Apristurus nakayai Iglésias, 2012

Ng, Shing-Lai, Liu, Kwang-Ming & Joung, Shoou-Jeng, 2023, Occurrence of the milk-eye catshark Apristurus nakayai (Carcharhiniformes: Pentanchidae) from the South China Sea, Zootaxa 5244 (1), pp. 51-60 : 52-59

Apristurus nakayai Iglésias, 2012

Figs. 1–7 ; Table 1

Diagnosis. Pre-outer nostril length 4.2–4.9 % TL; upper labial furrows longer than the lower furrows; first dorsal fin smaller than second dorsal fin; first dorsal-fin origin situated well behind pelvic-fin insertion, at or slightly behind a vertical line through pelvic-fin tip; second dorsal-fin insertion above or just slightly anterior to anal-fin insertion; snout rather long, tip rounded; abdomen very short; pectoral-fin tip at or slightly extending after the midpoint of P1- P2 space; intestinal spiral valves 14–16; monospondylous centra 32–36; precaudal diplospondylous centra 34–37; clasper hook present on the inner margin of exorhipidion; body uniformly brownish black to black; iris shiny white when fresh; maturing size at about 400 mm TL in both sex.

Description of the specimens from the South China Sea. Proportional measurements are given in Table 1 . Body slender, cylindrical before pelvic fins, gradually compressed laterally after pelvic fins. Caudal-peduncle height 49.2–67.6% head height. Abdomen narrow, pectoral-fin free rear tip to pelvic-fin origin 54.3–86.2% internarial width; pectoral-fin tip at or slightly extending after midpoint of space between pectoral and pelvic fins. Midpoint of the total length well behind cloaca.

Head dorsoventrally flattened, its height 66.9–98.6% greatest width of head. Snout rather long, tip rounded ( Fig. 3 ). Pre-outer nostril length 42.2–49.4% preorbital length, 83.6–101.3% interorbital width. Nostril large, its length subequal to orbit length and internarial width, expanding obliquely inward towards mouth. Mouth considerably arched, its length 58.4–72.3% width. Labial furrows well developed, length of lower labial furrows 44.5–77.3% upper furrows; length of upper furrows 51.3–57.8% mouth width. Orbit narrowly elliptical, with a poorly developed subocular fold. Spiracle small, bean-shaped, situated below a horizontal line through mid-eye. Five gill slits present; the 5th gill slit located above pectoral-fin origin.

Pectoral fin broadly triangular, its width subequal to base length. Pelvic fin slightly elongated, its length 59.2– 65.7% (92.2% for the female specimen) distance between pelvic-fin and anal-fin origin. Anal fin long, narrowly triangular, anal-fin base length (ceratotrichia) longer than distance between origins of the first and the second dorsal fins, its height (muscle) 21.0–33.0% base length (muscle). First dorsal fin situated well behind pelvic-fin insertion, at or slightly behind a vertical line through pelvic-fin tip; first dorsal fin much smaller than second dorsal fin, the height of first dorsal fin 41.2–62.2% height of second dorsal fin; apexes of dorsal fin rounded. Second dorsal-fin length 66.9–88.7% distance between origins of the first and the second dorsal fins. Second dorsal-fin insertion above or just slightly anterior to anal-fin insertion. Caudal fin very long, its length 43.0–46.5% precaudal length; ventral lobe broad, apex rounded. Caudal terminal lobe well developed, its length 69.5–93.9% caudal-fin height.

Teeth small and numerous on both jaws, 28–37 + 32–36 = 61–73 on upper jaw and 28–31 + 28–31 = 56–60 on lower jaw. Each tooth with a long central cusp, with mostly a pair of smaller lateral cusps, or additional rudimentary lateral cusps. Streaks present on central cusps. No naked space at symphysis of upper jaw. Naked space present at symphysis of lower jaw in males, but absent in the female specimen ( Fig. 4 ).

Denticles small, overlapping each other, tricuspid, central cusp with strong median ridge and distinctly longer than lateral cusps; lateral cusps with weak ridges; surface of denticles with reticulations ( Fig. 5 ). Denticles present around gill openings, but absent from palate and tongue, and on dorsal margin of caudal fin.

Clasper robust at base, gradually tapering posteriorly; ventral and outer surface densely covered with denticles; dorsal surface naked, except for outer half of exorhipidion; exorhipidion flat, its inner margin with small clasper hooks; pseudosiphon narrow and deep; cover rhipidion not developed; pseudopera fairly long ( Fig. 6 ).

Duodenum developed but not especially elongated. The sole female specimen with ripe ova in the left ovary only. No egg case was found.

Intestinal spiral valves 14–16; Monospondylous vertebrae 32–34; precaudal diplospondylous vertebrae 34–37.

Coloration. (When fresh) body and fins uniformly brownish black to black; palate and tongue black; iris distinctly shiny white ( Fig. 7 ). (In preservative) Body color less dark; iris pale.

Size. The largest known specimen is the holotype, which is a mature male measuring 676 mm TL ( Iglésias 2012). The sole female specimen measuring 408 mm TL, and the smallest male specimen measuring 415 mm TL were mature.

Distribution. Known from western Pacific, including New Caledonia ( Iglésias 2012), Papua New Guinea ( White et al. 2017; Ebert et al. 2021), and the South China Sea. This species may eventually be found in other regions within the western Pacific.

Remarks. Apristurus nakayai can be assigned to the ‘ brunneus group’ as defined by Nakaya & Sato (1999) by having the following combination of characters: wide and relatively short snouts, pre-outer nostrils length less than 6% total length TL; intestine spiral valves 13–22; and upper labial furrows longer than lower furrows. Within this group, A. nakayai superficially resembles A. platyrhynchus , which also co-occurs in all known distributions of A. nakayai . Both species share the combination of characteristics: abdomen short, space between pectoral and pelvic fins much shorter than anal-fin base length (ceratotrichia); first dorsal fin much smaller than second dorsal fin; and origin of the first dorsal fin situated well behind pelvic-fin base ( Iglésias 2012; Nakaya & Kawauchi 2013). Iglésias (2012) found some morphometrical differences between the two species (i.e., pre-branchial length; mouth width; nostril length; distance between origins of dorsal fins; distance between insertions of dorsal fins; and pelvic-fin length). However, when comparing all the known A. nakayai specimens to the data of A. platyrhynchus in Nakaya & Kawauchi (2013), the above morphometrics significantly overlapped, thus solely using morphometrics may not separate the two species. Counts of intestinal spiral valves (14–16 vs. 16–20 in A. platyrhynchus ; Nakaya & Kawauchi 2013), monospondylous vertebrae (32–34 vs. 33–40), and precaudal diplospondylous vertebrae (34–37 vs. 37–45) may separate the two species, but the values still overlap slightly. Nevertheless, the two species can be readily distinguished by several aspects: iris shiny white when fresh in A. nakayai (vs. black in A. platyrhynchus ); body brownish black to black (vs. light to dark brown); second dorsal-fin insertion above or slightly in front of the anal-fin insertion (vs. well before the anal-fin insertion); denticles absent inside mouth (vs. present in specimens larger than 500 mm TL); and maturing at about 400 mm TL in both sexes (maturing size larger than 550 mm TL) (Data of A. platyrhynchus are modified from Nakaya & Kawauchi 2013).

The specimens from the South China Sea are mostly consistent with the holotype of A. nakayai in morphometrics, but are slightly different in counts. For example, the holotype has slightly more monospondylous centra (36, Iglésias 2012 vs. 32–34); and higher tooth counts (79/72, Iglésias 2012 vs. 61–73/56–60). The difference may be attributed to intraspecific or geographical variations, which require more specimens from different localities to confirm. In addition, the sole female specimen (NMMB-P36993) differs from the male specimens by lacking a naked space at symphysis of lower jaw ( Fig. 4 ), which may be a character of sexual dimorphism.

Apristurus nakayai was formerly known from three specimens, one from New Caledonia ( Iglésias 2012), and two from Papua New Guinea ( White et al. 2017; Ebert et al. 2021). The occurrence of A. nakayai in the South China Sea is very far away (ca. 4665 km) from its previous record in Papua New Guinea ( White et al. 2017), which is also the first record in the northern hemisphere. Most of the Apristurus species show high level of endemism, which are often confined to one single ocean region, with only a few exceptions (e.g., A. macrostomus , A. melanoasper , A. laurussonii , A. platyrhynchus ; Ebert et al. 2021). In addition, no evidence shows that the deepwater catsharks, including genus Apristurus , have large scale horizontal migration or high dispersal ability.. This example suggests that at least some Apristurus species may have a much wider distributional range than previously recognized. Further exploration of remote deepsea habitats may further elucidate the distribution of deepsea chondrichthyans, including those of the genus Apristurus .

Materials examined. A. nakayai: EBFS 00138 , 425 mm TL, adult male, NMMB-P 36993 , 408 mm TL, adult female, NMMB-P 36994 , 451 mm TL, adult male, NMMB-P 36995, 415 mm TL, adult male, NMMB-P 36996 , 424 mm TL, adult male, NMMB-P 36997 , 435 mm TL, adult male, ca. 19°N, 114°E, off western Dongsha Atoll , South China Sea, ca. 500 m depth, bottom trawl, 12 May 2022.

Figures

FIGURE 1 Fresh coloration (previously frozen) of Apristurus nakayai specimens collected from the South China Sea.A. EBFS 00138, 425 mm TL, adult male. B. NMMB-P36993, 408 mm TL, adult female. Scale bar = 10 cm.

FIGURE 2 Preserved Apristurus nakayai specimens. A. EBFS 00138, 425 mm TL, adult male. B. NMMB-P36993, 408 mm TL, adult female. Scale bar = 10 cm.

FIGURE 3 Dorsal and ventral head view. (A–B) EBFS 00138, 425 mm TL, adult male. (C–D) NMMB-P36993, 408 mm TL, adult female. (A, C) Dorsal view. (B, D) Ventral view.

FIGURE 4 Symphysis of lower jaws of (A) EBFS 00138, 425 mm TL, adult male, and (B) NMMB-P36993, 408 mm TL, adult female; and anterolateral teeth from the right side of the jaw of NMMB-P36993, 408 mm TL, adult female. (C) Upper jaw. (D) Lower jaw. Red arrow indicates the naked area.

FIGURE 5 Denticles from below first dorsal fin of NMMB-P36993, 408 mm TL, adult female. Scale bar = 1 mm.

FIGURE 6 Dorsal view of the right clasper of EBFS 00138, 425 mm TL, adult male. CD = clasper denticles; CG = clasper groove; EN = envelope; ER = exorhipidion; PS = pseudosiphon; PV = pelvic fin; RH = rhipidion. Scale bar = 1 cm.

FIGURE 7 Detail of eye of EBFS 00138, 425 mm TL, adult male, highlighting the shiny white iris when fresh.

Tables

TABLE 1 Morphometrical measurements of Apristurus nakayai. Data of the holotype and non-type (NTUM 11488) were from Iglésias (2012) and White et al. (2017), respectively.

	Non-types			Holotype
Catalog number	NMMB-P36993 (n = 1)	EBFS 00138, NMMB- P36994–36997 (n = 4)	NTUM 11488 (n = 1)	MNHN 2003-1983
Sex	Mature female	Mature males	Mature male	Mature male	SD
Total length (mm)	408	415–451	559	659
Measurements (%TL)		Mean (Range)
Pre-D2 insertion	66.7	68.0 (66.7–69.9)	68.5	68.3	1.0
Pre-D2 origin	60.5	61.4 (59.9–63.0)	62.1	61.5	0.9
Pre-D1 insertion	53.2	53.2 (52.0–55.2)	52.6	52.2	1.0
Pre-D1 origin	49.8	48.5 (47.2–49.9)	47.4	47.6	1.0
Pre-P1 length	21.6	20.8 (19.5–21.6)	21.4	20.2	0.7
Pre-P2 length	38.0	37.1 (36.6–37.4)	37.2	37.2	0.4
Pre-vent length	43.0	41.0 (40.0–41.7)	41.9	40.4	0.9
Preanal length	52.2	51.5 (49.4–53.9)	47.4	51.6	1.9
Precaudal length	67.6	70.1 (68.6–71.8)	69.2	68.6	1.4
Pre-branchial length	18.4	17.2 (15.8–18.8)	16.9	15.9	1.0
Pre-spiracular length	13.8	13.4 (13.0–13.9)	11.9	12.1	0.7
Pre-orbital length	10.1	9.8 (9.2–10.5)	7.8	8.5	0.8
Pre-outer nostril length	4.9	4.6 (4.4–4.8)	4.5	4.2	0.2
Pre-inner nostril length	7.8	7.6 (7.3–8.1)	6.4	6.5	0.6
Pre-oral length	8.5	8.6 (8.3–8.9)	8.1	7.7	0.3
Head length	21.6	21.5 (21.2–22.1)	22.1	20.3	0.5
Head height	9.1	7.7 (7.1–8.3)	8.9	7.0	0.8
Head width at mouth corners	8.9	9.7 (9.5–9.9)	9.8	10.0	0.3
Head width max	9.2	10.2 (9.9–10.6)	10.1	10.6	0.4
Mouth width	6.4	6.6 (6.0–6.8)	8.3	6.8	0.6
Mouth length	3.8	4.3 (3.8–4.9)	2.6	2.4	0.8
Internarial width	3.6	3.4 (3.2–3.6)	3.4	3.3	0.1
Upper labial furrow length	3.3	3.6 (3.5–3.7)	2.9	3.1	0.2
Lower labial furrow length	2.5	1.8 (1.6–2.0)	2.1	2.1	0.3
Orbit length	3.8	3.7 (3.4–3.9)	3.3	3.2	0.2
Orbit height	1.5	1.1 (0.9–1.3)	0.9	1.1	0.2
Nostril length	4.0	3.6 (3.5–3.6)	3.2	3.0	0.3
Nostril -mouth space	1.8	1.8 (1.7–2.0)	1.7	1.7	0.1
Interorbital width	5.2	5.0 (4.7–5.5)	5.6	5.3	0.3
1st gill height	1.8	1.5 (1.2–1.7)	1.5	1.3	0.2
3rd gill height	2.0	2.0 (1.6–2.5)	1.4	1.7	0.3
5th gill height	1.5	1.7 (1.3–2.0)	1.5	1.5	0.2
D1-D2 space	6.7	8.9 (8.4–9.8)	9.5	9.4	0.9
D1-D2 origin	11.6	13.6 (12.4–15.0)	14.3	14.0	1.0
D1-D2 insertions	13.1	14.8 (13.4–15.9)	15.8	16.2	1.2
P1-P2 space	9.5	8.7 (8.1–9.6)	8.4	9.0	0.5
P1 tip to P2 origin	2.9	2.1 (1.8–2.8)	2.1	3.3	0.5

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