Ilyophis arx Robins, 1976

Tighe, Kenneth A., Smith, David G., Merrett, Nigel R., Frable, Benjamin W. & Zajonz, Uwe, 2024, Redescription of the eel Ilyophis arx Robins, 1976 (Anguilliformes: Synaphobranchidae: Ilyophinae) with a description of a new species of Ilyophis, and a taxonomic synopsis of the Ilyophinae, Zootaxa 5506 (1), pp. 35-57 : 37-41

Ilyophis arx Robins, 1976

Figures 1‒3 , Table 1‒2

Ilyophis arx Robins, 1976 in Robins & Robins 1976:265, Figs. 4 , 6 , 7b , 8 (original description, Southeastern Pacific, 1° 48' S, 90° 19' W, depth 3225 meters, Holotype: ANSP 133808). Böhlke 1984:158 (listed in type catalog). Merrett & Saldanha 1985: 730‒735 (comparison of central Pacific and northeastern Atlantic specimens). Robins & Robins 1989:238 (compiled). Sulak & Shcherbachev 1997: 1163, 1172, 1188 (key, comparison with Ilyophis nigeli , distribution). Karmovskaya & Parin 1999: 360 (comparison with Ilyophis saldanhai ). Smith 1999 (compiled). Causse et al. 2005: 414 (comparison with Ilyophis saldanhai ). Leitner et al. 2021a: (occurrence on abyssal seamounts) [ Fig. 1 ].

Diagnosis. A species of Ilyophis with the following characters. Body without scales. Trunk relatively long (about 21‒29% of TL). Dorsal-fin origin (DFO) above posterior third of pectoral fin. Gape of mouth moderate, extending to near rear margin of eye. Teeth not compound. Intermaxillary teeth conical with approximately 16‒24 teeth in the tooth patch, inner teeth on the patch nearly twice as large as the outer teeth. Vomerine teeth contiguous with intermaxillary teeth with approximately 25‒30 teeth irregularly biserial to triserial anteriorly and uniserial posteriorly. Both intermaxillary and vomerine teeth set in beds of papillose tissue. Maxillary teeth small, set in irregular rows, 3 rows anteriorly to 6 rows posteriorly. Dentary teeth similar to maxillary teeth except they are slightly larger and more uniform in size anteriorly. Lateral line short, confined to anterior half of body (ca. 35‒45% of TL). Cephalic lateralis pores: supraorbital (SO) 3, infraorbital (IO) 5 (including adnasal, AD), preoperculomandibular (POM) 6‒10. Total vertebrae 129‒136.

Description. Body moderately elongate, laterally compressed posteriorly ( Fig. 2 ). Body tapers gradually to caudal, deepest body depth anterior to anus. Dorsal-fin origin near posterior half or just behind tip of pectoral fin, predorsal length ca. 14–18 % TL. Trunk length moderate, contained about 4 times in TL. Preanus length approximately 3 times in TL, preanal-fin length about 36% of TL. Head length moderate (ca. 10–14 % TL). Gill slits crescentic and horizontal, located ventrally just anterior to and below pectoral fin base. Anterior nostril tubular, directed anterolaterally. Posterior nostril round, just in front of eye. Eye relatively small (9–14 % HL). Pectoral fin moderate in size (3.2–3.7 % TL, 10–27 % HL) with 15 rays. Gape extends to near posterior half of eye. Snout plicate, two main plicae on each side of mid-line, outermost least developed. Tip of lower jaw also plicate, two main plicae on each side of mid-line and occasionally one smaller plica lateral to the main plicae for a total of six. Teeth conical, arranged in bands along maxillary, dentary and intermaxillary/vomer. Teeth not compound. Maxillary teeth in two to four rows (medially) reducing to one irregular row posteriorly, with anterior teeth somewhat enlarged. Dentary teeth in three to five irregular rows, with anterior ones (in two rows) larger. Intermaxillary teeth with 14–28 strong teeth concentrically arranged around tooth patch with inner teeth slightly larger. Vomerine teeth abutting intermaxillary tooth patch with two irregular rows anteriorly (with ca. 10–16 teeth each) and converging to single row posteriorly (with two to three teeth). Both intermaxillary and vomerine teeth surrounded by papillae. Cephalic lateralis pores: supraorbital pores three, first located between first and second plicae on tip of snout, second on snout at level of the dorsal rim of anterior nostril, third on dorsal of snout above adnasal pore; infraorbital pores five, first (adnasal pore) just posterior to dorsal rim of anterior nostril, second on lip just posteroventral to base of anterior nostril, third on lip halfway between anterior and posterior nostrils, forth on lip ventral to posterior nostril, fifth just ventral to the posterior margin of the eye; preoperculomandibular pores seven to ten, six to eight pores along mandible with zero to two pores in preopercular position, first very small and located within plicae at the tip of lower jaw, remaining mandibular pores spaced progressively far apart along jaw, last mandibular pore located approximately below rictus, preopercular pores (if present) located posteriorly in the opercular region. Lateral line incomplete, relatively short; pores extend to approximately 35% length of body. Lateral line numbering two to six pores anterior to pectoral-fin base, 8–14 pores anterior to dorsal-fin origin, 31–39 pores anterior to anus, 31–47 total pores. Vertebral numbers: predorsal 9–13, preanal 37–42, total 128–137.

Comparative remarks. Ilyophis arx can be distinguished from all members of the genera Atractodenchelys , Dysomma , Dysommina , and Linkenchelys by the lack of compound teeth in the dentition (versus at least vomerine teeth compound). Head length differentiates I. arx from Meadia abyssalis ( Kamohara, 1938) and M. roseni Mok, Lee & Chan, 1991 (head length less than trunk in I. arx versus greater than trunk in M. abyssalis and M. roseni ). Vertebral counts also distinguish these species (128‒137 versus 173–178 in M. abyssalis and 198–206 in M. roseni ). The third species of Meadia , M. minor Vo & Ho, 2021 , which has a head length less than trunk, can be differentiated from I. arx by its lower vertebral count (128‒137 versus 118–122 in M. minor

Morphometric, meristic, and morphological data for all described species of the genus Ilyophis are presented in Table 2 . The lack of scales distinguishes Ilyophis arx from I. brunneus , l. blachei , l. nigeli and I. maclainei sp. nov. Ilyophis arx can also be differentiated from three of these species by its lower vertebrae number (128‒137 versus 145–151 in I. brunneus , 177–188 in l. blachei and 140–152 in l. nigeli ). It can be distinguished from I. maclainei sp. nov. (which has a similar range of vertebral number) by its lower lateral-line pore count (31–47 versus 88–94 in I. maclainei sp. nov.). Among the species without scales, I. arx can be differentiated from I. singularis and I. robinsae by vertebral count (128‒137 versus 116–118 in I. singularis and 141 in the unique holotype of I. robinsae ).

The last species of Ilyophis without scales, I. saldanhai , is the most difficult to distinguish from I. arx . There is almost a total overlap in meristics and morphometrics between the two species. Karmovskaya & Parin (1999) compared their specimens to two specimens from the northeastern Atlantic (described as a new species below) and the data for the type series presented in Robins & Robins (1976). Length of the lateral line differentiates I. saldanhai (ca. 35–46% TL in I. arx versus 57‒62% in I. saldanhai ). The total number of lateral-line pores also separates the 2 species (31–47 in I. arx versus 66–75 in I. saldanhai ). Karmovskaya & Parin (1999) also distinguished I. arx from I. saldanhai based on the post-orbital distance (52.9‒62.6% HL in I. arx versus 43.6‒52.3% HL in I. saldanhai ), but this character shows more variation and does not differentiate the species. Another character used by Karmovskaya & Parin (1999) to distinguish I. saldanhai was the number of pores in the preoperculomandibular series (POM). They indicated that I. saldanhai had 8 to 9 pores, 7‒8 mandibular pores and 1 preopercular pore while I. arx had only 7 mandibular pores. However, reexamination of three of the types of I. arx using carmine blue to highlight the pores showed that all three specimens had 7 mandibular pores and 1 preopercular pore. Examination of all 21 additional specimens showed variation in the number of POM pores (6‒9 mandibular and 0‒2 preopercular) and therefore this character should not be used as diagnostic between the two species. Karmovskaya & Parin (1999) also reported an osteological difference between I. arx and I. saldanhai . Ilyophis arx has 0‒1 branchiostegals attached to the ceratohyal, 2 branchiostegals attached to the intercalary cartilage, and the remainder articulating with the epihyal. Ilyophis saldanhai has 3 branchiostegals attached with the ceratohyal, 2 branchiostegals attached with the intercalary cartilage, and the remainder articulating with the epihyal. The combination of longer lateral line and higher lateral line pore counts do differentiate I. saldanhai from I. arx . However, the difference in branchiostegal attachment should be investigated further when additional specimens of I. saldanhai become available for osteological examination.

In addition to the morphological differences above, there is apparently a habitat difference between Ilyophis arx and I. saldanhai . Iyophis saldanhai was described from 12 specimens collected at the Broken Spur hydrothermal vent field on the Mid-Atlantic Ridge.All subsequent specimens or sightings including Biscoito et al. (2002), Biscoito et al. (2006) and Causse et al. (2005) are also associated with hydrothermal vent environments. However, none of the specimens of I. arx cited here are associated with hydrothermal vents.

Distribution. Ilyophis arx is widely distributed in the eastern and central Pacific Ocean from the Nazca Ridge off central Peru to seamounts northwest off Hawaii, between 30 o N and 15 o S ( Fig. 3 ). However, it is not uniformly distributed within this area. The species seems to be associated with underwater topography such as ridges and seamounts and is not found far away from these features on the abyssal plain. Leitner et al. (2021a) reported large numbers of I. arx attracted to baited cameras placed near the summits of three seamounts within Areas of Particular Environmental Interest (APEI) within the western CCZ in the central Pacific Ocean, but none attracted to cameras placed on the nearby abyssal plain. Leitner et al. (2021a) also reported on 12 specimens collected in a baited trap on the summit of the seamount in APEI (7). In addition, Leitner et al. (2021b), in analysis of deep-sea videos and time-lapse still photography from these cameras, found that I. arx was only seen in samples on the seamounts, and not in the associated samples from the near-by abyssal plain, concluding that I. arx may be a specialist of seamounts, ridges and island slopes and suggesting that the species may only be found associated with rough topographies.

Material Examined. ANSP 133808 (holotype, 439 mm TL) , ANSP 133809 (paratypes, 2, 344 and 366 mm TL) , ANSP 133810 (paratype, cleared and stained, 401 mm TL); Eastern Pacific Ocean , south of Galapagos Islands, 01° 48' S, 90° 19' W, 3225 m . BMNH 1998.11 .12.1 (1, 455 mm TL); Eastern Pacific Ocean , ca. 750 km SSE of Galapagos Islands, 7° 3.74' S, 88° 26.33' W, 4157 m . SIO 68-462 (2, 430 and 541 mm TL), North Pacific Ocean , Hamilton Seamount, 15° 33' 30" N, 179° 13' 42" W, 3017 m . SIO 68-494 (1, 610 mm TL); North Pacific Ocean , seamount S of Palmer Seamount, 28° 43' N, 177° 52.5'W, 2375 m . SIO 72-189 (1, 516 mm TL); South Pacific Ocean , Nasca Ridge, off Punta San Juan, 15° 39' 12" S, 76° 13' 36" W, 3475 m . SIO 88-149 (1, 383 mm TL); Pacific Ocean , Magellan Rise, at summit, 7° 16' N, 176° 7’ W, ca 3150 m . SMF 28622 (1, 430 mm TL) , SMF 28623 (1, 470 mm TL) , SMF 28624 (1, 481 mm TL); Eastern Pacific Ocean , ca. 750 km SSE of Galapagos Islands, 7° 3.74' S, 88° 26.33' W, 4157 m . USNM 443827‒443846 (10, 215‒ 536 mm TL) , BPBM 42216 ( KM 1808 TR 03 # 11, 268 mm TL) , BPBM 42217 ( KM 1808 TR 03 # 16, 511 mm TL); Central Pacific Ocean , western Clarion Clipperton Zone, 4° 52' 57.36" N, 141° 46' 45.48" W, 3203 m .

Figures

FIGURE 1. Ilyophis arx. Swarm of eels attracted to a baited camera near summit of seamount APEI 7 (4° 52' 57.36" N, 141° 46' 45.48" W, 3203 m) (photo grab from https://oceanexplorer.noaa.gov/explorations/18ccz/logs/photolog/photolog.html#cbpi=../ may25/media/DC03.html

FIGURE 2. Ilyophis arx. Pictures of fresh specimens after capture and before preservation. Upper specimen: USNM 443829. Lower specimen: BPBM 42217. (photos supplied by Dr. Jeffrey C. Drazen)

FIGURE 3. Distribution of Ilyophis arx in the eastern and central Pacific Ocean. Open star is the type locality. Closed stars are other study material.

FIGURE 4. Ilyophis maclainei sp. nov. Holotype, BMNH 1995.4.19.31 (male, 242 mm TL)

FIGURE 6. Ilyophis maclainei sp. nov. Holotype, BMNH 1995.4.19.31(DISCOVERY Collection, Station 50710), 240 mm TL (A) Snout ornamentation. (B) Lower jaw tip ornamentation (from Merrett & Saldanha 1985)

FIGURE 7. Ilyophis maclainei sp. nov. Referred Specimen, DISCOVERY Collection, Station 50517, 237 mm TL (A) Dentition of upper jaw. (B) Dentition of lower jaw (from Merrett & Saldanha 1985)

FIGURE 8. Ilyophis maclainei sp. nov. (A) Referred Specimen, DISCOVERY Collection, Station 50517. Distribution of type α and ss squamation on left and right side of specimen. (B) Same specimen. Detail of squamation from area of trunk region (scales of type α ventrally). (C) Same specimen. Detail of squamation from anal region (scales of type ss). (D) Holotype, BMNH 1995.4.19.31. Detail of squamation from region of end of caudal (scales of type ss) (modified from Merrett & Saldanha 1985)