Eustomias (Nominostomias) robertsi, Stewart & Kenaley & Sutton, 2024

Stewart, Andrew L., Kenaley, Christopher P. & Sutton, Tracy, 2024, A new species of Eustomias (Nominostomias) (Stomiiformes: Stomiidae) from the Kermadec Ridge-Tonga Trench region, western South Pacific Ocean, with notes on the barbel morphology of E. trewavasae, Zootaxa 5458 (1), pp. 93-107 : 94-99

Eustomias (Nominostomias) robertsi sp. nov. Figs 1–5 ; Table 1

Holotype NMNZ P.013765, 232.1 mm SL, gravid female; Kermadec Trench / Tonga Trench, 24° 07.0’–25° 22.0’S, 175° 15.0’–11.0’W; fine mesh midwater trawl, depth n.r., RV James Cook, Stn. JCO 7617/063, 13 Dec 1976.

Paratype NMNZ P.058340, 144.6 mm SL, female?; east of Macauley Island , Kermadec Islands, 30° 15.88’– 16.03’S, 178° 11.57’–09.22’W; meso–pelagic trawl, 100–1000 m over 1399 m , RV Tangaroa, Stn. TAN 1612/069, 28 Oct 2016 .

Diagnosis: The new species is diagnosed from others in Eustomias (Nominostomias) Group III by the combination of the barbel morphology of: A very small distance between proximal and distal bulbs (0.1–0.2 % SL; 0.01–0.07 times distal bulb length), and three long terminal filaments, longest 30.4–42% SL.

Description: Holotype, paratype in parenthesis. D 25 (24), A 37 (39), P 3 (3), V 7 (7). OV 33 (33), VAL 19 (20), OA 52 (53); IP 7 (7), PV 34 (27), VAV 19 (20), AC 19 (21), IC 79 (75).Barbel length 42.1% SL (45.9% SL), axil internally pigmented (slightly faded on holotype); two terminal bulbs, the proximal small, round 0.5% SL (0.6% SL), position on the barbel at 40.2% SL (43.1% SL) length, distal bulb larger, oval, length 1.5% SL (1.7% SL), width 42.8% (42%) length. Proximal bulb 34.3% (36%) length of the distal bulb. Bulbs separated by a very short distance, 0.1% SL (0.2% SL). Distal bulb with three long filaments, longest 42% SL (30.4% SL), others 28.3% SL (15.6% SL). The holotype has a simple branch at about. 69% the length of the longest filament, about. 20.2% the length of the branch, with a slight elongate thickening, its length 0.2% SL (0.9% SL) (see Fig. 2 ). The paratype has two short filaments arising from a very short common stem, visible under magnification. The terminal filaments have granular inclusions along their lengths, most pronounced in the longest filament of the holotype. Post orbital light organ 21.9% ED (17.5% ED).

Head moderate, 14.3% SL (16.8% SL); snout pronounced, 2.8% SL, 19.8% HL (5.5% SL, 32.5% HL), eye round, large, 2.9% SL, 20.9% HL (2.3% SL, 16.5% HL). The anterior–most fangs in the maxilla and dentary, the two smaller teeth in front of these, and two tiny ones behind fixed, all the other teeth depressible ( Fig. 3 ). Suborbital photophore small, elongate, greatest length 21.9% ED (17.5% ED). Pre–dorsal length 85.9% SL (84.6% SL), pre–anal length 73.8% SL (73.7% SL), anal fin length 23.6% SL (25% SL). Pre–pelvic length 56.6% SL (52.3% SL). Caudal peduncle depth 1.2 % SL (1.2% SL). Ventral groove absent.

Both specimens of Eustomias robertsi have suffered some damage making accurate counts of the number of pre–dorsal spots difficult and unreliable. The range for this character amongst species of Group III has been recorded as seven to nine ( Gibbs et al. 1983).

Colour: ( Fig. 4 ) Head and body skin uniform black. The barbel has slight external dark pigmentation at base; a narrow but distinct internal axis of dark pigmentation, breaking up into a line of melanophores distally (axis pigment faded on holotype), terminating in a small black pigment cap and streak on the proximal end of the proximal bulb (present on both specimens). Proximal bulb pale grey-green. The distal bulb grey-green with a slight violet flush on distal end. Terminal filament inclusions and bulblet grey-green; rapidly fading to a pale yellow–tan on death. Following preservation the entire body of the holotype faded to light brown; body photophores black.

Discussion: The new species is a member of the Nominostomias sub-genus sensu Gibbs et al. (1983), which are diagnosed by a combination of: Pectoral fin with three separated rays, seven pelvic fin rays, barbel without external pigmentation (axis slightly pigmented), branches or rows of dark spots proximal to terminal bulb absent; one or two terminal bulbs. Wide ventral body groove absent, or groove short, narrow, shallow. Photophore counts high (IC seldom less than 72; see Table 2 ).

Within Nominostomias , this new species is a member of Group III sensu Gibbs et al. (1983), diagnosed by having two terminal bulbs; three or more filaments emerging from distal bulb, simple to long with prominent bulblets and/or inclusions.

With the addition of E. robertsi ¸ Nominostomias Group III contained 10 species (see Gibbs et al., 1983, and Table 1 ). Within this group, E. robertsi is most similar to E. suluensis in barbel morphology, having a slightly rounded proximal bulb followed by a larger, elongate distal bulb, and three simple terminal filaments arising from the end of the distal bulb. However, the distance between the proximal and terminal bulb of E. robertsi is much less (0.1–0.2% SL cf. 2.4–2.8% SL), and the distal bulb is slightly larger (1.5–1.7% SL cf. 1.2–1.3% SL), relative to the proximal (2.8–2.9 x proximal bulb diameter cf. 1.5–1.8 x). The barbel length (to the distal margin of the distal bulb) of E. robertsi is shorter that E. suluensis (42.1–45.9% SL cf. 52–59% SL) but the terminal filaments are longer (30.4–42% SL cf. 15–20% SL). There are small but clearly discernible bulblets (towards the distal end of one of the longest filaments) on both specimens of E. robertsi (cf. no discernible bulblets), and the longest filament branches with a slight swelling distal to the separation. The terminal filaments of E. robertsi arise independently from the terminal face of the distal bulb whereas in E. suluensis two filaments come off near the base of the central (longest) filament. The axil pigmentation of E. suluensis extends beyond the proximal bulb to end in a pigment cap on the proximal end of the distal bulb whereas in E. robertsi the axil pigmentation terminates in a cap marking on the proximal bulb. Some tiny diffuse dark spots are on the distal bulb.

The inter-bulb distances are smaller in E. robertsi compared with the other nine species: 0.1–0.2 % SL cf. 0.5– 3.5 % SL, and 0.01–0.07 of the distal bulb length cf. 0.4–3.6 ( Gibbs et al., 1983; Table 4). In the overall length of the barbel, E. robertsi is similar to E. bertelseni , E. kreffti , E. medusa , E. multifilis and E. posti , however these species generally have more and/or different terminal filaments (occasionally 3, usually 4–16) that are much shorter (5–35% SL) cf. 3 filaments, 30.4–42 % SL. Only E. kreffti has terminal filaments with lengths overlapping with E. robertsi , however the number and morphology of those filaments is markedly different comprising two short plus three to nine long filaments in E. kreffti ( Gibbs et al 1983; fig. 18b–c). Prokofiev (2020b) reported on a species he referred to as E. gr. melanostigma but the inter-barbel distance was 1.6% SL, and 1.6 times the length of the distal bulb.

Most members of Eustomias (Nominostomias) Group III are tropical to subtropical, and often restricted in distribution. Only E. melanostigma is widespread, from the Caribbean Sea to the western central Pacific Ocean but is not known to occur south of 10° S. Eustomias kreffti , E. melanonema , and E. posti are restricted to the tropical Atlantic Ocean. The other species are similarly tropical, in the eastern Indian Ocean, to the western and central Pacific. This is the first record of a species of E. ( Nominostomias ) Group III species from the New Zealand EEZ, and one of the southern–most species in the group. Only the holotype of E. multifilis was taken farther south, off southern Western Australia at 33° 34’S. For other values see Table 1 .

Distribution: Eustomias robertsi is known only from two specimens taken on the Kermadec Ridge, and in the southern Tonga Trench region of the western South Pacific Ocean ( Fig. 5 ).

Maximum size: 232.1 mm SL

Etymology: We take great pleasure in naming this species after our friend and colleague Dr. Clive Roberts, the former Curator of Fishes at the Museum of New Zealand Te Papa Tongarewa. Clive conceived, secured funding for, and directed a 20–year project to describe the fish fauna of New Zealand, culminating in publication of the awardwinning multi-volume 2,000 page The Fishes of New Zealand ( Roberts et al. 2015).

Figures

FIGURE 1. Eustomias robertsi n. sp. holotype, NMNZ P.013765.; 232.1 mm SL. Illustration: Michelle Freeborn.

FIGURE 2. Terminal barbel morphology of Eustomias robertsi n. sp. holotype, NMNZ P.013765. Illustration: Michelle Freeborn.

FIGURE 3. Teeth of Eustomias robertsi n. sp. holotype, NMNZ P.013765. Fixed teeth (black), remaining teeth depressible. Illustration by Michelle Freeborn.

FIGURE 4. Freshly caught paratype of Eustomias robertsi n. sp. NMNZ P. 058340; 144.6 mm SL. Photo Carl Struthers/Thom Linley.

FIGURE 5. Capture locality of Eustomias robertsi; red star = holotype. Prepared by Thom Linley.