Coelorinchus amirantensis 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 : 234-238

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

Figures 2A , 3 , 4 , 5A–C , 6A–B , 7A–B , 8 , 9 . Coelorinchus sp. A : Shcherbachev et al. 1986:201 (6°20´S, 54°23´E). Coelorinchus amirantensis Iwamoto, Golani, Baranes, and Goren, 2006:438 , figs. 4–5 (original description,

Seychelles and Mascarene Plateau, 950–1900 m).

Holotype. — TAU P.11600 (157 mm HL, tail incomplete), Seychelles between Alphonse and Bijoutier islands, by trammel net in 1900 m, R / V Sea Surveyor, 17 Dec. 1998, collector M. Goren.

Paratypes. — TAU P.11603 (139.6 mm TL, 420+ mm TL), same data as for holotype . CAS 223467 (3, 96–126. 5 mm HL, 285+– 374 mm TL) , IOM M.007–001 (1, 126 mm HL, 424+ mm TL) and IOM M.008–002 (1, 108 mm HL, 343+ mm TL), Mascarene Ridge, 8°26.4´S, 59°29´E, 1300– 1260 m, FRV Fiolent, cruise 7(9), trawl 52, 2 Sept. 1977 . IOM M.005–020 (1, 79 mm HL, 250+ mm TL), Mascarene Ridge, 8°08´S, 59°37.6´E, 1247–1269 m, FRV Zvezda Kryma, cruise 1/6, trawl 210, 9 Nov. 1976 . IOM M.006–019 (1, 85 mm HL, 260+ mm TL), Mascarene Ridge, 8°29´S, 59°35´E, 950–1150 m, FRV Zvezda Kryma, cruise 1/6, trawl 4, 19 June 1976 . IOM M.009–010 (1, 41 mm HL, 147+ mm TL) and IOM M.010–009 (1, 80 mm HL, 265+ mm TL), Mascarene Ridge, 8°07´S, 59°18.6´E, 1300– 1240 m, FRV Fiolent, cruise 7(9), trawl 53, 3 Sept. 1977 .

DIAGNOSIS.— A species of the Coelorinchus acanthiger –group with light organ not evident externally; snout length 46–51 % HL, with sides in dorsal view straight to slightly concave and strongly converging anteriad in front of nasal fossa; medial element of terminal scute long and diamond-shaped, 9.9–11.1 % HL; preoral region partially to almost completely scaled at 79 mm HL or greater; nasal fossa mostly naked, area between nasal fossa, orbit, and suborbital ridge fully scaled; scales over interorbital region with spinules arranged in distinct keel-like rows; scales on body firmly attached, bearing spinules with buttresses strongly developed, forming concentric ridges even on scales below and in front of first dorsal fin, but weaker on tail scales; 3.0–4.5 and 4.0–5.5 scales between midbase of first dorsal fin and between origin of second dorsal fin and lateral line, respectively; body dark-colored, fins darker, lips and gums pale.

DESCRIPTION.— Based on specimens 79–157 mm HL. General features of fish seen in Figs. 2A , 3 , 4 . Counts: first dorsal-fin rays ii + 7–8 [ii + 7]; pectoral-fin rays i + 16–19 [i + 17]; pelvic-fin rays 7; gill-rakers (inner) on 1st arch 6–8 [8]; gill-rakers on 2nd arch 5–7 (outer) / 6–8 (inner) [6 / 8]; transverse scale rows below origin of first dorsal fin 3.5–6.5 [5.5]; ditto, below midbase of first dorsal fin 3.0–4.5 [4.5]; ditto, below origin of second dorsal fin 4.0–5.5 [4.5]; ditto, between origin of anal fin and lateral line 10.0–10.5 [not counted]; lateral-line scales before origin of second dorsal fin 12(13); pyloric caeca not counted (stomachs everted).

Measurements shown in Table 2 . Width of body across pectoral bases 79.3–84.5 % of greatest body depth (at dorsal-fin origin), 1.1–1.3 times smaller than greatest width of head (at preopercles). Head c. 3.0–3.6 times in TL. Snout long, 2.0–2.2 times in HL, 1.4–1.7 times greater than postorbital length of head.

Snout acute and horizontal in lateral view, with dorsal contour gently sloping toward the tip ( Fig. 3A ); usually distinctly convex at sides in dorsal view, with sides much more convergent toward the tip in anterior half to two-thirds of length of lateral nasal ridges; however, in some specimens (holotype and paratype IOM M.008–002) sides of snout regularly convergent toward the tip ( Figs. 3B, 3C , 4 ). Snout tipped with long diamond-shaped terminal scute, which lateral elements form small (sometimes indistinct) “shoulders” on both sides of medial element at midlength of the latter. Anterolateral margins of snout not completely supported by bone. Orbit elliptical, its greatest diameter 4.0–4.9 times in HL, 1.8–2.3 times in snout. Suborbital shelf moderately angulated; shelf depth 1.4–1.5 times in suborbital depth. Lateral nasal ridge 2.7–2.9 times in length of suborbital ridge. Mouth large, posterior tip of maxilla extending to level of posterior quarter of orbit (rarely, to posterior third, but nearly to posterior border of orbit in largest IOM specimen), rictus below middle to posterior third of orbit. Preopercle inclined backward at about 50–70º, with moderately long, angularly rounded posteroventral lobe having crenate margin. Subopercle terminates ventrally in slender tip that extends somewhat beyond preopercle or completely hidden by preopercular lobe. Chin barbel short, slender, 3–4 times in orbit. Free neuromasts on snout, along head ridges and on underside of head prominent, more or less infuscated (more prominently on snout), surrounded by black hair-like papillae, poorly expressed on dorsal surface of head but densely covering preoral portion of snout, areas along upper jaw, and lower jaws. Inner side of gill opening usually without protrusion in its lower half (but present in 126-mm HL IOM specimen). Anus close to anal-fin origin. Light organ not externally visible.

Jaw teeth thin, conical, in narrow bands; premaxillary teeth larger than those on dentary; outermost teeth of premaxilla weakly enlarged. Premaxillary tooth band short, 1.9–2.2(2.6) times in length of rictus; dentary tooth band reaching end of rictus.

First dorsal-fin base 0.9–1.3 times shorter than the interdorsal space; second dorsal-fin spine with broken tip in all specimens examined. Pectoral fin narrow based, rather short, falling from just before anus ( IOM M.007–001) to just behind anal-fin origin. Pelvic fins originate on same vertical with pectoral fins or (more often) behind it. Outermost pelvic-fin ray filamentous and extending to anus.

Squamation ( Figs. 5A–C , 6A–B , 7A–B ). All scales strong and adherent, those on body large. Scales on flanks, even in front of first dorsal-fin origin, with spinules arranged in parallel or slightly divergent rows; those on scales on top and lateral sides of head, in predorsal area, and on isthmus arranged in divergent rows. Spinules on scales below first dorsal-fin base arranged in 3–8 rows (usually 5–7), those on scales below origin of second dorsal fin arranged in 5–7 (smaller specimens) to 7–12 rows (largest specimens); predorsal scales with more erect and more separated spinules, arranged in 3–5 rows ( Figs. 5A–C ); lateral rows of spinules less complete on scales from anterior third of trunk and in specimens of smaller size; middle row of spinules distinctly enlarged, giving appearance of horizontal striations on body surface; spinules in rows gradually increasing in length posteriad, those of middle row longer and more erect in smaller specimens. Each spinule with prominent lateral buttresses often joined with those of neighboring rows to form low transverse ridges across exposed field of scale; buttresses strongly developed on scales elsewhere on trunk, in predorsal region, breast and belly, on top of head, on cheeks and gill cover ( Figs. 5A–C , 6B , 7A ), but becoming weaker on scales from tail region behind anterior third of anal-fin base, these scales show more depressed spinules with last spinule of medial row more greatly produced backward ( Fig. 7B ). Buttresses are better developed in larger specimens. Scales over interorbital region bearing spinules arranged in keel-like rows ( Fig. 6A ). Area between the orbit, nasal fossa, and suborbital ridge fully scaled, with spinules on scales arranged in 1–3 rows in larger specimens, in clusters to one short row in smaller specimens. Narrowest portion of suborbital shelf bearing 2–4 rows of tightly joined scales between lower rim of orbit and upper edge of suborbital ridge, scales in lowermost row much larger than 1–3 rows of small scales above it; number of rows decreasing with growth. Nasal fossa scaleless except for some cycloid scales along margins and sometimes a few spinulose scales in front of and/or below nostrils (from one side only in 126-mm HL IOM specimen, fully naked in holotype). Top of snout densely scaled. Underside of head usually densely scaled, but not gular region, branchiostegal membranes and small triangular area in front of premaxillary symphysis ( Fig. 4A, 4B ). Scales firmly adherent. In IOM specimens of 79 and 108 mm HL, underside of snout largely scaleless over anterior third to two-thirds of preoral length ( Figs. 4C ). Spinules on scales from underside of snout arranged in a single short row (up to 3–4 spinules in number in largest specimens). Density of spinulation of scales on top of head and arrangement of spinules variable. Scutes of head ridges strongly armed with numerous sharp, comparatively long, and rather stout conical spinules (thinner and longer in smaller specimens). Scutes forming medial nasal ridge 8–9 in number (including terminal scute), each with radiating rows of spinules bearing prominent buttresses. Supraoccipital scute small, coarsely spinulated, usually as large as neighboring scales but distinctly larger than same in 126-mm HL IOM specimen (slightly larger in 79-mm HL IOM specimen); postoccipital scute small, coarsely spinulated, about same size as anteriormost flank scales.

Body color brown, without markings; dark coloration formed by dense aggregations of brownish melanophores on epithelium covering exposed field of scales. When scales removed, scale pockets pure white with narrow dark-brown margins and sparse concentrations of brownish melanophores, or rarely, isolated melanophores, much less expressed and much smaller on scale pockets from dorsal half of body and from caudal region. Distal third of scale pockets around periproct heavily pigmented by confluent brownish to blackish melanophores. Orbit encircled by dark-brown ring; anterior border of second naris blackish. Gular region and branchiostegal membranes darker than other parts of underside of head, upper reaches of branchiostegal membrane blackish. Lips and gums pale; mouth cavity pale to dusky; gustatory papillae pale, but minute papillae along margins of lower lip sometimes blackish; branchial cavity and peritoneum blackish; stomach pale. All fins dark or dusky; skin at bases of anal-fin rays sometimes darker than on adjacent scales.

VARIATION.— Specimens examined are variable in shape of snout, degree of scale covering of preoral region, and distribution of spinules on scales atop head. The holotype (157 mm HL) and paratype IOM M.008–002 (108 mm HL) show sides of snout regularly converging toward the tip, in contrast to the other specimens with sides of snout distinctly convex in dorsal view (compare Figs. 3B and 3C ). However, this character does not correlate with other variable features. Paratype IOM M.005–020 (79 mm HL) is fairly distinct from the others in arrangement of scales in preoral region, which is largely scaleless along two-thirds of its length with scales cycloid laterally and behind scaleless area. However, in paratype IOM M.008–002, the anterior third of preoral region is also scaleless, though laterally and posteriorly from that area scales are spinulose. Paratype IOM M.005–020 shows less numerous spinules on scales on top of head in comparison to other specimens examined; however, density and arrangement of spinules on these scales are variable in all available specimens, and there are no specimens having these features nearly identical. We treat all these differences as individual variation.

The 41-mm HL paratype ( IOM M.009–010: Fig. 8 ) differs from the adult and subadult specimens described above in having underside of head completely devoid of scales ( Fig. 8B ). It has body scales ( Fig. 9 ) much less adherent than in larger specimens, with fewer but more raised spinules, 3–4 rows on scales in front and below first dorsal fin, in interdorsal area, and on isthmus; scales lost from other parts of body. The last spinule of medial row on scales from dorsal half of trunk conspicuously enlarged. Head ridges much more coarsely spinulated than in adults, but spinules on scales from head are less developed, and scales between the orbit and suborbital ridge are largely cycloid. Overall coloration is much paler than in adults but darkish circumorbital ring is already obvious. In proportions this juvenile is distant from all available larger specimens in the longer jaws and premaxillary tooth band, shorter terminal scute, much shorter lateral nasal ridges, and wider and somewhat shorter snout ( Table 2 ).

ETYMOLOGY.— The species name is derived from the Amirantes Basin, the type locality of the species ( Iwamoto et al. 2006).

COMPARISONS. — Coelorinchus amirantensis can be easily distinguished from all other Mascarene species of Coelorinchus by its distinctly longer and more acutely pointed snout, relatively shorter premaxillary tooth band, short-based first dorsal fin apparently lacking elongated filament of its second ray at all stages of growth, shorter interdorsal distance, and by details of squamation. Scales of C. amirantensis are strongly adherent with prominent lateral buttresses mostly confluent with each other to form low transverse ridges across exposed field of scale (vs. scales deciduous and buttresses weak or lacking in C. mascarenus ) ( Figs. 5A–F ); buttresses forming transverse ridges on scales from predorsal area and flanks above lateral line, and before second dorsal-fin origin (vs. not forming transverse ridges in C. yurii ) ( Figs. 5A–C, 5J–L , 6B, 6F ), becoming weaker on tail scales with transverse ridges disappearing, in contrast to C. paraboliceps ( Figs. 7B, 7D ). Scales on interorbital region possess spinules arranged in distinct keel-like rows in C. amirantensis instead of irregularly disposed spinules on most of scales in that area in the three other Mascarene species ( Figs. 6A, 6C, 6E ). Morphometric differences between four Mascarene Coelorinchus species are summarized in Table 2. A comparison with similar extralimital species was provided by Iwamoto et al. (2006), see keys to species in current paper for additional data.

The 41-mm HL juvenile of C. amirantensis has underside of head completely scaleless and can be confused with C. cf. labiatus , which has the scaleless underside of head at all stages of growth. The 38-mm HL juvenile of C. cf. labiatus ( IOM 00184) shows all lateral rows of spinules on scales in front, below and behind first dorsal fin complete and reaching distal margin of scale, in contrast to those in adults and similar to those of C. amirantensis . However, these spinules are much finer and more depressed than in juveniles of C. amirantensis , more numerous (usually in 5 rows, rarely in 4 or 6), with last spinule of medial row never much enlarged. The head ridges in juveniles of C. cf. labiatus are not so coarsely spinulated, and spinules on scales on top and sides of head are poorly developed (most scales cycloid and much more deciduous than in C. amirantensis ). The 40–45 mm HL juveniles of C. braueri ( IOM 00191) have underside of head also scaleless or with few scales at corners of mouth only, and the number and shape of spinules on their trunk scales is similar to those observed in the juvenile of C. amirantensis . However, spinulation of head ridges is much harsher in C. amirantensis , and the spinules on scales on top of head are arranged in parallel rows vs. rather irregularly disposed in juveniles of C. braueri . Furthermore, the juveniles of C. braueri possess a small naked dermal window in front of anus and a filamentous second dorsal-fin ray (both absent in C. amirantensis ). We have no juveniles of three other Mascarene species for comparison. The 40–50-mm HL juveniles of C. flabellispinis and C. trunovi already have underside of head partially scaled.

Figures

FIGURE 2. Coelorinchus spp., habitus, lateral view. A. C. amirantensis, IOM M.008–002, paratype, 108 mm HL. B. C. mascarenus, IOM M.011–007, holotype, 91 mm HL. C. C. paraboliceps, IOM M.004–003, holotype, 120 mm HL. D. C. yurii, IOM M.003–017, non-type specimen, c.156 mm HL.

FIGURE 3. Coelorinchus amirantensis, paratypes, IOM M.007–001, 126 mm HL (A, B), IOM M.008–002, 108 mm HL (C). A. Lateral view of head. B, C. Dorsal view of head.

FIGURE 4. Coelorinchus amirantensis, paratypes, ventral view of head. A. IOM M.007–001, 126 mm HL. B. IOM M.008–002, 108 mm HL. C. IOM M.010–009, 80 mm HL.

FIGURE 5. Coelorinchus spp., isolated scales from predorsal region (A, D, G, J), from flank between first dorsal-fin origin and lateral line (B, E, H, K), and from flank between second dorsal-fin origin and lateral line (C, F, I, L) (stained by alizarine red S for contrast). A–C. C. amirantensis, IOM M.007–001, paratype. D–F. C. mascarenus, IOM M.011–007, holotype. G–I. C. paraboliceps, IOM M.004–003, holotype. J–L. C. yurii, IOM M.003–017, non-type specimen. Scale bars: A, J, 2.5 mm. B, C, E, F, H, K, 3.0 mm. D, 1.5 mm. G, 2.0 mm. I, L, 3.5 mm. Scale bar is common for E and F.

FIGURE 6. Coelorinchus spp., squamation of interorbital region (A, C, E) and of flank between mid-base of first dorsal fin and lateral line (B, D, F). A, B. C. amirantensis, IOM M.007–001, paratype. C, D. C. paraboliceps, IOM M.004–003, holotype. E, F. C. yurii, IOM M.003–017, non-type specimen. Scale bars: A, C, E, 2.5 mm. B, D, F, 5.0 mm.

FIGURE 7. Coelorinchus spp., squamation of flank between origin of second dorsal fin and lateral line (A, C, E) and of tail behind anterior third of second dorsal-fin base (B, D, F). A, B. C. amirantensis, IOM M.007–001, paratype. C, D. C. paraboliceps, IOM M.004–003, holotype. E, F. C. yurii, IOM M.003–017, non-type specimen. Scale bars: 5.0 mm.

FIGURE 8. Coelorinchus amirantensis, IOM M.009–010, juvenile paratype, 41 mm HL. A. Dorsal view of head. B. Ventral view of head. C. Habitus, lateral view.

FIGURE 9. Coelorinchus amirantensis, IOM M.009–010, juvenile paratype, 41 mm HL, isolated scales from predorsal region (A) and from flank between mid-base of first dorsal fin and lateral line (B). Scale bars: 0.5 mm