Coelorinchus yurii Iwamoto, Golani, Baranes, and Goren, 2006

Prokofiev, Artem M. & Iwamoto, Tomio, 2020, Revision of the Grenadier Genus Coelorinchus (Teleostei: Macrouridae) from the Mascarene Ridge, Western Indian Ocean, with Description of Two New Species, Proceedings of the California Academy of Sciences 66 (9), pp. 231-273 : 243-247

Coelorinchus yurii Iwamoto, Golani, Baranes, and Goren, 2006

Figures 2D, 5J–L, 6E–F, 7E–F, 12A, 12C, 16, 17

Coelorinchus yurii (partim., holotype only) Iwamoto, Golani, Baranes, and Goren, 2006:434, figs. 2–3 (original description, Seychelles, 1900 m; all paratype specimens from Saya de Malha Bank herein referred to C. mascarenus ) .

1 An anonymous reviewer stated that the "unusual character of [the] terminal snout [scute] is obviously attributable to a damage during early ontogeny. Fig. 14 clearly indicates that its snout is in [an] abnormal condition." Furthermore, the reviewer suggests that the specimen is an "unusual variant of C. amirantensis ." We strongly disagree with that interpretation. The specimen is in good condition despite having been trawled from a bottom depth of 1260 to 1300 m; it suffered no major external damage, the snout is fully intact with associated bones unbroken, and the head overall is in a normal condition. The terminal scute is notably small and blunt, but there is nothing to suggest that an acuminate, sharly pointed scute such as found in the other species here treated could have been present at any earlier stage. Other differences supporting our species hypotheses are enumerated in the comparisons sections of each species and in the key to Mascarene species.

Holotype. — TAU P.11602 (154 mm HL, 541+ mm TL), Seychelles between Alphonse and Bijoutier islands, by trammel net in 1900 m, R / V Sea Surveyor, 17 Dec. 1998, collector M. Goren.

Other material examined. — IOM M.003–017 (c. 156 mm HL [tip of snout damaged], 520+ mm TL), Mascarene Ridge, 8°29´S, 59°35´E, 950–1150 m, FRV Zvezda Kryma, cruise 1/6, trawl 4, 19 June 1976 .

DIAGNOSIS.— A species of the Coelorinchus acanthiger –group with light organ not visible externally; snout 38–42 % HL, sides weakly convex in dorsal view; medial element of terminal scute diamond-shaped (tip broken in both known specimens); preoral region almost fully scaled, nasal fossa largely scaleless, area between nasal fossa, orbit, and suborbital ridge fully scaled; most scales over interorbital space with spinules irregularly disposed; scales on body firmly attached; flank scales bearing spinules with buttresses strongly developed, but not forming transverse ridges, at least on scales from below and in front of first dorsal fin, weakly developed to absent on scales from last three-quarters of tail; 4.5–5.0 and 5.5 scales between midbase of first dorsal fin and between origin of second dorsal fin and lateral line, respectively; body dark-colored, fins darker, gums and lower lip pale, upper lip pale to blackish.

DESCRIPTION.— General features of fish seen in Figs. 2D, 16A, 17. Counts: first dorsal-fin rays ii + 8; pectoral-fin rays i + 18; pelvic-fin rays 7; gill-rakers (inner) on 1 st arch 8; gill-rakers on 2 nd arch 6 (outer) / 8 (inner) in holotype, not counted in IOM specimen (gill arches damaged); transverse scale rows below origin of first dorsal fin 4.5 [5.0]; ditto, below midbase of first dorsal fin 4.5–5.0 [4.5]; ditto, below origin of second dorsal fin 5.5; ditto, between origin of anal fin and lateral line 12–15; lateral-line scales before origin of second dorsal fin 13; pyloric caeca not counted (stomach everted).

Measurements shown in Table 2 ( IOM M.003–017; for holotype, see Iwamoto et al. 2006: Table 1 ). Width of body across pectoral bases 85.7 % of greatest body depth (at dorsal-fin origin), 1.2 times less than greatest width of head (at preopercles). Head c. 3.3 times in TL. Snout long, 2.6 times in HL, equal to postorbital length of head. Snout acute and horizontal in lateral view, with dorsal contour gently declining toward the tip; weakly convex at sides in dorsal view, with sides rather regularly convergent toward the tip ( Figs. 16A–B, 17). Snout tipped with moderately long, somewhat diamond-shaped terminal scute, which lateral elements forming distinct “shoulders” on each side of medial element. Anterolateral margins of snout not completely supported by bone. Orbit elliptical, 4.3 times in HL, 1.7 times in snout. Suborbital shelf rather steep and feebly angulated ( Fig. 13B); shelf depth 1.4 times in suborbital depth. Lateral nasal ridge 3.4 times shorter than suborbital ridge. Mouth large, posterior tip of maxilla extending below posterior quarter of orbit, rictus below middle of orbit. Preopercle inclined backward at about 60º, with moderately long, broadly and somewhat angularly rounded posteroventral lobe. Subopercle terminates ventrally in slender tip that extends somewhat beyond preopercle. Chin barbel short, slender, 2.1 times in orbit. Free neuromasts on snout, along head ridges, and on underside of head prominent, more or less infuscated (brownish to blackish), surrounded by isolated or scattered black hair-like papillae, more densely aggregated along anterior and lower margins of nasal fossa, in scaleless preoral area in front of premaxillary symphysis, along suborbital ridge below its posterior quarter, in area between the ends of upper jaw and horizontal limb of preopercle, and in scaleless area above gill cover. Inner side of gill opening lacking protrusion in its lower half.

Jaw teeth stout, conical, in narrow bands; premaxillary teeth larger than those on dentary; outermost teeth of premaxilla weakly enlarged. Premaxillary tooth band short, 1.5 times in length of rictus; dentary tooth band reaching rictus.

First dorsal-fin base 1.7 times shorter than the interdorsal space (equal in holotype); second dorsal-fin spine with broken tip in IOM specimen (1.3 times shorter than postrostral length in holotype). Pectoral fin narrow based, rather short, falling more or less before anal-fin origin. Pelvic fin originates on the same vertical with pectoral fins, or in the holotype, behind it. Outermost pectoral-fin ray filamentous and extending to anus. Anus close to anal-fin origin. Light organ not externally visible ( Fig. 16C).

Squamation ( Figs. 5J–L, 6E–F, 7E–F, 12A, 12C, 16D–E). All scales strong and adherent, those on body large; those on flanks in front of first dorsal fin and below its base above lateral line, in predorsal area, and on isthmus with spinules in more or less diverging rows; those on flanks below lateral line, behind first dorsal-fin base, and on belly with spinules in more or less parallel rows. Spinules on scales below first dorsal-fin base arranged in 5–8 rows, most rows complete ( Fig. 5K); those on scales below beginning of second dorsal fin arranged in 7–10 [8–12 in holotype] rows, some of rows often incomplete ( Fig. 5L); middle row of spinules somewhat enlarged, giving appearance of horizontal striations on body surface; spinules in rows gradually increasing in length posteriad (less conspicuous on anteriormost trunk scales); spinules of tail scales thinner than those of flank scales and somewhat needle-like, especially in lateral rows. Each spinule with prominent lateral buttresses, those of spinules of neighboring rows usually not joined with each other and not forming transverse ridges across exposed field of scale ( Figs. 5J–K) (except some scales on flanks and abdomen: Fig. 7E); buttresses becoming weakly developed to absent on tail scales ( Fig. 7F). Predorsal scales with more raised and more separated spinules, arranged in 3–5 rows ( Fig. 5J). Scales on top of head, cheeks, and gill cover with diverging rows of spinules ( Fig. 12A); most scales over interorbital space with irregularly disposed spinules not forming clear rows ( Fig. 6E); area between the orbit, nasal fossa, and suborbital ridge densely scaled, with spinules on scales arranged in rows. Narrowest portion of suborbital shelf bearing about 4 rows of tightly joined scales between lower rim of orbit and upper edge of suborbital ridge; scales in lowermost row being much larger ( Fig. 12C). Nasal fossa with few spinulose scales along anterior and lower margins ( Figs. 16D–E). Top of snout densely scaled. Underside of head densely scaled, except gular region, branchiostegal membranes, and a small triangular area in front of premaxillary symphysis; scales firmly adherent ( Fig. 16B). Spinules on scales from underside of head arranged in a cluster or in 1–2 short rows. Scutes of head ridges strongly armed with numerous stout conical spinules with pointed tips; scutes of medial nasal ridge with radiating rows of spinules bearing conspicuous buttresses, 9 scutes (including terminal scute). Supraoccipital and postoccipital scutes small (indistinctly larger than neighboring scales), coarsely spinulated.

Body color dark to medium brown without markings. Brown coloration formed by dense aggregations of melanophores on epithelium covering exposed field of the scales. When scales removed, scale pockets pure white with narrow dark-brown margins and sparse concentrations of brownish melanophores or rare isolated melanophores, often indistinct on flanks, most prominent on belly. Orbit encircled by dark-brown ring; anterior border of second naris blackish. Middle part of preoral area, gular region, and especially branchiostegal membranes darker than rest of head, distal margin of branchiostegal membrane much more infuscated in its upper half. Lips and gums pale, with a row of blackish papillae inside of premaxillary tooth band [upper lip blackish in holotype]; mouth cavity dusky; branchial cavity and peritoneum blackish; stomach pale. All fins dark; skin at bases of anal-fin rays much darker than on adjacent scales.

ETYMOLOGY.— The species was named in honor of the Soviet/Russian ichthyologist Yuri N. Shcherbachev ( Iwamoto et al. 2006).

REMARKS.— Within the specimens listed in the original description ( Iwamoto et al., 2006) only the holotype actually belongs to this species. All the paratypes are the new species C. mascarenus . However, we have discovered a second, previously overlooked specimen of C. yurii in the IOM collection; its characters match well with those of the holotype.

The photograph of the head in ventral view of the holotype of C. yurii was erroneously replaced by that of the holotype of C. amirantensis in the original description ( Iwamoto et al. 2006:fig. 2c). Herein we publish a correct picture ( Fig. 17).

COMPARISONS — The most obvious feature distinguishing C. yurii from the similar and co-occurring species C. amirantensis and C. paraboliceps is the absence of the continuous transverse ridges formed by buttresses of the spinules on the scales in front of and below the first dorsal fin, even in very large specimens ( Figs. 5, 6). In addition, C. yurii can be easily distinguished from C. amirantensis in its shorter snout (38–42 % vs. 46–51 % HL), and from C. paraboliceps in the preoral region almost completely scaled (vs. scales present in small area behind terminal scute and in lateral quarters of posterior third of preoral length only). See Table 4 for more thorough comparison with the aforementioned species.

Within the extralimital species, C. yurii shares many common features with C. lasti from the Australasian waters (Iwamoto and Williams 1999), but can be easily distinguished from that species in the parallel to slightly divergent and more numerous rows of spinules on body scales (vs. strongly divergent, 3–5 in number in C. lasti ), in the short (vs. high and blade-like in C. lasti ) spinules on the scales from the underside of head, and in the somewhat longer snout (38–42 % HL, preoral length 35–37 % HL vs. 37–38 and 30–32 % HL in C. lasti ). Although the body coloration is similarly dark in both these species, the lips, gums and mouth are generally light-colored in C. yurii (only upper lip can be conspicuously blackish) instead of blackish in C. lasti . Another similar species, C. trachycarus from the South Australian – New Zealand region ( Iwamoto et al. 1999), can be distinguished by its much more extensive naked areas along the suborbital and the lower preopercular margins, more harsh and coarse spinulation of scales and head ridges, and by its smaller ratio of orbit into postorbital (1.0–1.2 vs. 1.5– 1. 7 in C. yurii ).

Key to the species of Coelorinchus from the western Indian Ocean (eastward to Ninety East Ridge) with long snout (clearly exceeding orbital diameter and upper jaw length) incompletely supported by bone, and with light organ externally invisible or represented by a cleft or lens-like fossa in front of anus [NB: May not work for juvenile specimens less than 45 mm HL.]

1a. Underside of head fully naked.................................................. 2

1b. Underside of head at least partially scaled........................................ 3

2a. Trunk completely encircled by a broad dark band; body scales with median spinule row scarcely if at all larger than lateral rows; spinule rows complete and extending to distal margin of scale................................................... Coelorinchus kaiyomaru

2b. Trunk not encircled by a dark band; body scales with median spinule row much larger than lateral rows; lateral rows incomplete, mostly not reaching scale margin......................................................................... Coelorinchus cf. labiatus

3a. Body scales with median spinule row scarcely if at all larger than lateral rows; all spinule rows complete and extending to scale margin; premaxillary tooth band extending nearly to end of rictus..................................................................... 4

3b. Body scales with median spinule row much larger than lateral rows; lateral rows variably expressed; premaxillary tooth band ending far before end of rictus................... 5

4a. Posterior nostril large, 2.3-2.7 times in orbit; outer premaxillary teeth not enlarged; scale spinules inclined; light organ not visible externally............... Coelorinchus flabellispinis

4b. Posterior nostril smaller, about 3.5 times in orbit; outer premaxillary teeth enlarged; scale spinules raised; light organ present as scaleless fossa in front of anus..... Coelorinchus trunovi

5a. Light organ externally not visible; buttresses of spinules of flank scales strongly developed at 79 mm HL or more.......................................................... 6

5b. External light organ present; buttresses of spinules of flank scales at most weakly developed even in largest fishes........................................................ 8

6a. Snout length 46–51 % HL; spinules on scales over interorbital region arranged in keel-like rows; buttresses of spinules on tail scales less conspicuous than on trunk scales, but similarly developed on predorsal scales and on flank scales ( Figs. 6B, 7A–B)....... C. amirantensis

6a. Snout length 38–44 % HL; spinules on scales over interorbital region mostly irregularly disposed; buttresses of spinules strongly developed on scales throughout the body or less conspicuous both on predorsal and on tail scales ( Figs. 6D, 6F, 7C–F).................... 7

7a. Snout strongly and regularly convex toward blunt tip in dorsal view, convex at level of nasal fossa and gently declining toward the tip in lateral view; medial element of terminal scute small and rectangular; preoral region scaleless except small area behind terminal scute and lateral quarters of posterior third of preoral length; buttresses of spinules forming transverse ridges on scales in front of and below first dorsal-fin origin and on tail ( Figs. 5G–I, 7D).......................................................... C. paraboliceps sp. nov.

7b. Snout weakly convex and more rapidly convergent toward acuminate tip in dorsal view, almost horizontal in lateral view; medial element of terminal scute diamond-shaped; preoral region scaled except small area in front of premaxillary symphysis; buttresses of spinules not forming transverse ridges on scales in front and below first dorsal-fin origin and on tail ( Figs. 5J– L, 7F)............................................................... C. yurii

8a. Light organ with dermal window representing oval naked black fossa in front of anus ( Fig. 11B); terminal snout scute longer, 9.5–12.2 % HL; scales below midbase of first dorsal fin and lateral line 3.5–5.0 (usually 4.5); scales between origin of second dorsal fin and lateral line 4.5–6.5 (usually 5.0–5.5).............................................. C. braueri

8b. Light organ cleft-like, without dermal window in front of anus ( Fig. 11A); terminal snout scute shorter, 6+–8.8 % HL; scales below midbase of first dorsal fin and lateral line 3.0–4.5 (usually 3.5); scales between origin of second dorsal fin and lateral line 4.0–5.0 (usually 4.5)........................................................... C. mascarenus sp. nov.