HUMZ 149389, adult male, 590 mm TL, Okinawa Trough, 25°37'28"N, 126°05'35"E to 25°38'12"N 126°07'83"E. Collected on 2 August 1994 by unknown collector.

HUMZ 80329, adult female, 770 mm TL, East China Sea, 25°33.8'N, 126°25.2'E, 310 m depth, longline, collected on 4 December 1978 by T. Kanayama; HUMZ 80330, juvenile male, 365 mm TL, locality and collection data same as HUMZ 80329; HUMZ 80331, adult female, 755 mm TL, locality and collection data same as HUMZ 80329; HUMZ 101718, adult male, 685 mm TL, off north-west of Okinawa-jima Island, Japan, collected in July 1983 by K. Nakaya; HUMZ 146165, adult male, 600 mm TL, 31°58.5'N, 173°09.5'E, 390 m depth, longline, collected on 21 February 1997; HUMZ 149391, adult male, 623 mm TL, locality and collection data same as holotype; HUMZ 149392, adult male, 595 mm TL, locality and collection data same as holotype; HUMZ 149394, adult male, 623 mm TL, locality and collection data same as holotype; HUMZ 149395, adult male, 595 mm TL, locality and collection data same as holotype; HUMZ 149426, adult male, 630 mm TL, locality and collection data same as holotype; HUMZ 149434, adult male, 580 mm TL, Okinawa Trough, 25°40'32"N, 126°14'18"E to 25°41'43"N, 126°16'08"E, collected on 2 August 1994 by unknown collector; HUMZ 149438, adult male, 605 mm TL, locality and collection data same as HUMZ 149434.

Okinawa Trough, Japan, 25°37'28"N, 126°05'35"E to 25°38'12"N, 126°07'83"E, 2 August 1994, unknown collector.

HUMZ 495, juvenile female, 350 mm TL, unknown locality; HUMZ 39991, juvenile male, 260 mm TL, unknown locality.

Single values correspond to the holotype and range to paratypes, respectively. A Squalus species that is separated from its regional congeners by: first dorsal fin conspicuously upright (vs. first dorsal fin prone); pectoral-fin posterior margin falcate (vs. not falcate); upper and lower caudal lobes markedly slender with dorsal and ventral caudal tips pointed and broadly white (vs. upper and lower caudal lobes wide with dorsal and ventral caudal tips rounded and greyish in colour); dermal denticles lanceolate, except for S. brevirostris (vs. tricuspidate); smaller claspers with clasper outer length 3.3%, 3.3%–3.4% TL and clasper inner length 6.3%, 5.9%–6.3% TL (vs. 4.5%–4.9% TL, 6.9%–8.4% TL for S. mitsukurii vs. 4.8%, 4.3%–5.6% TL and 7.5%, 7.2%–9.1% TL for S. japonicus; vs. 3.9%, 3.9%–5.1% TL and 7.1%, 7.0%–8.1% TL for S. brevirostris). Squalus shiraii sp. nov. further differentiates from S. mitsukurii by more elongate eyes, its length 4.9%, 4.4%–4.9% TL (vs. 3.6%, 3.1%–4.0% TL) and smaller fifth gill slit, its length 1.7%, 1.7%–2.1% TL (vs. 2.3%, 2.2%–2.6% TL). It is separated from S. japonicus and S. brevirostris by prenarial length 5.5%, 5.4%–5.6% TL (vs. 5.9%, 5.9%–6.8% TL for S. japonicus vs. 4.0%, 3.8%–4.3% TL for S. brevirostris) and width of first dorsal-fin spine 0.9%, 0.9%–0.9% TL (vs. 0.7%, 0.5%–0.8% TL for S. japonicus vs. 0.7%, 0.5%–0.7% TL for S. brevirostris). Squalus shiraii sp. nov. further differs from S. japonicus by shorter snout, its preorbital length 7.9%, 7.4%–7.9% TL (vs. 8.7%, 8.7%–9.3% TL for S. japonicus), smaller preoral length 10.4%, 9.4%–10.4% TL (vs. 10.7%, 10.7%–12.2% TL), larger first dorsal fin, its base length 7.7%, 7.7%–8.7% TL and first dorsal-fin posterior margin length 9.2%, 9.0%–9.3% TL (vs. 6.7%, 6.6%–7.5% TL and 7.8%, 6.7%–8.6% TL for S. japonicus) and wider pectoral fin, its posterior margin length 10.0%, 10.0%–11.7% TL (vs. 8.6%, 7.8%–9.2% TL). It is separated from S. brevirostris by wider internarial space, its width 4.8%, 4.1%–4.8% TL (vs. 3.6%, 3.4%–3.8% TL for S. brevirostris), smaller second dorsal fin with inner margin length 3.6%, 3.2%–4.1% TL (vs. 5.0%, 4.8%–5.7% TL), shorter pectoral-fin inner margin, its length 7.3%, 7.3%–8.3% TL (vs. 10.3%, 9.8%–11.2% TL) and narrower caudal fin, its width at caudal fork 6.4%, 5.7%–6.4% TL (vs. 6.6%, 6.6%–7.3% TL). Squalus shiraii is distinguished from S. formosus regardless of maturity by having more elongate precaudal (78.3%, 78.6%–80.5% TL), pre-second (60.5%, 61.1%–63.4% TL) and pre-first (30.5%, 29.6%–31.2% TL) dorsal length (vs. 77.6%–78.3% TL, 59.4%–59.7% TL and 28.1%–29.0% in S. formosus).

Single values correspond to the holotype and range to paratypes, respectively.

External morphology. Body elongate (590–770 mm maximum TL in adults), fusiform and robust, arched dorsally from anterior margin of the eye to insertion of first dorsal fin, turning straight from abdomen to caudal fin (Fig. 1A–C); body equally deep from head to abdomen with head height 0.9 (0.9–1.0) times trunk height and 0.9 (0.8–1.0) times abdomen height; body with its greatest width at head (head width 1.1 (1.0–1.2) times trunk width and 1.3 (1.2–1.5) times abdomen width. Head flattened anterior-dorsally, arched posterior-dorsally and elongate, its length 24.1% (21.9%–23.9%) TL. Snout conspicuously obtuse at tip and large (preorbital length 7.9%, 7.4%–8.1% TL) (Fig. 1D–E); anterior margin of nostrils strongly broad and bi-lobed, its distance to snout tip 1.1 (1.1–1.3) times its distance to upper labial furrow; prenarial length 0.5 (0.5–0.6) times preoral length; internarial space 1.0 (0.8–1.1) times eye length. Eyes oval and enlarged, its length 3.1 (1.9–3.4) times eye height; eyes with anterior margin convex and posterior margin slightly notched. Prespiracular length 1.7 (1.7–1.7) times preorbital length and 0.6 (0.6–0.6) times prepectoral length. Spiracles crescent and large, its length 1.5% (1.0%–1.3%) TL, located laterally behind the eyes. Gill slits tall and somewhat vertical, placed anteriorly to pectoral-fin origin; fifth gill slit height 1.4 (1.1–1.7) times first gill slit height.

Figure 1. 

Squalus shiraii sp. nov.: lateral (A–C) and ventral (D, E) views; first (F) and second (G) dorsal fins; pectoral (H) and caudal (I) fins. HUMZ 149389 (holotype), adult male, 590 mm TL (A, D, F–I); HUMZ 80329 (paratype), adult female, 770 mm TL (B); HUMZ 80330 (paratype), juvenile male, 365 mm TL (C, E). Scale bars: 50 mm (A–E); 20 mm (F–I).

Preoral length 1.3 (1.3–1.5) times mouth width and 0.4 (0.4–0.5) times head length. Upper labial furrow short (its length 0.5, 0.4–0.5 times eye length) with thick and small fold; lower labial furrow elongate with inconspicuous fold. Mouth arched and broad, its width 1.5 (1.3–1.4) times prenarial length and 1.7 (1.5–1.7) times internarial space. Teeth similar in both jaws; upper teeth smaller than lower teeth; teeth with cusp oblique, thick and small; mesial cutting edge slightly convex; distal heel rounded; mesial heel notched; apron short and heavy (Fig. 3A, B). Two and two to three series of functional teeth in the upper and lower jaws, respectively, 13–14 for holotype (13–14 for paratypes) teeth rows on upper jaw and 11–11 (11–12) teeth rows on lower jaw.

Origin of first dorsal fin over the vertical line traced at pectoral-fin insertion. First dorsal fin elongate, its length 1.5 (1.4–1.7) times height of first dorsal fin; first dorsal fin conspicuously tall, its height 1.7 (1.5–1.8) times first dorsal-fin inner margin length; first dorsal-fin anterior margin convex, posterior margin straight in the upper half and conspicuously concave in the lower half; first dorsal-fin apex rounded and evidently slender at the fin web; first dorsal-fin free rear tip triangular and first dorsal-fin inner margin short, its length 0.6 (0.5–0.7) times first dorsal-fin base length (Fig. 1F, J). First dorsal-fin spine thick with base width 0.9% (0.9–0.9%) TL and elongate (first dorsal-fin spine length 0.5, 0.4–0.5 times height of first dorsal fin), although never reaching first dorsal-fin apex. Interdorsal space 1.0 (1.0–1.2) times prepectoral length and 2.3 (2.2–2.8) times dorsal-caudal space. Pre-second dorsal length 2.0 (2.0–2.1) times pre-first dorsal length. Second dorsal fin elongate, its length 0.9 (0.7–0.9) times length of first dorsal fin; second dorsal fin tall, its height 1.6 (1.4–1.8) times second dorsal-fin inner margin length; second dorsal-fin anterior margin convex; second dorsal-fin posterior margin strongly concave and falcate; second dorsal-fin apex rounded and lobe-like; second dorsal-fin inner margin small, its length 0.5 (0.5–0.6) times second dorsal-fin base length (Fig. 1G, K). Second dorsal-fin spine thick with base width 0.7% (0.7%–0.8%) TL; second dorsal-fin spine elongate, its length 0.9 (0.7–0.9) times height of second dorsal fin and 1.3 (1.2–1.6) times length of first dorsal-fin spine, although not reaching second dorsal-fin apex.

Prepectoral length 3.0 (2.8–2.9) times preorbital length. Pectoral fins with anterior and inner margins conspicuously convex; pectoral-fin posterior margin markedly concave; pectoral-fin apex and free rear tips rounded and lobe-like; pectoral-fin apex conspicuously transcending the horizontal line traced at pectoral-fin free rear tip (Fig. 1H, L); pectoral fins markedly falcate and broad with pectoral-fin posterior margin 1.0 (0.4–1.0) times trunk height; pectoral-fin anterior margin 2.0 (1.7–2.0) times pectoral-fin inner margin length and 1.4 (1.4–3.2) times pectoral-fin posterior margin length. Prepelvic length 1.8 (2.0–2.2) times prepectoral length. Pelvic fins located slightly nearer to the first dorsal fin than the second dorsal fin. Pelvic fins with margins straight and apex rounded; pelvic-fin free rear tips rounded to slightly pointed, thin and lobe-like. Adult males with clasper somewhat transcending the pelvic fin; clasper inner length 1.2 (0.3–1.2) times pelvic-fin inner margin length; clasper groove medial-dorsally and vertical, profound and elongate; apopyle broad, located anteriorly in the clasper groove; hypopyle broad, located anterior to rhipidion; rhipidion blade-like, thin and small, placed at the medial-distal end of the clasper.

Pelvic-caudal space 1.4 (1.1–1.6) times pectoral-pelvic space. Caudal keel evident laterally in the caudal peduncle from second dorsal fin free rear tip to caudal-fin origin. Caudal fin with conspicuously thin upper and lower caudal lobes (Fig. 1I, M); caudal fork concave with its width corresponding to 6.4% (5.7%–6.4%) TL; dorsal caudal margin straight and markedly elongate, its length 0.9 (0.9–1.0) times head length and 2.1 (1.9–2.1) times length of preventral caudal margin; upper postventral margin straight on its upper half and slightly convex on its lower half; lower postventral margin convex; preventral caudal margin convex and short, its length 1.8 (2.1–2.3) times length of pelvic-fin inner margin; dorsal caudal tip rounded; ventral caudal tip markedly pointed.

Squamation (Fig. 7A, B). Dermal denticles lanceolate and imbricate, broad at the crown base (conspicuously broad in females); length of dermal denticles larger than its width (although it is about equal to its width in females); denticles with median cusp slightly pointed lateral cusps inconspicuous; median and lateral ridges thick; median and lateral ridge bifurcated anteriorly; anterior furrow conspicuously profound and oval.

Colouration (Fig. 1). Body brown dorsally, light brown laterally, although whitish latero-ventrally from the pelvic fin to the caudal fin and pale ventrally. Dorsal fins brownish, somewhat light brown at dorsal fin base; first dorsal-fin inner margin white; second dorsal-fin posterior margin fairly white at its lower half; second dorsal-fin apex somewhat dark brown. Dorsal-fin spines brownish anterior-laterally and white at the tip. Pectoral fins dark brown, whitish ventrally at pectoral-fin base; pectoral-fin posterior margin fairly white (not uniform); pectoral-fin apex broadly white. Pelvic fins brown and whitish at the pelvic-fin base; pelvic-fin anterior and posterior margins slightly white. Caudal fin dark brown at upper and lower caudal lobes, whitish over vertebral column; postventral caudal margins white, except at the caudal fork where it is dark brown; dorsal and ventral caudal tips broadly white; preventral caudal margin white; dark caudal stripe and bar inconspicuous; black upper caudal blotch somewhat evident in adults. Juveniles with body light brown dorsally, pale lateral and ventrally; first and second dorsal-fin posterior margins slightly white at its lower half; dorsal-fin apex blackish to dark brown at the tip; pectoral fins light brown to reddish-brown with pectoral-fin posterior margins and apex broadly white; pelvic fins light brown to reddish-brown; caudal fin dark brown with postventral caudal margins broadly white, except at the caudal fork; dorsal and ventral caudal tips broadly white; conspicuous upper black caudal blotch in the midline of dorsal caudal margin; lower caudal lobe with oblique black marking at the origin of lower caudal lobe; small black caudal bar at the caudal fork.

Vertebral counts (Table 6). 47 for holotype (44–48 for paratypes) monospondylous vertebrae; 93 (91–94) precaudal vertebrae; 122 (120–123) total vertebrae.

This species is named after Dr. Shigeru M. Shirai, Japanese ichthyologist from Tokyo University of Agriculture, for his valuable contributions to Systematics of Squaliformes.

Shirai’s spurdog; Hiretaka-tsunozame (Japanese).

This species is apparently a Japanese endemic, occurring in the shallow waters of the upper continental slope off Southern Japan in the North-western Pacific Ocean at 310–390 m depth (Fig. 8A).

Squalus shiraii sp.nov. is often misidentified with S. mitsukurii due to similarities concerning the general shape of body, pectoral and caudal fins and snout length. However, it clearly differs from this species in addition to the characters provided in the diagnosis above by having an obtuse snout, dorsal and ventral caudal tips conspicuously pointed, upper and lower caudal lobes markedly tapered, pectoral fins conspicuously falcate, dermal denticles lanceolate and unicuspidate (vs. snout rounded, dorsal and ventral caudal tips rounded, upper and lower caudal lobes broad, pectoral fins not falcate, dermal denticles rhomboid and tricuspidate in S. mitsukurii). Squalus shiraii sp. nov. has body brown in colour, postventral and preventral caudal margins whitish, dorsal and ventral caudal tips broadly white and black upper caudal blotch evident in adults. S. mitsukurii has body conspicuously black to dark grey and caudal fins black throughout with post-ventral caudal margin fairly whitish and black upper caudal blotch not evident in adults.

Two recently described species, S. hawaiiensis and S. boretzi, are also morphologically similar to S. mitsukurii, but were supported as distinct according to molecular and morphological data in Daly-Engel et al. (2018) and Dolganov (2019). These species are apparently sympatric in the Hawaiian-Emperor Seamount Chain that runs across the North Pacific Ocean on about 6000 km from the Kamchatka Peninsula, Russia down to Hawaii, USA (Domier et al. 2017). Squalus shiraii sp. nov. share with S. hawaiiensis a more elongate snout that is more evident when considering the ratio prenarial length/inner-nostril labial furrow space (1.2, 1.1–1.3 for S. shiraii sp. nov. vs. 1.3, 1.1–1.1 for S. hawaiiensis), body robust and presence of black upper caudal blotch, but morphometrics and vertebral counts support that they are conspicuously distinct as provided in the diagnosis above. Squalus shiraii sp. nov. may be further distinguished from the Hawaiian species by shorter pre-second dorsal length 60.5%, 61.1%–63.4% TL (vs. 63.6%–67.0% TL for S. hawaiiensis), pre-vent length 47.3%, 46.8%–48.7%TL (vs. 50.4%–53.6% TL for S. hawaiiensis), prepelvic length (44.1%, 44.5%–46.8%TL (vs. 48.6%–52.4% TL for S. hawaiiensis) and interdorsal space (23.8%, 24.0%–25.7%TL (vs. 26.7%–30.0% TL for S. hawaiiensis). Other differences include height of second dorsal fin, clasper length and head width at mouth (refer to Daly-Engel et al. 2018 for comparisons). Squalus shiraii sp. nov. has lanceolate dermal denticles and dorsal fins markedly tall and upright (vs. tricuspidate, low and oblique dorsal fins in S. hawaiiensis). It also has shorter pectoral-caudal space than S. hawaiiensis and S. boretzi (19.6%, 17.1%–22.9% TL vs. 23.6%–27.7% TL for S. hawaiiensis vs. 25.2%–30.6% TL for S. boretzi). More detailed comparisons with S. boretzi were not accomplished here because we could not yet have access to the full original description of this species that is so far available in Russian only.

Some characteristics of S. shiraii are very similar to those of S. formosus. However, the Japanese species bears much smaller dorsal and pelvic fins and it has larger pre-first, pre-second and pre-caudal length. Other differences with the Taiwanese species include larger snout (preorbital and prenarial length), smaller and thinner second dorsal-fin spine, smaller spiracle and lower gill slits and dermal denticles lanceolate (vs. weakly tricuspidate; Fig. 7C, D). Squalus formosus is easily identified by the presence of white anterior margin of the first dorsal fin which is absent in the Japanese species. Vertebral counts further help to separate Squalus shiraii sp. nov. from other congeners, including precaudal vertebrae in S. mitsukurii, S. japonicus and S. brevirostris, as well as S. blainvillei, S. lalannei, S. montalbani, S. grahami, S. bahiensis, S. bassi, S. hawaiiensis and S. boretzi and total vertebrae in S. mitsukurii, S. japonicus, S. brevirostris, S. blainvillei, S. lalannei, S. montalbani, S. grahami, S. bahiensis, S. melanurus, S. hawaiiensis and S. boretzi. Other morphometric differences are also observed between S. shiraii and S. formosus and S. hawaiiensis regardless of maturity stage by more elongate snout (prenarial length 5.5%, 5.4%–5.6% TL vs. 4.5%–4.9% TL for S. formosus vs. 4.8%–5.1% TL for S. hawaiiensis), first dorsal-fin height 8.1%, 7.9%–8.8% TL (vs. 9.0%–10.0% TL for S. formosus vs. 6.9%–7.7% TL for S. hawaiiensis), second dorsal-fin anterior margin, its length 10.7%, 9.7%–11.3% TL (vs. 11.9%–12.6% TL for S. formosus vs. 6.7%–7.4% TL for S. hawaiiensis).

Squalus hawaiiensis: USNM 62450, adult female, unknown TL, Honolulu market, Oahu Island, Hawaii, USA; USNM 62467, adult female, 745 mm TL, locality same as USNM 62450. Type material of Squalus formosus: CSIRO H 6816-01 (holotype), adult male, 720 mm TL, Tashi fish Market, North-eastern Taiwan; CSIRO H 6292-10 (paratype), juvenile female, 335 mm TL, same locality as holotype.

AMNH 8822 (paratype), female, 248 mm TL; SU 7184 (paratype, identified as S. acanthias), male, 277 mm TL; SU 7748 (paratype), two females, 240–243 mm TL, male, 247 mm TL (All embryo paratypes taken from the holotype); SU 12793 (holotype), adult female, 710 mm TL, Honshu Island, Misaki, Japan, 35.159430°, 139.493865°, 14104 m depth (uncertain, according to CAS), collectors D. S. Jordan and J.O. Snyder in 1900; SU 12794 (paratype), adult male, 770 mm TL, data same as holotype.

HUMZ 33680, adult female, 760 mm TL, East China Sea, 29°38'N, 134°E; HUMZ 79798, adult female, 835 mm TL, Kyushu-Palau Ridge, 26°46.5'–26°46.6'N, 135°20.3'–135°20.8'E, 340–640 m depth; HUMZ 89858, juvenile female, 705 mm TL, off Hachijo-jima Island, Tokyo, Japan; HUMZ 97463, adult female, 800 mm TL, Ishikari Bay, Tokyo, Japan, 43°12.4'N, 141°08.5'E, 22 m depth; HUMZ 102986, adult female, 1005 mm TL, Central Pacific Ocean, near Northern Mariana Islands, 17°39.4'N, 145°50.3'E, 450 m depth; HUMZ 102987, adult female, 970 mm TL, Central Pacific Ocean, near Northern Mariana Islands, 19°09'N, 142°59'E, 520 m depth; HUMZ 102988, adult female, 1025 mm TL, same locality as HUMZ 102987; HUMZ 113586, adult female, 1120 mm TL, off Shirahama, Shimoda, Shizuoka Prefecture, Japan, 45 m depth; HUMZ 113587, adult female, 990 mm TL, same locality as HUMZ 113586; HUMZ 113588 adult female, 960 mm TL, same locality as HUMZ 113586; NSMT-P 44097, adult female, 740 mm TL, Izu, Honshu, Japan; NSMT-P 44381, adult male, 770 mm TL, off Koura, Suruga Bay, Honshu, Japan; NSMT-P 65518, adult female, 1119 mm TL, Suno-saki Point, Boso Peninsula, Chiba Prefecture, Honshu, Japan, 25 m depth; NSMT-P 77187, adult male, 1000 mm TL, unknown locality; NSMT-P 97762, neonate male, 278 mm TL, Tsushima, Nagasaki Prefecture, Kyusyu, Japan; SU 14245, adult female, 580 mm TL, Hong Kong, China; ZUMT 1360, female embryo, 178 mm TL, off Atami, Kanagawa Prefecture, Japan; ZUMT 21114, female embryo, 190 mm TL, Nagasaki, Japan.

Large-sized (710–1120 mm TL for adults) Squalus species that is distinguished from its regional congeners by: body dark grey to black in colour vs. body grey to light grey for S. japonicus vs. brownish-grey for S. brevirostris vs. brown for S. shiraii sp. nov.; dermal denticles tricuspidate and rhomboid (except for S. japonicus) vs. unicuspid and lanceolate. Squalus mitsukurii is separated from S. japonicus and S. brevirostris when adults by prenarial length 5.6%, 4.1%–5.6%TL (vs. 5.9%, 5.9%–6.8% TL for S. japonicus vs. 4.0%, 3.8%–4.3% TL for S. brevirostris) and from S. brevirostris and S. shiraii sp. nov. by smaller eyes, its length 3.6%, 3.1%–4.0% TL (vs. 5.0%, 4.2%–5.1% TL for S. brevirostris vs. 4.9%, 4.4%–4.9% TL for S. shiraii sp. nov.) and length of second dorsal-fin spine 0.8–1.1 times length of first dorsal-fin spine (vs. 1.3–1.8 times for S. brevirostris vs. 1.2–1.3 times for S. shiraii sp. nov.). It is further distinct from S. japonicus by smaller preoral length and distance nostril-upper labial furrow (10.3%, 7.6%–10.3% TL and 4.3%, 3.9%–4.5% TL vs. 10.7%, 10.7%–12.2% TL and 4.6%, 4.6%–5.1% TL for S. japonicus), more elongate first dorsal fin, its base length 8.2%, 7.6%–9.0% TL (vs. 6.7%, 6.6%–7.5% TL for S. japonicus), broader pectoral fins and head (pectoral-fin posterior margin length 11.7%, 9.9%–12.5% TL and head width at mouth 12.2%, 10.9%–12.2% TL vs. 8.6%, 7.8%–9.2% TL and 9.7%, 9.7%–10.6% TL for S. japonicus). Squalus mitsukurii differs from S. brevirostris by: pectoral fin with free rear tips rounded and posterior margin straight to weakly concave vs. free rear tips pointed and posterior margin conspicuously concave for S. brevirostris; shorter pectoral-fin inner margin, its length 9.5%, 7.7%–9.5% TL vs. 10.3%, 9.8%–11.2% TL for S. brevirostris; postventral caudal margins not uniformly white vs. uniformly white for S. brevirostris.

External morphology. Body fusiform, markedly humped anteriorly from posterior margin of the spiracle to pelvic fin origin, turning slender posteriorly; body conspicuously robust and stout with head height 1.2 (0.8–1.4) times trunk height and 1.6 (0.9–1.7) times abdomen height and head width 1.2 (1.0–1.6) times trunk width and 1.5 (1.1–2.6) times abdomen width (Fig. 2A–C). Head flattened anteriorly, conspicuously elongate, its length 24.2% (20.4%–24.4%) TL and broad, its width at mouth 12.2% (10.4%–12.8%) TL. Snout rounded at the tip and noticeably elongate, its preorbital length 7.3% (6.9%–10.7%) TL; prenarial length 1.3 (1.1–1.4) times distance between nostrils to upper labial furrow and 0.5 (0.5–0.6) times preoral length; anterior margin of nostrils wide and bi-lobed; internarial space 1.3 (0.8–1.4) times eye length (Fig. 2D, E). Eyes elliptical with anterior margin concave and posterior margin notched, placed laterally; eyes large, its length 4.1 (1.6–3.0) times eye height. Prespiracular length 0.5 (0.5–1.0) times prepectoral length and 1.8 (1.5–2.8) times preorbital length. Spiracles crescent, located laterally and behind the eyes; spiracles elongate, its length 0.4 (0.2–0.4) times eye length. Prebranchial length 1.6 (1.0–1.8) times prespiracular length. Gill slits vertical and concave, placed anterior to origin of pectoral fins; gill slits very tall with first gill slit height 1.7% (1.5%–2.8%) TL; fifth gill slit 1.3 (0.7–1.7) times taller than first gill slit.

Figure 2. 

Squalus mitsukurii: lateral (A–C) and ventral (D, E) views; first (F, J) and second (G, K) dorsal fins; pectoral (H, L) and caudal (I, M) fins. SU 12794 (paratype), adult male, 770 mm (A, D, F–I); HUMZ 102988, adult female, 1025 mm TL (B, E, J–M); NSMT-P 97762, neonate male, 278 mm TL (C). Scale bars: 50 mm (A, B, D, E); 25 mm (C); 20 mm (F–M).

Preoral length 1.2 (1.0–1.5) times greater than mouth width. Mouth somewhat straight and conspicuously broad (mouth width 1.8, 1.5–2.1 times internarial width and 1.5, 1.2–1.8 times prenarial length); upper labial furrow elongate, its length 2.5% (2.1%–2.7%) TL, bearing a thin fold; lower labial furrow markedly elongate, lacking fold. Teeth unicuspid, flattened labial-lingually, similar in both jaws; teeth somewhat rectangular, very broad at the crown; cusp short, thick, pointed and oblique; mesial cutting edge convex and oblique; distal heel conspicuously rounded; mesial heel sharp; apron thick and small, placed more laterally in the upper teeth and in the midline of crown base in the lower teeth, slightly more elongate in the lower teeth than in the upper teeth; median teeth present on upper jaw in adult paratype only and distinct from the subsequent teeth by: teeth hexagonal with cusp and apron placed more medially; both mesial and distal heels pointed (Fig. 3C, D). One to two series of functional teeth on upper jaw and two series in lower jaw; 13–0–15 for holotype (13–1–13) teeth rows on upper jaw; 12–0–12 (11–0–11) teeth rows on lower jaw.

Figure 3. 

Upper (A, C, E, G, I) and lower (B, D, F, H, J) teeth of S. shiraii sp. nov. (A, B; HUMZ 80329), S. mitsukurii (C, D; HUMZ 33680), S. japonicus (E, F; HUMZ 189737), S. brevirostris (G, H; HUMZ 189762) and S. suckleyi (I, J; NSMT-P 92640). Scale bars: 1 mm.

Interdorsal space 2.0 (1.9–3.0) times larger than dorsal-caudal space and 0.9 (0.9–1.3) times prepectoral length. Origin of first dorsal fin located prior to vertical traced at pectoral-fin free rear tips. First dorsal fin elongate, its length 1.1 (1.0–1.3) times length of second dorsal fin; first dorsal-fin base length 0.8 (0.7–1.3) times height of first dorsal fin. First dorsal fin conspicuously broad at fin web; first dorsal-fin anterior margin markedly convex and elongate, its length 12.0% (9.3%–13.2%) TL; first dorsal-fin posterior margin concave, its length 9.3% (7.0%–10.6%) TL; first dorsal-fin inner margin elongate, its length 6.2% (5.0%–6.6%) TL; first dorsal-fin apex broadly rounded; first dorsal fin vertical and low, its height corresponding to 1.6 (1.1–1.6) times length of first dorsal-fin inner margin and 1.3 (0.8–1.4) times preorbital length (Fig. 2F, J). Origin of first dorsal-fin spine anterior to vertical traced at pectoral-fin free rear tips. First dorsal-fin spine thick, its width at base 1.0% (0.5%–1.0%) TL; first dorsal-fin spine elongate, its length 3.9% (2.1%–5.4%) TL) and corresponding to 0.4 (0.3–0.6) times first dorsal fin height, although not reaching first dorsal-fin apex. Pre-second dorsal length 2.5 (2.4–3.1) times prepectoral length. Origin of second dorsal fin behind a vertical traced at pelvic-fin free rear tips. Second dorsal fin broad at fin web with second dorsal-fin apex conspicuously rounded and lobe-like; second dorsal fin small, its length at base 0.9 (0.7–1.3) times first dorsal fin base length; second dorsal fin oblique and low with second dorsal-fin height 1.3 (0.8–1.4) times length of second dorsal-fin inner margin; second dorsal-fin anterior margin convex and elongate, its length 10.2% (7.7%–11.9%) TL; second dorsal-fin posterior margin concave and somewhat falcate, its length 6.3% (4.5%–6.3%) TL; second dorsal-fin inner margin length 5.3% (4.1%–5.9%) TL (Fig. 2G, K). Second dorsal-fin spine elongate, its length 0.6 (0.5–1.0) times second dorsal fin height and 1.1 (0.8–1.9) times length of first dorsal-fin spine, although never reaching second dorsal-fin apex; second dorsal-fin spine thick, its base width 0.9% (0.7%–1.1%) TL.

Pectoral fins markedly broad and elongate (pectoral-fin posterior margin length 11.7%, 9.2%–12.5% TL), although never extending up to trunk height when adpressed laterally on body; pectoral-fin anterior margin straight; pectoral-fin inner margin convex; pectoral-fin posterior margin weakly concave medially; pectoral-fin apex rounded and lobe-like; pectoral-fin free rear tips rounded, reaching the same length when a horizontal line is traced at pectoral-fin apex (Fig. 2H, L); pectoral-fin anterior margin large, its length 1.6 (1.4–2.3) times greater than pectoral-fin inner margin length and 1.3 (1.1–1.5) times pectoral-fin posterior margin length. Pectoral-pelvic space 0.9 (0.7–1.1) times pelvic-caudal space. Pelvic fins broad with pelvic-fin anterior and posterior margins convex; pelvic-fin free rear tips rounded and lobe-like; pelvic fin elongate, its length 11.5% (8.4%–12.6%) TL; pelvic fins nearer to second dorsal fin than to first dorsal fin. Adult males with clasper extremely robust and thick; clasper groove medial-dorsally and longitudinal, profound and short, not extending forward to inner margin of the clasper; clasper inner margin length 0.7–1.3 times length of pelvic-fin inner margin; apopyle broad, placed anteriorly in the clasper groove; hypopyle wide, located just above the rhipidion; rhipidion blade-like and evidently slender and elongate, placed at the medial-distal end of the clasper.

Caudal keel prominent and situated laterally in the precaudal peduncle since insertion of second dorsal fin to behind origin of caudal fin. Caudal fin conspicuously slender in the upper caudal lobe; dorsal caudal margin somewhat convex proximally and conspicuously straight distally (Fig. 2I, M); caudal fin elongate with dorsal caudal margin length 1.0 (0.9–1.1) times head length and 2.0 (1.7–2.0) times larger than length of preventral caudal margin; upper postventral caudal margin straight; lower postventral caudal margin somewhat convex; preventral caudal margin convex, its length 1.9 (1.5–3.8) times length of pelvic-fin inner margin; both posterior and ventral caudal tips rounded and thin; caudal fork discontinuous and conspicuously concave, its width 7.0% (6.1%–7.7%) TL.

Squamation (Fig. 7E, F). Dermal denticles tricuspid, rhomboid to heart-shaped and imbricate, conspicuously broad at the crown, its length equal to its width; cusps pointed and posterior; lateral cusps often inconspicuous and much shorter than median cusp; median cusp conspicuous and wide; median ridge prominent, tall and thick with anterior furrow oval and profound; anterior margin of the crown broad, convex and arrow-shaped with one small ridge on each side; one to two lateral ridges, thin and smaller than median ridge. Dermal denticles slightly larger and more imbricate in adults than juveniles.

Colouration (Fig. 2). Body black dorsally, dark grey ventrally and laterally since first dorsal fin insertion to origin of caudal fin. Dorsal fins blackish, dark grey at fin base near the dorsal-fin spines; lower half of anterior margin of dorsal fins whitish. Dorsal-fin spines brownish anteriorly and white posteriorly and at the tips. Pectoral fins black dorsally and ventrally, although dark grey near pectoral-fin inner margin; pectoral-fin posterior margin fairly white. Pelvic fins dark grey dorsally and ventrally, somewhat dark brown at the fin base; pelvic-fin posterior margins slightly white. Caudal fin black, dark grey near caudal fin origin and proximally over vertebral column; dorsal caudal margin dark grey proximally; postventral caudal margins slightly white, except near the caudal fork; preventral caudal margin blackish. Embryos, neonates and young juveniles differ from adults by having body light grey, dorsal fins light grey with dorsal-fin apex blackish and dorsal-fin free rear tips white, caudal fin light grey with postventral caudal margins broadly white, dark caudal bar conspicuous, black upper caudal blotch prominent and black caudal blotch at the base of lower caudal lobe, located horizontally.

Vertebral counts (Table 6). 68 (holotype) 70–72 (other material) diplospondylous vertebrae; 30 (26–31) caudal vertebrae; 116–117 total vertebrae.

Squalus mitsukurii is a regional endemic to the North-western Pacific Ocean with records along the coasts of China, Marianna Islands and Japan. It inhabits continental shelves and upper continental slopes within depth ranges of 22 to 640 m (Fig. 8B). This species possibly reaches depths of up to 980 m in the Western Pacific Ocean, but it is often caught between 180–300 m depth (Chen et al. 1979).

Squalus mitsukurii has been misidentified with many nominal species in all oceans and, more particularly, with S. acutirostris, S. hawaiiensis, S. boretzi and S. shiraii sp. nov. in the North Pacific Ocean. Squalus mitsukurii exhibits smaller interdorsal space than S. hawaiiensis (21.1%, 21.1%–25.9% TL vs. 26.7%–30.0% TL), larger first and second dorsal fins (first dorsal-fin base length 8.2%, 7.6%–9.0% TL and second dorsal-fin base length 7.2%, 6.8%–7.9% TL vs. 6.2%–7.4% TL and 4.9%–5.5% TL in S. hawaiiensis), which corroborates the findings of Daly-Engel et al. (2018). Torso measurements that are suggested as being diagnostic to separate these species in the referred study show minor overlapping and they must be used with caution despite range values of the direct measurements (e.g. pre-first and pre-second dorsal lengths, precaudal length) are somewhat higher in S. hawaiiensis. The Japanese species further exhibit more elongated pectoral fins, its inner margin 9.5%, 7.7%–9.5% TL (vs. 6.4%–7.4% TL in S. hawaiiensis). Vertebral counts are congruent between these species, which is contrary to Daly-Engel et al. (2018) when comparing data taken from Chen et al. (1979). The latter work possibly included specimens of S. shiraii sp. nov. which is the only Japanese species with the highest number of vertebral counts. Morphologically similar Squalus species tend to exhibit overlapping of vertebral counts as seen here between S. mitsukurii, S. hawaiiensis and S. boretzi, with a few exceptions. Squalus mitsukurii may be separated from S. brevirostris, S. melanurus S. blainvillei and S. grahami by monospondylous vertebrae and from S. blainvillei, S. melanurus and S. montalbani by diplospondylous vertebrae and S. brevirostris, S. shiraii sp. nov., S. blainvillei and S. lalannei by total vertebrae. Precaudal vertebrae separates it from S. shiraii sp. nov., S. blainvillei, S. lalannei and S. montalbani.

NSMT-P 44380, adult male, 645 mm TL, Senoumi Bank, Suruga Bay, Honshu, Japan, approximate coordinates 34°43'N and 138.33E, 270–300 m depth. Collected on 13 April 1982, collector Kazunari Yano.

CSIRO H 6294-26, juvenile male, 440 mm TL, Tashi fish market, near I-Lan (NE coast), Taiwan; CSIRO H 6294-27, juvenile female, 360 mm TL, locality same as CSIRO H6294-26; CSIRO H 6294-31, adult female, 540 mm TL, locality same as CSIRO H6294-26; HUMZ 40026, juvenile male, 215 mm TL, unknown locality; HUMZ 80223, adult male, unknown TL, near Okinawa, Japan, 25°33.8'N, 126°25.2'E, 310 m depth; HUMZ 95213, unknown sex and TL, East China Sea, 27°46'N, 126°15.3'E, 296–465 m depth; HUMZ 189642, adult male, 530 mm TL, East China Sea; HUMZ 189673, juvenile female, 430 mm TL, East China Sea; HUMZ 189675, juvenile female, 277 mm TL, East China Sea; HUMZ 189676, juvenile male, 288 mm TL, East China Sea; HUMZ 189678, adult male, 545 mm TL, East China Sea; HUMZ 189682, juvenile male, 365 mm TL, East China Sea; HUMZ 189685, adult female, 512 mm TL, East China Sea; HUMZ 189687, juvenile male, 380 mm TL, East China Sea; HUMZ 189689, juvenile male, 276 mm TL, East China Sea; HUMZ 189693, juvenile female, 380 mm TL, East China Sea; HUMZ 189695, juvenile female, 290 mm TL, East China Sea; HUMZ 189696, juvenile male, 291 mm TL, East China Sea; HUMZ 189701, juvenile male, 287 mm TL, East China Sea; HUMZ 189705, juvenile male, 282 mm TL, East China Sea; HUMZ 189735, adult male, 518 mm TL, East China Sea; HUMZ 189737, adult male, 560 mm TL, East China Sea; HUMZ 189738, adult male, 535 mm TL, East China Sea; HUMZ 189739, adult male, 530 mm TL, East China Sea; HUMZ 191688, juvenile female, 340 mm TL, Okinawa, Japan; HUMZ 191689, juvenile male, 281 mm TL, Okinawa, Japan; HUMZ 191691, juvenile male, 338 mm TL, Okinawa, Japan; HUMZ 191693, juvenile female, 410 mm TL, Okinawa, Japan; HUMZ 191694, juvenile female, 440 mm TL, Okinawa, Japan; HUMZ 191695, juvenile male, 338 mm TL, Okinawa, Japan; HUMZ 191697, juvenile female, 330 mm TL, Okinawa, Japan; HUMZ 191698, juvenile male, 388 mm TL, Okinawa, Japan; HUMZ 191699, juvenile female, 410 mm TL, Okinawa, Japan; NMMB P 15491, adult male, 593 mm TL, Pingtung county, Taiwan; NMMB P 15691, adult male, 540 mm TL, locality same as NMMB P 15491; NMMB P 15696, adult male, 580 mm TL, locality same as NMMB P 15491; NMMB P 15698, juvenile female, 448 mm TL, locality same as NMMB P 15491; NMMB P 15699, adult male, 513 mm TL, locality same as NMMB P 15491; NMMB P 15700, juvenile female, 443 mm TL, locality same as NMMB P 15491; NSMT-P 47384, juvenile female, 433 mm TL, Central Pacific Ocean; NSMT-P 67530, adult female, 560 mm TL, East China Sea, 31°21.31'N, 128°19.12'E, 254–260 m depth; NSMT-P 91127, juvenile female, 410 mm TL, Mimase Fishing Port, Kochi City, Kochi Prefecture, Shikoku, Japan; UF 44332, adult female, 490 mm TL, Seno-Umi, Honshu Island, Suruga Bay, Japan, 195 m depth.

Single value corresponds to neotype and range values include all examined material from which data was taken. Species of Squalus clearly distinct from all its congeners (except with S. melanurus Fourmanoir, 1979 and S. nasutus) by a combination of characters: snout obtuse at tip and conspicuously elongate (prenarial length 5.9%–6.8% TL in adults); mouth markedly narrow, its width 1.6, 1.0–1.6 times prenarial length; pectoral fins conspicuously tapered (pectoral-fin posterior margin length 8.6%, 7.8%–9.2% TL in adults); markedly elongate preoral length 10.7%, 10.7%–12.2% TL. Squalus japonicus is differentiated from its regional congeners S. mitsukurii, S. brevirostris, S. shiraii sp. nov. and S. acutirostris by head markedly tapered with head width at mouth 9.7%, 9.7%–10.6% TL (vs. 10.9%–12.2% TL for S. mitsukurii vs. 10.7%–12.1% TL for S. brevirostris vs. 11.2%–11.6% TL for S. shiraii sp. nov. vs. 10.7%–11.6% TL for S. acutirostris) and caudal fin with conspicuous black caudal bar and black upper caudal blotch (vs. black caudal bar and black upper caudal blotch absent). It is separated from S. brevirostris and S. shiraii sp. nov. by more elongate pre-first dorsal length (31.8%, 31.8%–33.0% TL vs. 29.6%, 28.9%–31.7% TL for S. brevirostris and 30.5%, 29.9%–31.2% TL for S. shiraii sp. nov.), larger preorbital length (8.7%, 8.7%–9.3% TL vs. 6.7%, 6.5%–7.5% TL for S. brevirostris vs. 7.9%, 7.4%–7.9% TL for S. shiraii sp. nov.) and from S. mitsukurii and S. shiraii sp. nov. by smaller first dorsal fin, its base length 6.7%, 6.6%–7.5% TL (vs. 8.2%, 7.6%–9.0% TL for S. mitsukurii vs. 7.7%, 7.7%–8.7% TL for S. shiraii sp. nov.). It is also distinct from S. mitsukurii by larger inner nostril-labial furrow space (4.6%, 4.6%–5.1% TL vs. 4.3%, 3.9%–4.5% TL for S. mitsukurii) and from S. brevirostris by larger internarial space (4.1%, 4.1%–4.7% TL vs. 3.6%, 3.4%–3.8% TL for S. brevirostris). It differs from S. shiraii sp. nov. by lower first dorsal fin, its height 6.6%, 6.6%–7.5% TL (vs. 8.1%, 7.9%–8.2% TL for S. shiraii sp. nov.) and lower number of precaudal and total vertebrae (87, 80–88 and 116, 104–116 vs. 93, 91–94 and 122, 120–123 for S. shiraii sp. nov.). Squalus japonicus also differs from S. brevirostris by dermal denticles tricuspidate, pectoral-fin free rear tips rounded, postventral caudal margins not uniformly white (vs. unicuspid dermal denticles, pectoral-fin free rear tips pointed, postventral caudal margins uniformly white).

. Single values correspond to the neotype and ranges to all other examined material from which data were obtained.

External morphology. Medium sized species (512–645 mm maximum TL in adults), with body fusiform and skinny, somewhat arched from trunk to abdomen, turning slim to caudal fin; head height 0.9 (0.8–1.1) times trunk and abdomen heights (Fig. 4A–C). Head compressed and very narrow, its width 1.2 (1.1–1.3) times trunk width and 1.3 (1.0–1.7) times broader than abdomen width; head elongate, its length 23.6% (23.0%–24.5% TL). Snout conical and obtuse at the tip, conspicuously elongate (preorbital length 9.4%, 8.3%–10.1% TL); anterior margin of nostrils bi-lobed with first lobe larger than second lobe; nostrils much closer to the mouth than to the tip of the snout, its prenarial length 1.4 (1.2–1.6) times the distance from nostril to upper labial furrow (Fig. 4D, E). Eyes oval with anterior margin convex and posterior margin notched; eyes elongate, its length 2.5 (2.0–3.5) times greater than its height. Prespiracular length 0.6 (0.6–0.7) times prepectoral length. Spiracles crescent, placed laterally just posterior to the eyes; spiracles small, its length 0.3 (0.2–0.4) times eye length. Prebranchial length 1.4 (1.4–1.5) times larger than prespiracular length. Gill slits convex, located anterior to origin of pectoral fins; gill slits vertical and low with height of fifth gill slit 1.2 (0.9–1.6) times height of first gill slit.

Figure 4. 

Squalus japonicus: lateral (A–C) and ventral (D, E) views; first (F) and second (G) dorsal fins; pectoral (H) and caudal (I) fins. NSMT-P 44380 (neotype), adult male, 645 mm TL (A, D, F–I); HUMZ 189738, adult male, 535 mm TL (B); HUMZ 189687, juvenile male, 380 mm TL (C, E). Scale bars: 50 mm (A–E); 20 mm (F–I).

Preoral length 1.8 (1.4–2.7) times greater than mouth width. Mouth markedly arched and narrow, its width 1.5 (1.0–1.6) times broader than internarial width and 1.0 (0.6–1.3) times prenarial length; upper labial furrow elongate, its length 2.1% (1.9%–2.4% TL), with fold very thin; lower labial furrow also elongate, although without fold. Teeth unicuspid and tiny on both jaws, although upper teeth smaller than lower teeth; cusp short and oblique, thicker on lower teeth than upper teeth; mesial cutting edge convex; mesial heel markedly notched; distal heel rounded; apron thick and short, somewhat conical in lower teeth (Figs 3E–F). Two series of functional teeth on each jaw; 13–0–13 teeth rows for neotype (13–0–13 for other material) in upper jaw; 10–0–10 (11–0–11) teeth rows in lower jaw.

Pre-first dorsal length 1.4 (1.3–1.5) times larger than prepectoral length. Origin of first dorsal fin prior to the vertical line traced at pectoral-fin free rear tips. First dorsal fin evidently slender at fin web (broader in young juveniles); first dorsal-fin anterior margin convex; first dorsal-fin posterior margin almost straight, although falcate distally; first dorsal-fin apex markedly rounded; first dorsal-fin free rear tip pointed (Fig. 4F); first dorsal fin short, its base length 1.0 (0.8–1.1) times height of first dorsal-fin, its length 1.0 (0.8–1.1) times length of second dorsal fin; first dorsal fin low, its height 1.3 (1.1–1.6) times length of first dorsal-fin inner margin; first dorsal-fin inner margin small, its length 5.6% (5.1%–6.6%) TL. First dorsal-fin spine narrow, its base width 0.6% (0.5%–0.8%) TL and very short (its length 2.9%, 2.0%–3.5% TL), comprising 0.4 (0.3–0.5) times first dorsal-fin height. Interdorsal space 1.0 (0.8–1.1) times prepectoral length and 2.2 (1.8–2.4) times larger than dorsal caudal space. Pre-second dorsal length 2.7 (2.6–2.8) times prepectoral length. Second dorsal fin slender at fin web; second dorsal-fin anterior margin concave; second dorsal-fin posterior margin concave and deeply falcate; second dorsal-fin apex rounded and lobe-like; second dorsal-fin free rear tip pointed (Fig. 4G); second dorsal fin short, its length 2.1 (1.8–2.5) times second dorsal-fin height; second dorsal-fin inner margin small, its length 5.0% (4.3%–5.5%) TL; second dorsal fin low, its height 1.3 (1.2–1.4) times length of second dorsal-fin inner margin. Second dorsal-fin spine slender, its base width 0.8% (0.7%–1.0%) TL; second dorsal-fin spine small, its length 4.9% (4.1%–5.6%) TL, corresponding to 0.8 (0.6–1.0) times height of second dorsal fin, almost reaching its apex; second dorsal-fin spine 1.7 (1.3–2.2) times larger than first dorsal-fin spine.

Pectoral fins conspicuously narrow (pectoral-fin posterior margin length 7.9%, 6.8%–10.1% TL); pectoral-fin anterior and inner margins convex; pectoral-fin posterior margin concave (deeply concave in juveniles); pectoral-fin anterior margin length 1.6 (1.4–1.7) times length of pectoral-fin posterior margin and 1.5 (1.3–1.7) times length of pectoral-fin inner margin; pectoral-fin apex and free rear tips rounded; pectoral-fin free rear tips lobe-like and reaching the horizontal line traced at pectoral-fin apex (in juveniles, pectoral-fin free rear tips transcend the pectoral-fin apex) (Fig. 4H). Pectoral-pelvic space 0.8 (0.7–0.8) times pelvic-caudal space. Pelvic-caudal space 1.2 (1.1–1.4) times larger than interdorsal space; pelvic fins in the midline between two dorsal fins, slightly nearer to first dorsal fin. Pelvic fins narrow with margins straight; pelvic-fin free rear tips pointed; pelvic fins short, its length 10.8% (9.6%–12.4%) TL. Clasper thin and small, slightly transcending pelvic-fin free rear tips; clasper inner margin length 0.8 (0.5–1.3) times length of pelvic inner margin; clasper groove dorsal and longitudinal, elongate and open; apopyle rounded, located anteriorly in the clasper groove; hypopyle constricted, located posteriorly in the clasper groove; rhipidion very thin and soft, flap-like, placed medially at distal end of the clasper.

Caudal keel prominent and lateral since insertion of second dorsal fin to behind origin of caudal fin. Caudal fin small with dorsal caudal margin length 0.9 (0.8–0.9) times head length and 1.9 (1.8–3.1) times length of preventral caudal margin; dorsal and ventral caudal lobes slender (dorsal lobe rectangular in juveniles); dorsal caudal tip pointed; dorsal caudal margin slightly convex; upper postventral caudal margin straight; lower postventral caudal margin convex (Fig. 4I); preventral caudal margin convex and short, its length 2.0 (1.4–2.4) times greater than length of pelvic-fin inner margin; ventral caudal tip rounded; caudal fork discontinuous, markedly concave and narrow, its width 6.6% (6.0%–7.4%) TL.

Squamation (Fig. 7G, H). Dermal denticles tricuspid and markedly imbricated, its length slightly greater than its width; denticles conspicuously broad at the crown base; cusps posterior, prominent and pointed; lateral cusps shorter than median cusp, forming well-marked concavity with median cusp; median ridge thick and elongate; one to two lateral ridges, very thin, almost equal in length to median ridge; anterior furrow shallow with one small ridge on each side; anterior margin of the crown arrow-shaped. In juveniles, denticles are slender at the crown base, with cusps conspicuously pointed and more elongate and anterior margin of the crown tapered.

Colouration (Fig. 4). Body grey to light grey dorsally and white laterally from insertion of pectoral fins to origin of caudal fin. Dorsal fins greyish, darker at its apex to the upper half of the posterior margin, whitish at fin base and free rear tips; dorsal-fin spine brown anteriorly, white posteriorly and at the tips. Pectoral fins grey, slightly darker near pectoral-fin anterior margin; pectoral-fin posterior margin strongly white and uniform. Pelvic fins grey dorsally and completely pale ventrally; pelvic-fin posterior margin white. Caudal fin mostly greyish, light grey over vertebral column; dorsal caudal margin whitish proximally; upper postventral caudal margin white, lower postventral caudal margin broadly white; lower caudal lobe mostly white; preventral caudal margin whitish; black caudal stripe conspicuous; black upper caudal blotch conspicuous in the upper caudal lobe, nearest to dorsal caudal tip; black caudal bar oblique, placed in the caudal fork to midline of upper postventral caudal margin; dorsal and ventral caudal tips broadly white. Embryos, neonates and young juveniles exhibit body light grey with caudal bar and upper caudal blotch more conspicuous than in adults.

Vertebral counts (Table 6). 87 (neotype) 80–88 (other material) precaudal vertebrae; 73 (65–75) diplospondylous vertebrae; 115 (104–116) total vertebrae.

Squalus japonicus is a regional endemic species occurring in the North-western Pacific Ocean from Southern Japan, China to Taiwan (Fig. 8C). It is found on the upper continental slope of tropical and subtropical areas, ranging between 195–520 m depth, although it also has been reported at 52 m depth only in Weigmann (2016). This species also occurs in Korean waters (Chen et al. 1979).

Ishikawa (1908) mentioned three syntypes of S. japonicus that were deposited at the Zoological University Museum of Tokyo (formerly Imperial Museum of Tokyo). A visit to the fish collection from ZUMT was undertaken in 2013 to verify the existence of the type material. However, it was noticed that there was a single specimen of S. japonicus held in the museum, assuming that syntypes are lost and no longer exist. Dr. M. Nakae (NSMT, person. comm. 2014) stated that the NSMT fish collection has never been in possession of this material and ratified that the syntypes are lost. Squalus japonicus has been misidentified in waters outside Japan (e.g. Vietnam by Quang et al. 2013) and many morphological variations were reported earlier in Compagno et al. (2005b) and Last et al. (2007b) for material from Indonesia, Philippines and Western Australia, compromising the correct application of this nominal species. To clarify the taxonomic status of S. japonicus across the Western Pacific Ocean, the designation of a neotype along with a re-description of this species is provided herein. The neotype was collected from Southern Japan (Senoumi Bank, Suruga Bay), which is within the known range distribution of S. japonicus, based on the examined material, the original description of S. japonicus and information in literature. The designation of the neotype was further based on the sex, maturity and total length that were approximate to those provided for a syntype of S. japonicus in Ishikawa (1908). The neotype is securely deposited in the scientific fish collection from the NSMT in Tokyo at the Department of Zoology for preservation and accessibility purposes. This species is easily separated from its congeners through many characteristics of the external morphology, morphometrics and meristic data, including shape of head, snout and pectoral fins, length of snout, width of mouth and preoral length and several other external measurements as shown in the diagnosis above. The current diagnosis and re-description of S. japonicus are congruent with observations of Ishikawa (1908) and Chen et al. (1979) with the exception of the presence of two hooks in the clasper. These cartilages of the clasper probably concern the accessory terminal 3 cartilage (spur) and dorsal terminal cartilage (claw) that are commonly noticed in species of Squalus. Differences in morphometrics are observed between the data provided in Ishikawa (1908) and those from the present analysis, but this is due to divergences of the applied methodology.

This species together with S. melanurus and the Australian species S. nasutus, comprise the S. japonicus subgroup of species whose members are characterised by having a conspicuously elongate and obtuse snout, thin body, mouth very narrow and pectoral fins tapered (Viana and Carvalho 2018a). The Japanese species is more closely related to S. nasutus according to morphological and genetic data (Last et al. 2007b). Shape of dermal denticles, dentition and shape of dorsal, pectoral, pelvic and caudal fins are intrinsically similar between these two species. Morphometrics are also mostly congruent, although differences in measurements are apparent when adults are compared: second dorsal fin height 5.4%–6.5% TL for S. japonicus vs. 4.9%–5.3% TL for S. nasutus; length of second dorsal-fin anterior margin 10.8%–12.7% TL vs. 8.6%–10.5% TL; length of second dorsal-fin spine 4.7%–5.6% TL vs. 3.9%–4.4% TL; length of dorsal caudal margin 19.6%–20.4% TL vs. 18.1%–19.4% TL. Preoral length, eye length, pectoral-fin inner margin length, trunk height and width overlaps between these two species which contradicts Last et al. (2007b) in supporting these measurements as diagnostic characters for these species. Vertebral counts differ between S. japonicus and S. nasutus as well: monospondylous (41–43), precaudal (84–88) vertebrae (vs. 37–40; 80–83 for those from Australia). These results are congruent with recent molecular analyses of the cytochrome c oxydase 1 (COI) and NADH dehydrogenase 2 (NAD2) mitochondrial genes in Ward et al. (2007), Naylor et al. (2012), Straube et al. (2013), Veríssimo et al. (2017) and Daly-Engel et al. (2018) which support genetic divergence between S. japonicus and S. nasutus. The latter study also noticed high intraspecific genetic distances in S. japonicus when including data taken from populations from Japan and Taiwan, but variations in the species are not yet supported through morphological evaluation, except for the black caudal marking near the base of the lower lobe in Taiwanese specimens that is absent in other material. Compagno et al. (2005b) also noticed variations in this species when examining material from the Philippines and considered it to be a separated species (Squalus sp. 2) due to differences with S. japonicus and S. nasutus. In contrast, Last et al. (2007b) considered it to be con-specific with S. nasutus as the morphological differences pointed in Compagno et al. (2005b) and in their study represent intraspecific variations. Whether the Philippine form represents an undescribed species is still unclear, but we consider it here to be con-specific with S. nasutus for nomenclature stability.

Squalus nasutus. AMS I22817-008, juvenile male, 362 mm TL, Northwest Shelf, 240 km north of Port Hedland, Western Australia, 18°06'S, 117°45'E, 492–520 m depth; CSIRO H 4132-02, adult male, 465 mm TL, Bolinao evening market, Philippines; CSIRO H 4132-03, juvenile female, 475 mm TL, locality same as CSIRO H 4132-02; CSIRO H 4132-04, adult female, 540 mm TL, locality same as CSIRO H 4132-02; CSIRO H 5860-01, adult female, 540 mm TL, Cilacap (South coast, central Java), Indonesia, 07°40'S, 109°00'E; CSIRO H 5860-02, adult female, 570 mm TL, locality same as CSIRO H 5860-01; CSIRO H 5860-03, adult female, 545 mm TL, locality same as CSIRO H 5860-01; CSIRO H 6125-04, adult male, 465 mm TL, Kedonganan, Jimbaran Bay (South coast of Bali), Indonesia, 08°45'S, 115°10'E; CSIRO H 6413-01, adult female, 580 mm TL, West of Shark Bay, Western Australia, 25°03.8'S, 112°08.9'E, 315–340 m depth; CSIRO H 6484-01, adult female, 575 mm TL, locality same as CSIRO H 5860-01. Type material of S. nasutus: CSIRO H2590-12, adult female, 503 mm TL, west of Leander Point, Western Australia, 29°15'S, 113°56'E (holotype); Paratypes: CSIRO H 2608–15, juvenile female, 411 mm TL, Rottnest Canyon, Western Australia, 31°57'S, 115°08'E; CSIRO H 2598–07, juvenile female, 465 mm TL, west of Green Head, Western Australia, 29°58'S, 114°27'E; CSIRO H 2567–08, adult male, 470 mm TL, west of Dorre Island, Western Australia, 25°09'S, 112°09'E; CSIRO H 1652–01, juvenile female, 315 mm TL, northwest of Port Hedland, Western Australia, 18°25'S, 117°48'E; CSIRO H 1652–02, juvenile female, 459 mm TL, same locality as CSIRO H 1652–01; CSIRO H 1207–07, adult female, 537 mm TL, northwest of Port Hedland, Western Australia, 18°20'S, 117°50'E; CSIRO H 1207–08, juvenile female, 496 mm TL, same locality as CSIRO H 1207–07; CSIRO CA 3290, adult female, 549 mm TL, southwest of Rowley Shoals, Western Australia, 18°10'S, 118°20'E; CSIRO CA 4055, adult female, 527 mm TL, southwest of Rowley Shoals, Western Australia, 18°11'S, 118°04'E; CSIRO H 2898–07, juvenile male, 452 mm TL, north-northwest of Port Hedland, Western Australia, 18°07'S, 118°12'E; CSIRO CA 4110, adult male, 497 mm TL, east of Rowley Shoals, Western Australia, 17°18'S, 120°09'E; CSIRO H 1693–01, juvenile male, 361 mm TL, Rowley Shoals, Western Australia, 17°02'S, 120°05'E; CSIRO H 1693–02, juvenile female, 306 mm TL, same locality as CSIRO H 1693–01; CSIRO H 1694–01, juvenile female, 425 mm TL, Rowley Shoals, Western Australia, 16°57'S, 120°14'E; CSIRO H 2032–01, adult male, 461 mm TL, northeast of Mermaid Reef, Rowley Shoals, Western Australia, 16°54'S, 120°25'E; CSIRO H 2032–02, juvenile female, 404 mm TL, same locality as CSIRO H 2032–01; WAM P 28086-006, juvenile male, 380 mm TL; juvenile female, 450 mm TL, Rowley Shoals, Western Australia, 17°49'S, 118°41'E, 308–310 m depth.

ZUMT 7630 (holotype), adult male, 426 mm TL, collected at Tokyo fish market, Japan, unknown collecting date and collector.

CAS 15916, juvenile male, 380 mm TL, Formosa Strait, Taiwan, 26°N, 121°E, 90 m depth; CAS 30563, juvenile female, 270 mm TL, Formosa banks, Taiwan strait, Taiwan, 22°40'N, 118°30'E, 50 m depth; CSIRO H 6293-29, juvenile female, 404 mm TL, Tashi fish market, near I-Lan (NE coast), Taiwan; CSIRO H 6483-02, adult male, 450 mm TL, off Kasasa, Kagoshima, Japan, East China Sea, 31°29'N, 130°02'E, 145–150 m depth; HUMZ 33679, juvenile male, 390 mm TL, East China Sea, 29°54'N, 126°08'E; HUMZ 95065, adult female, 465 mm TL, East China Sea, 26°14.7'N, 124°42.5'E, 270–342 m depth; HUMZ 189743, adult male, 387 mm TL, East China Sea; HUMZ 189745, adult female, 450 mm TL, East China Sea; HUMZ 189747, adult male, 365 mm TL, East China Sea; HUMZ 189751, adult male, 395 mm TL, East China Sea; HUMZ 189756, adult female, 402 mm TL, East China Sea; HUMZ 189757, juvenile female, 357 mm TL, East China Sea; HUMZ 189758, adult female, 475 mm TL, East China Sea; HUMZ 189761, adult male, 400 mm TL, East China Sea; HUMZ 189762, adult male, 403 mm TL, East China Sea; HUMZ 189763, adult female, 433 mm TL, East China Sea; HUMZ 189767, juvenile male, 275 mm TL, East China Sea; KAUM-I 185, adult female, 500 mm TL, off Kasasa, Kagoshima, Japan, East China Sea, 31°29'N, 130°02'E, 145–150 m depth; KAUM-I 187, adult female, 578 mm TL, locality same as KAUM-I 185; KAUM-I 377, adult male, 377 mm TL, locality same as KAUM-I 185; NSMT-P 47378, adult female, 600 mm TL, Central Pacific Ocean; NSMT P-47379, adult male, 452 mm TL, Derwent-Hunter Guyot, 30°36.3'S, 156°12.2'E; NSMT-P 64979, two juvenile females, 480–520 mm TL, East China Sea, 29°0.57'N, 127°0.4'E, 167–170 m depth; SU 13468, adult female, 530 mm TL, Osaka market, Japan, 34.70N, 135.49E; SU 230373, neonate female, 270 mm TL, two neonate males, 200 mm TL, South China Sea, 19°6'30"N, 112°38'00"E, 111–119 m depth; ZUMT uncatalogued, female embryo, 147 mm TL, male embryo, 182 mm TL, Japan.

A small-sized (373–578 mm TL in adults) Squalus species that differs from its Japanese congeners by: snout conspicuously short, its prenarial length 4.0%, 3.8%–4.3% TL (vs. 5.4%–5.6% TL for S. shiraii sp. nov. vs. 4.1%–5.6% TL for S. mitsukurii vs. 5.9%–6.8% TL for S. japonicus vs. 4.7%–5.1% TL for S. acutirostris); dermal denticles unicuspid and lanceolate; pectoral-fin free rear tips conspicuously pointed; more elongate pectoral fins with its inner margin length 10.3%, 9.8%–11.2% TL (vs. 7.3%–8.3%TL for S. shiraii sp. nov. vs. 7.7%–9.5% TL for S. mitsukurii vs. 7.5%–8.8% TL for S. japonicus vs. 7.3%–8.7% TL for S. suckleyi vs. 6.6%–8.3% TL for S. acutirostris). It is further separated from S. japonicus and S. shiraii sp. nov. by narrower internarial space (3.6%, 3.4%–3.8% TL vs. 4.1%, 4.1%–4.7% TL for S. japonicus vs. 4.8%, 4.1%–4.8% TL for S. shiraii sp. nov.) and from S. shiraii sp. nov. by larger second dorsal and pelvic fins (second dorsal-fin inner margin length 5.0%, 4.8%–5.7% TL and pelvic fin length 11.7%, 10.9%–13.6% TL vs. 3.6%, 3.2%–4.1% TL and 10.7%, 9.5%–10.7% TL for S. shiraii sp. nov.) and wider caudal fin, its width at caudal fork 6.6%, 6.6%–7.3% TL (vs. 6.4%, 5.7%–6.4% TL for S. shiraii sp. nov.). It is distinct from S. japonicus by snout rounded at tip, shorter preorbital (6.7%, 6.5%–7.5% TL vs. 8.7%, 8.7%–10.3% TL for S. japonicus) and preoral length (9.1%, 8.4%–9.5% TL vs. 10.7%, 10.7%–12.2% TL for S. japonicus). Squalus brevirostris is easily separated from S. mitsukurii and S. shiraii sp. nov. by mononspondylous and total vertebrae (40, 37–42 and 110, 105–114 vs. 44–46 and 116–117 for S. mitsukurii vs. 47, 44–48 and 122, 120–123 for S. shiraii sp. nov.). It also exhibits lower number of precaudal vertebrae than S. shiraii sp. nov. (83, 78–89 vs. 93, 91–94 for S. shiraii sp. nov.).

External morphology. Small size species (147.0–578.0 mm TL) of Japanese dogfish. Body fusiform and slender, equally deep from head to trunk (head height 0.9, 0.9–1.0 times trunk height and 1.0, 0.8–1.2 times abdomen height) (Fig. 5A–C). Head very short, its length 21.9% (20.0%–29.6%) TL; head width 1.4 (1.0–1.4) times broader than trunk width and 1.1 (1.1–1.6) times abdomen width. Snout somewhat obtuse ventrally, very short (preorbital length 6.7%, 6.5%–7.5% TL); anterior nasal flap broad and bifurcated, nearer to snout tip than to mouth with prenarial length 0.8 (0.8–0.9) times distance from nostril to upper labial furrow (Fig. 5D–F). Eyes oval with anterior margin rounded and posterior margin notched; eyes large, its length 2.3 (1.8–2.5) times its height. Prespiracular length 0.6 (0.5–0.6) times prepectoral length and 1.8 (1.6–1.9) times preorbital length. Spiracle crescent and narrow, its length 0.3 (0.3–0.4) times eye length, nearer to posterior margin of the eye than to first gill slit. First and fifth gill slits somewhat concave and low; fifth gill slit 1.1 (0.9–1.6) times higher than first gill slit.

Figure 5. 

Squalus brevirostris: lateral (A–C) and ventral (D–F) views; first (G, K) and second (H, L) dorsal fins; pectoral (I, M) and caudal (J,N) fins. ZUMT 7630 (holotype), adult male, 426 mm TL (A, D, G–J); HUMZ 189762, adult male, 403 mm TL (B, E, K–N); HUMZ 189767, juvenile male, 275 mm TL (C, F). Scale bars: 50 mm (A–F); 20 mm (G–N).

Preoral length somewhat equal to mouth width, corresponding to 1.1 (1.1–1.2) times the latter. Mouth markedly arched and narrow, its width 2.3 (1.8–2.3) times broader than internarial space; upper labial furrow small, its length 2.6% (2.0%–2.7% TL) with thin fold; lower labial furrow also short with fold subdivided into three small inter-digits. Teeth tiny and unicuspid, labial-lingually flattened and alternate, similar in both jaws, lower teeth slightly wider than the upper teeth; cusp short and oblique; mesial cutting edge convex; both distal and mesial heels rounded; apron elongate in upper and lower teeth (Fig. 3G, H). Three and two series of functional teeth on upper and lower jaws for holotype (two series for non-type specimens); 12–0–13 (12–0–12) teeth rows in upper jaw; 10–0–10 (10–0–10) teeth rows in lower jaw.

Pre-first dorsal length 1.4 (1.3–1.6) times prepectoral length. First dorsal fin short, its length 1.7 (1.5–1.9) times its height; first dorsal-fin low, its height 1.3 (1.2–1.5) times greater than its inner margin length; first dorsal-fin anterior margin concave and posterior margin straight; first dorsal-fin free, rear tip pointed; first dorsal-fin apex rounded and slender at fin web (Fig. 5G, K). Origin of first dorsal-fin spine anterior to vertical traced at free rear tips of pectoral fins; first dorsal-fin spine short (broken in the holotype), never reaching first dorsal-fin apex, its length 0.3–0.6 times first dorsal-fin height. Interdorsal space 1.1 (1.1–1.3) times prepectoral length and 2.4 (2.2–2.6) times larger than dorsal-caudal space. Pre-second dorsal length 2.8 (2.7–3.3) times greater than prepectoral length and 3.0 (2.8–3.2) times the length of dorsal caudal margin. Second dorsal fin very short, its length 0.9 (0.8–1.0) times first dorsal-fin length; second dorsal fin low, its height 1.2 (0.8–1.4) times length of second dorsal-fin inner margin; second dorsal-fin anterior margin concave and posterior margin markedly falcate; second dorsal-fin apex rounded and evidently slender at fin web (Fig. 5H, L). Second dorsal-fin spine thick and elongate, although not reaching the apex, its length 0.8 (0.8–1.1) times the fin height; second dorsal-fin spine length 1.3–2.6 times greater than length of first dorsal-fin spine.

Pectoral fin conspicuously narrow with pectoral-fin anterior margin length 1.4 (1.3–1.9) times larger than pectoral-fin posterior margin length and 1.4 (1.1–1.5) times larger than pectoral-fin inner margin length; pectoral-fin anterior and inner margins convex; pectoral-fin posterior margin conspicuously concave; pectoral-fin apex rounded and lobe-like; pectoral-fin free rear tips markedly pointed and triangular, markedly lobe-like (Fig. 5I, M); pectoral-fin free rear tips reaching the horizontal line traced at pectoral-fin apex. Pectoral-pelvic space 0.8 (0.6–0.9) times pelvic-caudal space. Pelvic fins nearer to first dorsal fin than second dorsal fin. Pelvic fins very narrow with pelvic-fin margins straight; pelvic-fin apex rounded; pelvic-fin free rear tips triangular, pointed and lobe-like; pelvic fin length 1.2 (0.9–1.2) times length of preventral caudal margin. Clasper flattened ventrally and small, its outer length 3.9% (1.4%–5.1% TL), transcending the pelvic-fin free rear tips; clasper groove dorsal, longitudinal and short; apopyle and hypopyle with conspicuous apertures, near each other; rhipidion blade-like, located at the posterior end of the clasper; inner length of clasper 1.2 (0.5–1.2) times length of pelvic-fin inner margin.

Caudal keel prominent, reaching from forward second dorsal fin insertion to behind origin of caudal fin. Caudal fin slightly slender in the dorsal caudal lobe; dorsal caudal and upper postventral caudal margins convex; dorsal caudal tip rounded (Fig. 5J, N); dorsal caudal margin 0.9 (0.7–1.0) times head length and 2.0 (1.5–2.0) times larger than preventral caudal margin; lower postventral caudal margin convex (broken in holotype); preventral caudal margin strongly convex and short, its length 1.7 (1.6–2.3) times greater than length of pelvic-fin inner margin; ventral caudal tip rounded (broken in holotype); caudal fork discontinuous between lobes, its width 6.6% (6.6%–7.6%) TL.

Squamation (Fig. 7I, J). Dermal denticles unicuspid and lanceolate, not imbricated; cusp rounded and posterior; denticles conspicuously slender at crown, although markedly broad medial-anteriorly at its base; denticles with length much greater than its width; median ridge tall and thick, bifurcated anteriorly with profound anterior furrow; lateral ridges tall and thin, shorter than median ridge.

Colouration (Fig. 5). Body brownish-grey dorsally, white ventrally and latero-posteriorly Pectoral fins grey with pectoral-fin posterior and inner margins white. Pelvic fins white dorsally and ventrally. First dorsal fin brownish-grey, slightly white at first dorsal-fin posterior margin, free rear tip and fin base; second dorsal fin light brown, whitish at the fin base, somewhat white at second dorsal-fin apex, posterior margin and freer rear tip. Dorsal-fin spines grey, dark grey anteriorly and white at the tip. Caudal fin light brown, whitish near vertebral column; postventral caudal margins white, broadly white at dorsal caudal tip and from ventral caudal tip to caudal fork; light black caudal stripe.

Vertebral counts (Table 6). 83 (holotype) 78–89 (other material) precaudal vertebrae; 70 (66–72) diplospondylous vertebrae; 110 (105–114) total vertebrae.

This species occurs in tropical and subtropical areas of the North-western Pacific Ocean from Southern Japan, China and Taiwan on continental shelves and upper continental slopes between 50–342 m depth (Fig. 8D). It is also recorded from the Philippines in Manjaji-Matsumoto (2017).

Morphological variations in the shape of pectoral and dorsal fins, dermal denticles and colour of caudal fin are observed within S. brevirostris. First dorsal-fin posterior margin is somewhat straight near its apex like in the holotype, although other specimens exhibit first dorsal-fin posterior margin concave. Pectoral fins are slightly broad at posterior margin in specimens from the Central Western Pacific Ocean (except Taiwan), while it is narrow in others like that observed for the holotype. Pelvic fins are nearer to first dorsal fin, although it may be in the midline between the two dorsal fins in a few specimens. Caudal fin has postventral caudal margins white, although narrowly white at caudal fork in specimens from Kagoshima. Thickness of the dermal denticles varies from very slender to conspicuously broad at the crown in few specimens from Kagoshima. These variations are possibly not related to dimorphism or ontogeny and apparently it is scattered, as it has an unclear morphological pattern throughout the distributional range of this species in the North and Central Western Pacific Ocean. Duffy and Last (2007a) previously noticed differences in the dermal denticles, which was suggested to be due to ontogeny. Further comparative investigations are required to include more specimens from Taiwan and South Korea in order to better understand these variations within the species.

Squalus brevirostris exhibits low vertebral counts that are usually overlapped to those of morphologically similar species occurring elsewhere, such as S. megalops, S. acutipinnis and S. lobularis. However, it can be separated from other similar species by precaudal vertebrae with S. hemipinnis, S. notocaudatus and S. formosus, total vertebrae with S. hemipinnis, S. notocaudatus, S. formosus and S. albifrons and monospondylous vertebrae with S. notocaudatus, S. formosus, S. albifrons and S. bucephalus.

SCSFRI D01562 (holotype), adult male, 635 mm TL, South China Sea, 18°51'–18°47'N, 112°41'–112°33'E, 394 m depth, collected on 21 April 1982; SCSFRI D01548 (paratype), adult female, 975 mm TL, South China Sea, 18°50.8'–19°28.8'N, 112°47.3'–113°58.8'E.

HUMZ 74990, juvenile female, 536 mm TL, Kyushu-Palau Ridge, 26°14.1'–26°10.1'N, 135°47.5'–135°48.0'E, 360 m depth; NMMB P 15619 (photo only), adult female, 700 mm TL, Pingtung county, Taiwan.

Single values correspond to holotype first followed by a single paratype and a specimen from Japan between brackets. Squalus acutirostris is separated from its congeners by a combination of characters: large sized species (635–975 mm maximum TL in adults) with body moderately slender and fusiform (Fig. 6A), its greatest width at head (head width 1.4(1.4, 1.2) times trunk width and 1.3(1.1, 1.2) times abdomen width); head small, its length 24.4%(22.6%, 22.1%) TL; snout rounded at tip and short, preorbital length 8.2%(7.3%, 8.1%) TL; nostrils nearer to snout than to mouth (Fig. 6B); first dorsal fin oblique, wide at fin web and low, its height 6.0%(5.6%, 8.6%) TL; first dorsal fin concave at the lower half of posterior margin, not raked (Fig. 6C); second dorsal fin with raked posterior margin (Fig. 6D); dorsal-fin spines robust and short, never reaching the fin apex (first dorsal-fin spine length 2.8%(2.7%, 3.2%)TL; second dorsal-fin spine length 2.8%(1.9%, 4.8%) TL; pectoral fins moderately wide with posterior margin length 11.0%(10.3%, 10.1%) TL; pectoral-fin posterior margin concave, although not falcate; pectoral-fin free rear tips rounded; caudal fin rectangular with conspicuous caudal fork (Fig. 6E); caudal fin small with dorsal caudal margin length 0.7(0.9, 1.0) times head length; lower caudal lobe much shorter than upper caudal lobe (preventral caudal margin length 0.7(0.6, 0.5) times dorsal caudal margin length); body brownish-grey to dark grey dorsally and light brown ventrally; first and second dorsal-fin apex slightly black; conspicuous black upper caudal blotch and small black caudal bar; black basal marking conspicuous at the origin of caudal fin; lower caudal lobe mostly white.

Figure 6. 

Squalus acutirostris (A–E): HUMZ 74990, juvenile female, 536 mm TL in lateral (A) and ventral (B) views; first (C) and second (D) dorsal fins and caudal fin (E). S. suckleyi in lateral view (F): HUMZ 90963, adult female, 784 mm TL. Scale bars: 50 mm (A, B, F); 20 mm (C–E).

Squalus acutirostris occurs in the North-western Pacific Ocean with reports from Japan, Taiwan and China (Fig. 8E). It has been previously misidentified with S. japonicus in Vietnam as seen in Quang et al. (2013). It inhabits continental slopes between 360–525 m depth (Zhu et al. 1984; present study).

Squalus acutirostris, originally described from the South China Sea, was previously recognised in China and Western Australia in Zhu et al. (1984), Muñoz-Chápuli and Ramos (1989) and Zhu and Meng (2001). White and Last (2013) have proven that the Australian records correspond to a distinct species, S. grahami White, Last & Stevens, 2007. Furthermore, occurrence of S. acutirostris in the South China Sea was thought to be related to S. mitsukurii (White and Last 2013; Weigmann 2016) due to no apparent morphological and meristic distinction and, thus, this species was designated as a junior synonym. According to White and Last (2013), the differences in external measurements (e.g. precaudal length, interdorsal space, dorsal caudal margin length) between these species were due to the preservation condition of the holotype of S. acutirostris. However, the present analysis reveals many morphometric differences when adults are compared, such as: inner nostril-labial furrow space (6.9%–8.6% TL vs. 3.9%–4.5% TL for S. mitsukurii); first dorsal fin height (5.6%–6.0% TL vs. 6.9%–9.8% TL for S. mitsukurii); second dorsal-fin anterior margin length (6.8%–7.6% TL vs. 7.7%–10.2% TL for S. mitsukurii); second dorsal-fin base length (4.8%–5.8% TL vs. 6.8%–7.9% TL for S. mitsukurii); second dorsal fin height (3.3%–3.6% TL vs. 4.5%–6.8% TL for S. mitsukurii); second dorsal-fin spine length and base width (1.9%–2.8% TL, 0.5%–0.6% TL vs. 3.4%–5.3% TL, 0.7%–1.0% TL for S. mitsukurii). Further distinctive characteristics between S. mitsukurii and the types of S. acutirostris include conspicuously shorter snout with nostrils clearly nearer to the snout tip than to the mouth, its prenarial length 0.6–0.8 times nostril-labial furrow space (vs. elongate snout with nostrils nearer to the mouth, its prenarial length 1.1–1.4 times nostril-labial furrow space in S. mitsukurii) and narrower pectoral and pelvic fins (vs. conspicuously broad pectoral and pelvic fins in S. mitsukurii). These characteristics are congruent with observations of Zhu et al. (1984) and Zhu and Meng (2001) and are apparent, regardless of the preservation condition and which support the distinction between S. mitsukurii and the nominal species S. acutirostris.

Squalus acutirostris may be separated from the Japanese congeners by having more elongate second dorsal-fin length 63.9%–64.0% TL (vs. 60.5%–63.4% TL for S. shiraii sp. nov. vs. 58.4%–63.9% TL for S. mitsukurii vs. 60.7%–62.0% TL for S. japonicus vs. 61.1%–63.8% TL for S. brevirostris vs. 56.2%–62.3% TL for S. suckleyi) and larger inner nostril-labial furrow space 6.9%–8.6% TL (vs. 4.3%–4.9% TL for S. shiraii sp. nov. vs. 3.9%–4.5% TL for S. mitsukurii vs. 4.6%–5.1% TL for S. japonicus vs. 4.5%–5.1% TL for S. brevirostris vs. 3.9%–5.6% TL for S. suckleyi). It may also be distinguished from S. shiraii sp. nov., S. japonicus and S. brevirostris by pre-pelvic length 47.2%–49.3% TL and prenarial length 4.7%–5.1% TL (vs. 44.1%–46.8% TL and 5.4%–5.6% TL for S. shiraii vs. 43.7%–45.9% TL and 5.9%–6.8% TL for S. japonicus vs. 42.4%–46.9% TL and 3.8%–4.3% TL for S. brevirostris). It is also easily separated from S. brevirostris by having pectoral free rear tips rounded (vs. conspicuously pointed).

CAS 227267, adult male, 674 mm TL, Hood Canal, Puget Sound, Washington State, The United States of America, 47°22'N, 123°05'E, 30 m depth. Collected on 3 August 2007. Neotype designated in Ebert et al. (2010).

HUMZ 75718, juvenile female, 500 mm TL, Usujiri, Mimamikayakabie, Hokkaido, Japan; HUMZ 81094, adult female, 565 mm TL, Usujiri, Mimamikayakabie, Hokkaido, Japan; HUMZ 87733, juvenile male, 495 mm TL, off Shiretoko, Hokkaido, Japan; HUMZ 87752, juvenile female, 455 mm TL, off Muroran, Hokkaido, Japan; HUMZ 90963, adult female, 784 mm TL, Notori Misaki oki, Japan; HUMZ 107865, juvenile female, 465 mm TL, off Sekinai, Kumaishi, Hokkaido, Japan; HUMZ 107869, adult female, 650 mm TL, off Sekinai, Kumaishi, Hokkaido, Japan; HUMZ 123859, adult male, 815 mm TL, north Japan, 44°00.1'N, 155°00.1'E; NSMT-P 10540, adult female, 625 mm TL, Northern Japan, Japan; NSMT-P 42569, adult male, 597 mm TL, unknown locality; NSMT-P 61090, adult female, 915 mm TL, Northern Japan, Japan; NSMT-P 74887, juvenile male, 445 mm TL, Northern Japan, Japan; NSMT-P 77186, adult male, 835 mm TL, unknown locality; NSMT-P 79501, adult male, 740 mm TL, Northern Japan, Japan; NSMT-P 92640, adult female, 740 mm TL, Northern Japan, Japan; ZUMT 3231, neonate male, 272 mm TL, Nagasaki, Japan; ZUMT 4684, neonate male, 265 mm TL, Tokyo Fish Market, Tokyo, Japan; ZUMT 4685, neonate female, 270 mm TL, Tokyo Fish Market, Tokyo, Japan; ZUMT 10536, neonate female, 120 mm TL; ZUMT 10789, neonate female, 238 mm TL, Hokkaido, Japan; ZUMT 36806, neonate female, 202 mm TL, Sakhalin Island, Russia; ZUMT 36807, neonate female, 200 mm TL, Sakuharin, Japan; ZUMT 36825, neonate female, 202 mm TL, Sakhalin Island, Russia; ZUMT 36836, neonate female, 202 mm TL, Sakuharin, Japan; ZUMT 40117, neonate male, 257 mm TL, Uozu, Toyama Prefecture, Japan; ZUMT 41539, neonate male, 284 mm TL, Uozu, Toyama Prefecture, Japan; ZUMT 45801, neonate male, 270 mm TL, Uozu, Toyama Prefecture, Japan; ZUMT 46116, neonate male, 284 mm TL, Uozu, Toyama Prefecture, Japan; ZUMT 46123, neonate male, 254 mm TL, Uozu, Toyama Prefecture, Japan; ZUMT 46124, neonate male, 288 mm TL, Uozu, Toyama Prefecture, Japan; ZUMT 46151, neonate male, 272 mm TL, Uozu, Toyama Prefecture, Japan; ZUMT 46670, neonate male, 258 mm TL, Uozu, Toyama Prefecture, Japan; ZUMT 46671, neonate female, 250 mm TL, Uozu, Toyama Prefecture, Japan; ZUMT 46672, neonate male, 230 mm TL, Uozu, Toyama Prefecture, Japan; ZUMT 46673, neonate female, 260 mm TL, Uozu, Toyama Prefecture, Japan; ZUMT 46674, neonate male, 270 mm TL, Uozu, Toyama Prefecture, Japan; ZUMT 46675, neonate male, 245 mm TL, Uozu, Toyama Prefecture, Japan; ZUMT 51256, neonate female, 165 mm TL, East China Sea.

A large-sized Squalus species (650–952 mm maximum TL in adults) that can be differentiated from all its congeners (except S. acanthias) and including the Japanese species by: presence of rows of white spots dorsally on each side of the body (vs. absence of white spots) (Fig. 6F); anterior nasal flap unilobed (vs. bi-lobed); first dorsal fin origin located conspicuously behind the vertical line traced at pectoral-fin free rear tips (vs. first dorsal fin origin located before the vertical line); dermal denticles arrow-shaped (Fig. 7K, L) (vs. denticles rhomboid or lanceolate). It is further distinguished from the Japanese species by: shorter distance between pelvic and caudal fins 20.7%–22.6% TL, except with S. acutirostris (vs. 25.5%–27.3% TL for S. shiraii vs. 23.7%–27.0% TL for S. mitsukurii vs. 26.4%–28.0% TL for S. japonicus vs. 24.6%–29.4% TL for S. brevirostris); small first dorsal fin (except for S. japonicus), its length 11.7%–12.4% TL (vs. 12.6%–13.4% TL for S. shiraii vs. 12.7%–14.9% TL for S. mitsukurii vs. 12.3%–13.6% TL for S. japonicus vs. 13.0%–14.7% TL for S. brevirostris vs. 14.1%–15.0% TL for S. acutirostris); lower dorsal fins with first dorsal fin height 5.9%–6.7% TL, except with S. japonicus and S. acutirostris (vs. 7.9%–8.2% TL for S. shiraii vs. 6.9%–9.8% TL for S. mitsukurii vs. 7.3%–8.2% TL for S. brevirostris) and second dorsal fin height 3.6%–4.9% TL (except for S. mitsukurii and S. acutirostris) (vs. 5.6%–5.9% TL for S. shiraii vs. 5.4%–6.5% TL for S. japonicus vs. 5.7%–6.9% TL for S. brevirostris); conspicuously smaller dorsal-fin spines with first dorsal-fin spine length 1.0%–2.0% TL (vs. 3.6%–4.3% TL for S. shiraii vs. 3.1%–5.4% TL for S. mitsukurii vs. 3.0%–3.8% TL for S. japonicus vs. 3.1%–4.2% TL for S. brevirostris vs. 2.7%–2.8% TL for S. acutirostris) and second dorsal-fin spine 1.9%–3.2% TL, except with S. acutirostris (4.3%–5.1% TL for S. shiraii vs. 3.4%–5.3% TL for S. mitsukurii vs. 4.7%–5.6%TL for S. japonicus vs. 4.6%–6.6% TL for S. brevirostris). It also has smaller interdorsal space 19.2%–22.1% TL than from S. shiraii sp. nov. (23.8%–25.7% TL), S. japonicus (22.5%–24.8% TL) and S. brevirostris (23.3%–27.2% TL) and S. acutirostris (24.6%–28.3% TL) and more elongate pre-first dorsal length 32.6%–34.8% TL than S. shiraii sp. nov. (29.9%–31.2% TL), S. mitsukurii (28.3%–32.4% TL), S. brevirostris (28.9%–31.7% TL, and S. acutirostris (31.8%–32.3% TL). It is further separated from its regional congeners by smaller number of precaudal (71–73) and diplospondylous vertebrae (58–62) (vs. larger number of vertebrae for S. shiraii sp. nov. 91–94 and 72–77 vs. 86–90 and 70–72 for S. mitsukurii vs. 80–88 and 65–75 for S. japonicus, vs. 78–89 and 66–72 for S. brevirostris).

This species is found in temperate and adjacent Arctic waters of the North Pacific Ocean from Washington state and California, USA to Canada (east side) and South Korea to Russia (west side) (Veríssimo et al. 2010; Lee et al. 2016; Dyldin and Orlov 2018). In Japan, reports are from Hokaiddo, Notori Misaki oki and Uozu as well as Tokyo from where specimens were obtained in the local fish market. It occurs more commonly in temperate waters of Northern Japan (Fig. 8E).

Molecular genetic data has supported the taxonomic separation between the morphologically similar species S. acanthias Linnaeus, 1758, that bears circumglobal distribution and S. suckleyi (e.g. Ward et al. 2007; Veríssimo et al. 2010; Ebert et al. 2010; Naylor et al. 2012; Lee et al. 2016). The latter species is a regional endemic to the North Pacific Ocean with distribution ranging from Korea to Japan (western side) and from California to Canada (east side). Morphological separation between these two species is still unreliable as no efficient diagnostic features were provided as yet. Lindberg and Legeza (1956) suggested that the ratio pelvic-fin midpoint to first dorsal-fin insertion (PDI) and pelvic-fin midpoint to second dorsal-fin insertion (PDO) comprise diagnostic characters as well as the first dorsal-fin midpoint to pectoral-fin insertion (DPI) and first dorsal-fin midpoint to pelvic-fin origin (Ebert et al. 2010); however, these parameters are subject to bias due to the midpoint definition that relies on subjectivity. Squamation, colouration and vertebral counts, however, separate S. acanthias and S. suckleyi (Table 6). The latter species exhibits dermal denticles with a median ridge forming an acute angle or right angle when perpendicular to the body axis (vs. in parallel for S. acanthias). It is easily distinguished from S. acanthias by having body conspicuously dark grey dorsally with very few white spots distributed dorsally in a single row on each side of the body (vs. light grey body with many white spots in two rows). Specimens of S. suckleyi from Japan are also distinct from S. acanthias occurring elsewhere by having smaller number of vertebrae (monospondylous, diplospondylous, precaudal and total vertebrae; see Springer & Garrick, 1964 for S. acanthias), corroborating Ebert et al. (2010).

Additional characteristics of S. suckleyi include: dermal denticles arrow-shaped and unicuspid, small and not imbricated with a single ridge (lateral ridges absent); ridge very prominent, narrower distally than proximally with furrow anterior and profound; ridge very tall and convex, forming 45 degrees angle with horizontal axis of the body; crown base strongly broad and diamond-like with four prominent pedicels; dermal denticles conspicuously expanded laterally at the crown (Fig. 7K, L). A single adult male shows few dermal denticles with weak lateral cusps, although lateral ridges are still absent. Colouration of Japanese specimens consists of body dark grey dorsally with very few and inconspicuous white spots (1–6 pairs), rounded, distributed symmetrically in a single row on each side and laterally at the body; white spots commonly absent in some large specimens, although clearly evident in young juveniles; white to greyish-white ventrally. Both dorsal fins grey, brownish in the apex, slightly darker from the apex tip to the midline of the posterior margin, white distally at posterior margin and free rear tips; fin base slightly white; dorsal spines dark brown, white at the tip. Pectoral fins dark grey dorsally and ventrally with white posterior margin, although not uniform. Pelvic fins also dark grey with posterior and inner margins slightly white. Caudal fin grey, darker at the tip of the dorsal lobe and in the lower caudal lobe, white over vertebral column; proximal end of dorsal caudal margin white; postventral caudal margins narrowly white; preventral caudal margin slightly white; black caudal stripe conspicuous above vertebral column.

Figure 7. 

Scanning electron microscopy of the dermal denticles. Squalus shiraii sp. nov. (A, B): HUMZ 149389 (holotype), adult male, 590 mm TL. S. formosus (C, D): CSIRO H-6816-01 (holotype), adult male, 720 mm TL. S. mitsukurii (E, F): HUMZ 102987, adult male, 970 mm TL. S. japonicus (G, H): NSMT-P 44380 (neotype), adult male, 645 mm TL (G); NSMT-P 91127, juvenile female, 410 mm TL (H). S. brevirostris (I, J): NSMT-P 47378, adult female, 600 mm TL (I); KAUM-I 187, adult female, 578 mm TL (J). S. suckleyi (K, L): NSMT P-92640, adult female, 970 mm TL. Scale bars: 50μm (B); 100 μm (D, F, L); 200 μm (A, C, E, G–K).

Variations of diet and reproduction, as well as migration pattern, spatial distribution and size composition, were noticed in populations of S. suckleyi between the East and West sides of the North Pacific Ocean (Orlov et al. 2012a,b; Yano et al. 2017), suggesting that this species may comprise more than one fishing stock rather than a single one as supported in Ebert et al. (2010). In contrast to S. acanthias, this species has low migration movements and usually inhabits the pelagic zones up to 200 m depth with two possibly distinct populations observed throughout the North Pacific Ocean (Yano et al. 2017). Emperor Seamout Chain works as a biogeographic barrier between the East and West sides whose belt-shaped pelagic zone exhibits low probability of presence of S. suckleyi, according to Yano et al. (2017).

A single nominal species of spotted spiny dogfish, Squalus wakiyae Tanaka, 1917, was described from the North-western Pacific Ocean and it has been placed under synonymy with S. acanthias and S. suckleyi in Fowler (1941), White et al. (2007c), Ebert et al. (2010) and the present study, as no apparent differences in morphology and genetic data are noticed between the west and east populations of S. suckleyi. Morphometric variations, however, are noticed when comparing the neotype (taken from Ebert et al. 2010) to specimens from off Japan and Russia: greater prepelvic length than in specimens from the NWPO (51.5% TL for neotype vs. 47.2%–50.6% TL); greater pre-vent length (54.9% TL vs. 50.1%–53.6% TL); narrower mouth (6.2% TL vs. 6.9%–8.2% TL); lower head height (6.5% TL vs. 7.8%–10.6% TL); pectoral fin broader at the base (6.4% TL vs. 3.5%–4.9% TL); lower second dorsal fin in S. suckleyi (3.0% TL vs. 3.6%–5.0% TL). A larger number of comparative representatives are required to re-evaluate these variations between populations of S. suckleyi and should include additional characteristics, such as dentition and vertebral counts.

Figure 8. 

Map of the North-western Pacific Ocean, showing the geographical distribution of Squalus species. A: S. shiraii sp.nov. (red); B: S. mitsukurii (orange); C: S. japonicus (brown); D: S. brevirostris (yellow); E: S. acutirostris (green); F: S. suckleyi (pink). Star: holotype/ neotype; Dotted circle: paratypes; Circle: other material.

S. acanthias: BMNH 1879.10.9.64, adult female, 620 mm TL, off east coast, The United States of America; BMNH 1929.10.20.1, adult female, 875 mm TL, United Kingdom; BMNH 1931.4.27.2, adult female, 800 mm TL, United Kingdom; BMNH 1936.8.26.17, adult male, 635 mm TL, near Strait of Magellan, Argentina; BMNH 1950.7.26.2, juvenile female, 570 mm TL, Republic of Ireland; BMNH 1976.7.30.20, juvenile female, 523 mm TL, France, Mediterranean Sea; BMNH 1999.5.4.4, juvenile male, 550 mm TL, Falkland Islands; NMW 50119, adult female, 910 mm TL, Dalmatien, Croatia; NMW 59659, juvenile male, 345 mm TL, The North Sea; CSIRO H 2921-01, adult female, 605 mm TL, upper Pitt Water, near Shark Point, Tasmania, Australia; NMW 84781, juvenile female, 525 mm TL, Trieste, Italy; NSMT-P 41928, adult female, 715 mm TL, Atlantic Ocean; HUMZ 151302, juvenile male, 520 mm TL, off Namibia; HUMZ 30295, adult male, 660 mm TL, off Patagonia, Argentina; HUMZ 65447, adult male, 755 mm TL, New Zealand; SAIAB 21877, juvenile male, 590 mm TL, Cape Columbine, South Africa; SAIAB 25317, adult male, 640 mm TL, west coast of South Africa.