Branchenchelys megacephala, Tighe & Kodeeswaran, 2025

Tighe, Kenneth A. & Kodeeswaran, Paramasivam, 2025, A new genus and species of ilyophine eel (Anguilliformes: Synaphobranchidae: Ilyophinae) from the Arabian Sea, western Indian Ocean, Zootaxa 5722 (4), pp. 555-569 : 556-568

Branchenchelys megacephala sp. nov.

Figures 1–14 , Table 1

Holotype: BMNH 2016.8 .25.788, 195 mm TL, collected off Oman, Arabian Sea , western Indian Ocean, 19° 12’ N, 58° 22’ E, depth 700–800 m, R. R. S. Discovery Cruise 211, Station 12714#01, Agassiz Trawl, 28 Oct 1994.

Paratypes: BMNH 2016.8 .25.789, 148 mm TL, same data as holotype ; IO/OV/ ANG/00024 , 168 mm TL, collected from Mangalore Fishing Harbour, off Karnataka, Arabian Sea, western Indian Ocean , Sanjay Kumar , 10 Jan 2024 . IO/SS/ ANG/00025 , 2 specimens, 155–166 mm TL, collected off Karnataka, Arabian Sea, western Indian Ocean, 13°55’ N, 72°38’ E, depth 366 m, Sagar Sampada Cruise 404, Epibenthic Dredge , 26 Mar 2024 .

Diagnosis. As for the genus.

Description. Table 1 gives a summary of meristics and morphometrics for the type series. Total vertebrae 138 (134–145), predorsal vertebrae 23 (21–23), preanal vertebrae 27 (27–28), precaudal vertebrae 56 (56). Dorsal-fin rays 231 (282), anal-fin rays 206 (238), anal origin at dorsal-ray 10 (15). Proportions as percent of TL: predorsal length 25.6 (24.3–26.4), preanal length 27.7 (26.5–28.8), trunk length 8.8 (6.6–10.5), tail length 72.3 (71.2–73.5), depth (at gill opening) 5.5 (2.3–4.9), depth (at anal origin) 5.6 (4.2–6.2), head length (HL) 18.9 (17.2–19.9). Proportions as percent of HL: eye diameter 4.3 (4.0–5.6), interorbital width 10.8 (8.8–11.0), snout length 13.0 (12.6–14.1), upper jaw length 29.3 (28.4–31.8), lower jaw length 27.4 (27.7–30.6), branchial aperture 19.0 (19.6–24.5).

Coloration. Fresh colors. Body brownish, with numerous melanophores; abdomen with almost blue tinge, reaches almost half body length ( Fig. 2 ); head dark pinkish to brown with numerous melanophores in the brachial region. Anus black. Anterior dorsal and anal fin pale, posterior dorsal and anal fin black, caudal fin black. In preservation ( Figs. 1–2 ). Body and head turned dark brown, blue tinged abdomen portion turns pale; posterior dorsal and anal fin black, caudal fin remains black.

Cephalic lateralis pores ( Fig. 3B ): Head pores minute. Supraorbital pores 3, first pore at ventral end of snout, second pore at snout tip before anterior nostril, third pore slightly larger than both, positioned above and before anterior nostril. Infraorbital pores 4, first pore immediately at base of anterior nostril, second pore between anterior nostril and posterior nostril, third pore behind posterior nostril and under anterior margin of eye, last pore below posterior margin of eye.

Postorbital and supratemporal pores absent. Lateral line 4–10-pores, restricted to the branchial region of the head. Single line of sensory papillae posterior to head, but without open pores.

Body moderately elongate, very compressed and tapering posterior to anus ( Figs. 1–2 ).

Head large (ca. 1/5 of total length) and relatively deep ( Fig. 3 ). Anterior nostril small and tubular, lateral on snout. Posterior nostril slightly larger, oval, just in front of eye. Gill openings large (ca 1/5 of head length), with anterior ends close together ventrally, diverging posteriorly and dorsally ( Fig. 4A ); gill filaments hypertrophied ( Fig. 4B ).

Dentition ( Figs. 5–6 ). There are two transversely oriented compound intermaxillary teeth. Four relatively large, compound vomerine teeth set in epithelial pads, first approximately the same size as intermaxillary teeth, second larger, third largest, and fourth about same size as first. The holotype ( Fig. 5A ) has 2 small teeth (possibly replacement teeth as they are not fused into a socket) in place of the fourth, while the paratype ( Fig. 5B ) has a small replacement tooth posterior to the second tooth. The maxillary teeth are conical and slightly recurved, set in a single irregular row of 38–40. There are a few isolated, single teeth set outside of the main row, resulting in a total number of maxillary teeth of 40–45. The dentary teeth ( Fig. 6 ) are set in a single row, teeth decreasing in size gradually from anterior to posterior. The holotype ( Fig. 6A ) has 33–37 teeth with the anterior 10–12 clearly compound with lateral grooves and figure eight shaped pulp cavities ( Fig. 6C–D ). The rest of the dentary teeth have round to oval pulp cavities but may be capable of changing into compound teeth, as several of the posterior teeth have small lateral grooves. Also, the paratype (BMNH 2016.8.25.789) is smaller and has fewer anterior compound teeth (ca. 6–7) than the holotype ( Fig. 6B ). Apparently, some of the teeth can change into compound teeth with growth.

Osteology ( Figs. 7–14). The osteology discussed below is based on the two specimens that were microCT scanned. Details of the osteology of Branchenchelys megacephala are described individually in the sections below, and differences between B. megacephala and the other species will be discussed in the individual sections below.

Fig. 7 shows the holotype with a radiograph of the entire specimen. Fig. 8 shows a microCT scan of the osteology of the head of the holotype in dorsal, lateral, and ventral views. As can be seen in these radiographs, the skull is relatively short, being ca. 1/3 the length of the head. The gill arches extend from beneath the posterior portion of the skull to a point just over 2/3 of the length of the head. Most of the head consists of the branchial chamber supported by the branchiostegals.

Cephalic lateralis ( Fig. 9 ). The figure shows the cephalic lateralis and lateral line ossicles of the holotype as well as a close-up of the cephalic ossicles of a paratype (BMNH 2016.8.25.789). There are 12 cephalic lateralis ossicles on the dorsal surface of the head, around the eye, and along the upper jaw. The anteriormost on the dorsal surface is the nasal ossicle, which supports the dorsal portion of the nasal chamber. There is no rostral ossicle, and the supraoccipital lateralis pores are anterior to the nasal ossicle. There are two very small ossicles along the portion of the supraorbital canal above the eye and two ossicles on the postocular portion of the infraorbital canal, but there are no pores associated with them. There are six ossicles associated with the infraorbital canal. The relationship of the infraorbital pores with the ossicles could not be determined due to their very small size.

Finally, there is a relatively large, hour-glass shaped ossicle within the orbit and anterior to the eye. This was reported by Robins and Robins (1970) in Attractodenchelys phrix , Dysommina rugosa and Dysomma anguillare although this element was cartilaginous in the latter two species. They considered this element to be possibly a prefrontal or lateral ethmoid. Gosline (1952) referred to this as an “antorbital strut” in Echelus myrus and considered it to be supportive for the maxilla. The senior author has seen this ossicle in microCT scans of several other species including two species of Ilyophis . The homology of this preorbital ossicle cannot be determined at the present time as its development and presence in other species of anguilliform fishes needs further study. There are 7–9 small lateral line ossicles in the dorsal branchial region posterior to the skull. These support the anterior portion of the lateralis nerve, where there are 4–5 small open pores in this region of the head.

Neurocranium ( Fig. 10 ). The skull of Branchenchelys megacephala is lightly ossified and shows a generalized anguilliform pattern with the fused ethmovomer. The ethmovomer comprises the anterior 1/3 of the skull. There is a single frontal that covers nearly half of the dorsal surface of the skull. The remainder of the dorsal surface of the skull is composed of paired parietals, pterotics, epioccipitals, and sphenotics. In the holotype, there is no ossified supraoccipital visible although there is an open space that may represent a cartilaginous supraoccipital since cartilage does not show up in a standard microCT scan. The paratype does have a single, median supraoccipital in the same area. The ventral portion of the skull is composed of the fused ethmovomer, parasphenoid, prootics, exoccipitals, and basioccipital. In addition, the lateral portions of the pterosphenoids, pterotics, and sphenotics are visible in the ventral view. Despite the relatively small eye (4.0–5.6% HL), the orbit is relatively large and deep ( Fig. 10B ). The dorsal portion of the orbit is formed primarily by the frontal. The ethmovomer forms the anterior margin of the orbit, while the ventral and posterior margins are formed primarily by the parasphenoid and ventral processes of the frontal.

A unique character in the osteology of the neurocranium is found in the optic foramen at the back of the orbit. The optic foramen is relatively small and rimmed entirely by the frontal. In the paratype ( Fig. 11 C–D ), the ventral processes of the frontal extend down and around the foramen, and meet on the ventral midline, totally excluding the parasphenoid. In the holotype, these ventral processes form a complete ring around the foramen ( Fig. 11A–B ) and are fused on the ventral midline. In all other eels examined by the senior author, the rim of the orbital foramen is formed from the frontal along with the basisphenoid (if present) and the parasphenoid.

Suspensorium ( Fig. 12 ). The left suspensorium with lower jaw and opercular series is shown in the figure. The hyomandibula is inclined posteriorly ca. 45 degrees. It articulates with the neurocranium anteriorly with a hemispherical “ball and socket” joint underneath the sphenotic and posteriorly in a groove along the ventral surface of the pterotic. The hyomandibula and quadrate articulate in a complex interdigitated suture, with a long triangular process of the hyomandibula extending nearly to the articulation of the quadrate with the anguloarticulo-retroarticular. There is no ectopterygoid. The opercular series is complete with all four elements present, although greatly reduced in size and in ossification, especially along the margins. The opercle articulates with the hyomandibula through a bottle-neck articular condyle. The subopercle has an anterior arm that does not appear to articulate with any other bone of the opercular series. It is a very small bone lying just under the ventral margin of the opercle. The preopercle is an elongate triangular bone extending along the posterior margin of the hyomandibula from just below the articulation of the opercle with the hyomandibular down to just above the articulation between the hyomandibular and the quadrate. The interopercle is a triangular-shaped bone that is medial to the rest of the opercular series, extending from behind the preopercle to the lower margin of the opercle. Given the reduced size and ossification of the entire opercle series, it is unlikely that it provides much support for the branchial chamber.

Hyoid arch ( Fig. 13 ). Basihyal and hypohyals are not present as ossified elements and no trace can be seen on the scans. The elements of the hyoid arch include anterior ceratohyals, posterior ceratohyals, and branchiostegal rays. The anterior ceratohyals are cylindrical tubes with a narrow splint of bone extending onto the dorsal surface of the posterior ceratohyals. The posterior ceratohyals are also cylindrical tubes but become laterally flattened and curve dorsally posteriorly. There are 23–25 branchiostegal rays; three to four originate on the anterior ceratohyal, three on the interspace between the anterior and posterior ceratohyals, and the rest on the posterior ceratohyal. An unusual aspect of the branchiostegals is the attachment of the posteriormost rays to the posterior ceratohyals. The last rays ( 5–7 in the holotype, 9 on each side of the paratype) have a single attachment point to the ceratohyal and then split distally into individual rays. There is no pectoral fin, but the remnants of the pectoral girdle include the cleithrum and supracleithrum which are found embedded within the branchiostegals near the posterior of the branchial chamber.

Branchial arches ( Fig. 14 ). Basibranchials are not present as ossified elements and no trace can be seen on the scans. Ossified elements of the gill arches are the following: two pairs of hypobranchials, five pairs of ceratobranchials, four pairs of epibranchials, one pair of pharyngobranchials, and two pairs of pharyngobranchial toothplates. Both lower and upper pharyngobranchial toothplates are composed of two plates. The anterior lower pharyngobranchial toothplate is an elongate, slightly pointed oval and has a small number of small, conical teeth arranged in two irregular rows. The posterior upper pharyngobranchial toothplate is slightly triangular and has three rows of small, conical teeth. The anterior upper pharyngobranchial toothplate is an elongate triangular element with a small number of small, conical teeth at the posterior end. The posterior lower pharyngobranchial toothplate is a longer, irregularly triangular element covered with small, conical teeth arranged in five irregular rows.

Distribution. ( Fig. 15 ). Western Indian Ocean: Arabian Sea; collected from two locations: off Oman and off Karnataka, southwest Indian coast. Given the large geographic range (ca. 1700 km) of the two localities, the species is probably more widely distributed within the Arabian Sea. However, since both localities are on the outer slopes of the continental shelf, the species may not be distributed within the deeper abyssal regions of the Arabian Sea.

Etymology. The species name megacephala is derived from Greek μέγας meaning “big” and κεφάλΙ meaning “’head” in reference to the large head of this species.

Remarks. The enlarged branchial chamber and greatly increased gill surface from the hypertrophied gill filaments are an adaptation to increase oxygen absorption in the oxygen-deficient waters of the Arabian Sea ( Banse et al., 2014). Gibbs and Hurwitz (1967) noted a large difference in the development of the gills between Chauliodus pammelas and C. sloani in the Indian Ocean. C. pammelas , which is distributed in the northern Arabian Sea, has much longer gill filaments with more numerous gill lamellae than C. sloani . Two other species of eels from the northern Indian Ocean also apparently have adaptations for the oxygen-deficient waters of the Arabian Sea and Bay of Bengal. Sauromuraenesox vorax Alcock, 1889 ( Anguilliformes : Muraenesocidae ) was described by Talwar (1977) as having a very large head (20.7–24.0% of total length) and relatively large gill openings (4.2–4.8% of total length). Dysomma bucephalus Alcock, 1889 , another species in the subfamily Ilyophinae , was described by Karmovskaya (2023). She reported that this species also has a large head (23.9–24.8 of total length) and ”gill openings greatly enlarged” (10.2–10.4 of head length, ca, 2.5 % of total length). Specimens of these two species were examined, and both species showed hypertrophied gill filaments, but not to the same degree as Branchenchelys .

Comparative material examined. Dysomma bucephalus : USNM 438265 ( 1, 191 mm TL); Indian Ocean , Bay of Bengal, off Myanmar, 16° 25’ 53” N, 93° 56’ 55” E, 277 m. USNM 438289 (2, 205– 220 mm TL); Indian Ocean , Bay of Bengal, off Myanmar, 16° 47’ 13” N, 94° 02’ 09” E, 158 m. Sauromuraenesox vorax : USNM 410638 (2, 326– 415 mm TL); Indian Ocean, Arabian Sea, S of Oman, 16° 23’ 19” N, 54° 34’ 52” E . USNM 438239 (2, 370– 450 mm TL); Indian Ocean , Bay of Bengal, off Myanmar, 18° 14’ 15” N, 93° 37’ 52” E, 466 m. USNM 438255 ( 1, 470 mm TL); Indian Ocean , Bay of Bengal, off Myanmar, 16° 25’ 53” N, 93° 56’ 55” E, 277 m.

Figures

FIGURE 1. A: Holotype of Branchenchelys megacephala: BMNH 2016.8.25.788, 195 mm TL. B: Paratype of Branchenchelys megacephala: BMNH 2016.8.25.789, 148 mm TL.

FIGURE 2. Paratype of Branchenchelys megacephala IO/OV/ANG/00024, 168 mm TL. (Photo credit Ravinesh R., CMLRE).

FIGURE 3. A. Head of BMNH 2016.8.25.788. B. Drawing of head of BMNH 2016.8.25.788 showing nostrils and cephalic lateralis pores.

FIGURE 4. A. Ventral surface of anterior of BMNH 2016.8.25.788 showing gill openings. B. Gill chamber of BMNH 2016.8.25.789 showing hypertrophied gill filaments.

FIGURE 5. MicroCT scan of the skull of Branchenchelys megacephala, ventral view showing the vomerine teeth and maxillary teeth. A. BMNH 2016.8.25.788. B. BMNH 2016.8.25.789.

FIGURE 6. MicroCT scan of the mandible of Branchenchelys megacephala, ventral view showing mandibular teeth.A. BMNH 2016.8.25.788. B. BMNH 2016.8.25.789. C. Cross-section of anterior teeth of left mandible, BMNH 2016.8.25.788. D. Crosssection of anterior teeth of right mandible, BMNH 2016.8.25.788.

FIGURE 8. MicroCT scan of the anterior portion of holotype of Branchenchelys megacephala: BMNH 2016.8.25.788. A. Dorsal view. B. Lateral view. C. Ventral view.

FIGURE 9. MicroCT scan of the anterior portion of Branchenchelys megacephala: A. Holotype BMNH 2016.8.25.788 showing cephalic and lateral line ossicles (white blotch-like areas). B. Paratype BMNH 2016.8.25.789, close-up showing morphology of cephalic ossicles.

FIGURE 10. MicroCT scan of the neurocranium of Branchenchelys megacephala. A–C: holotype BMNH 2016.8.25.788. A. Dorsal view. B. Lateral view. C. Ventral view. D. paratype BMNH 2016.8.25.789, dorsal view,.Abbreviations: BO, basioccipital; BS, basisphenoid; EP, epiotic; EV. Ethmovomerine; EX, exoccipital; FR, frontal; PA, parietal; PR, prootic; PS, parasphenoid; PT, pterotic; SO; supraoccipital; SP, sphenotic

FIGURE 11. MicroCT scan of the optic foramen of Branchenchelys megacephala: A–B: holotype BMNH 2016.8.25.788. A. External view. B. Internal view. C–D: paratype BMNH 2016.8.25.789. C. External view. D. Internal view.

FIGURE 12. MicroCT scan of the suspensorium, opercular series and lower jaw of holotype of Branchenchelys megacephala: BMNH 2016.8.25.788. Lateral view. Abbreviations:AR, angulo-articulo-retroarticular; CM, coronomeckelian ossification; DE, dentary; HY, hyomandibular; IO, interopercle; OP, opercle;PO, preopercle; Q, quadrate; SO, subopercle.

FIGURE 13. MicroCT scan of the hyoid arches of holotype of Branchenchelys megacephala: BMNH 2016.8.25.788.A. Dorsal view. B. Ventral view. C. Lateral view. D. Dorso-lateral view. Abbreviations: ACH, anterior ceratohyal; BH, basihyal; BR, branchiostegal rays; CL, cleithrum; HH, hypohyal; PCH, posterior ceratohyal; SC, supracleithrum.

FIGURE 14. MicroCT scan of the branchial arches of paratype of Branchenchelys megacephala: BMNH 2016.8.25.789. A. Branchial arches in dorsal view, with upper elements of left branchial arch flipped to allow ventral view. B. Lower pharyngeal toothplates. C. Upper right pharyngeal toothplates.Abbreviations: CB, ceratobranchial; EB, epibranchial; HB, hyobranchial; IP, infrapharyngobranchial; LP; lower pharyngeal toothplate; UP, Upper pharyngeal toothplate.

FIGURE 15. Distribution of Branchenchelys megacephala in the Arabian Sea, western Indian Ocean. Star is type locality, and circle is locality.