the Atlantic fauna. With respect to the sclerites, F. crossotus from the Scandinavian coast and F. gutturosus from the Mediterranean Sea are similar to those of F. ryokuyomaruae by having lanceolate scales with flared base in the midbody (see Salvini-Plawen 1968, 1972, 1996). It differs, however, by geographical provenance, and by the pedal shield which surrounds the mouth opening, narrower sclerites with waist in the foregut region, and proportionally wider prepallial sclerites. In the North Pacific, only one species, Falcidens salviniplaweni ( Ivanov, 1984) , is known from the Peter the Great Bay, Sea of Japan. The present species is distinguishable from the latter by having the tail-like narrowed posterior body.

Figures

Figure 6. Falcidens ryokuyomaruae sp. nov. (A) holotype, NSMT-Mo 78608, preserved condition; (B) paratype #8, NSMT-Mo 78616, specimen in life; (C) pedal shield of paratype #8; (D) radula of holotype. Scale bars: 1 mm for A; 0.5 mm for B; 200 µm for C; 100 µm for D.

Figure 7. Falcidens ryokuyomaruae sp. nov. Sclerites of holotype. (A, B) peribuccal region; (C, D) in foregut region; (E, F) in anterior midgut sac region; (G–I) in posterior midgut sac region; (J) in prepallial region; (K, L) at posterior margin of pallial region; (M) inside of posterior margin of pallial region. Scale bar: 100 µm.