Ophichthus naga McCosker & Psomadakis, 2018

Psomadakis, Peter N., 2018, Snake eels of the genus Ophichthus (Anguilliformes: Ophichthidae) from Myanmar (Indian Ocean) with the description of two new species, Zootaxa 4526 (1), pp. 71-83 : 75-78

Ophichthus naga McCosker & Psomadakis sp. nov.

New English name: Deepwater demonic snake eel (En)

( Figs. 4 – 6 )

Material examined. Holotype: USNM 438246 , 573 mm TL, a gravid female, off the Ayeyarwady Delta , Myanmar (14 o 0 3.73 N, 94 o 19.09 E), R/ V Dr. Fridtjof Nansen, Sta. 68, bottom trawl, 455 – 459 m depth, collected by P. N. Psomadakis.

Diagnosis. A moderately elongate species of Ophichthus with: tail 50%, head 11.7%, and body depth at gill opening 0.43% of TL; dorsal-fin origin behind pectoral fin by 2 HL; pectoral fin rounded, not elongate and lanceolate; posterior nostril a hole above upper lip, covered by a flap that extends to edge of mouth; barbels lacking between anterior and posterior nostrils; pores small but conspicuous, SO 1 + 4, IO 5 + 2, POM 6 + 2; upper jaw short, orbit ends above rictus; teeth small, numerous and conical, biserial on upper jaw, uniserial on lower jaw and vomer; coloration dark brown, median fins dark with a pale margin, pectoral fins dark, with a pale ventral margin; vertebral formula 15-65-153.

Counts and measurements of the holotype (in mm). Total length 573; head 67; trunk 218; tail 288; predorsal distance 126; pectoral-fin length 19.3; pectoral-fin base 5.0; body depth ca. 25 at gill openings; body width ca. 22 at gill openings; body depth at anus ca. 26.5; body width at anus ca. 23; body depth at branchial basket 26; snout 14.5; tip of snout to rictus 21.5; snout overhang beyond tip of lower jaw 2.3; eye diameter 6.5; interorbital width 8.9; gill opening height 11.5; isthmus width 17. Vertebral formula 15-65-153.

Description. Body moderately elongate, stout for an Ophichthus , subcircular to level of anus, then becoming more compressed, its depth at gill openings 23 in TL ( Figure 4 ). Branchial basket moderately expanded. Head 3.3 in trunk. Head and trunk 2.0, head 8.5, and tail 2.0 in TL. Snout short, rounded, moderately acute when viewed from above; upper lip not bisected on underside of snout. Lower jaw included, its tip extending slightly beyond bases of tubular anterior nostrils. Upper jaw not elongated, rictus beneath a vertical from posterior margin of eye. Eye enlarged, 3.3 in upper jaw and 10.3 in head. Anterior nostrils tubular, short, extending ventrally from snout, reaching upper lip and barely reaching tip of chin when directed forward. Posterior nostril a hole above upper lip, covered by a flap that extends below the edge of mouth. Barbels absent along upper lip. Dorsal-fin origin well behind pectoral-fin tips by about 2 HL from gill opening. Median fins low but obvious, ending in a shallow groove a little more than 0.8 eye diameters before the bluntly pointed tail tip. Pectoral fins rounded posteriorly, not elongate and lanceolate.

Head pores small but apparent ( Figure 5 ). Single median interorbital and temporal pores. Paired pores identical in number and position on each side. Supraorbital pores 1 + 4, infraorbital pores 5 + 2, mandibular pores 6, preopercular pores 2, supratemporal pores 3. Lateral-line pores apparent; 9 before gill opening in a high arching sequence, the remainder are difficult to accurately count due to the nature of the specimen (see Remarks),the last ca. 0.5 HL from the tail tip.

Teeth ( Figure 6 ) small, conical, numerous and slightly retrorse. Upper jaw teeth biserial, lower jaw and vomerine teeth uniserial. An intermaxillary rosette of 1 central and 2 on each side at tip of snout, followed by 5 irregular pairs of intermaxillary teeth, followed by a single row of ca. 16 small vomerine teeth, decreasing in size posteriorly. Maxillary with about 12 – 13 pairs of subequal irregularly biserial teeth, followed by 10 smaller uniserial teeth. Lower jaw with ca. 13 – 15 uniserial teeth, decreasing in size posteriorly.

Coloration in life ( Figure 4 ) and in ethanol preservative dark brown. Inner margins of lips pale. Median fins dark with a pale margin. Pectoral fins dark, with a pale ventral margin. Tail tip, anal opening, inside of mouth, and peritoneum pale.

Size. Known only from the holotype, a 573 mm TL gravid female.

Etymology. Named naga , a noun in apposition. Buddhist tradition believes that Nāga, a seagoing, serpentine dragon-like being has great powers. They are able to swim through the earth as if it was water, a behavior not unlike that of ophichthid snake eels.

Distribution. Known only from the holotype, collected by bottom trawl at 455 – 457 m depth off Myanmar.

Remarks. The single specimen is intact and in very good condition, however it is twisted and stiff resulting from its preservation, making linear measurements and tooth and pore counts difficult

This new species differs from the majority of its congeners by having its dorsal fin arising nearly two pectoral-fin lengths behind the gill opening, a condition shared by O. bicolor McCosker & Ho (2015) , O. macrorhynchus ( Bleeker, 1852) , O. megalops Asano 1987 , and Ophichthus retrodorsalis Liu, Tang & Zhang, 2010 .

The new species is most similar to Ophichthus bicolor , a deepwater (200-400 m) species captured by bottom trawl and hook-and-line from SW and SE Taiwan. They are very similar in most characters but differ in: the length of the lower jaw (the rictus of O. naga is beneath the posterior margin of the orbit vs. the rictus beneath and behind the rear orbital margin by the diameter of the orbit); in its body coloration O. naga is dark brown vs. the pale gray and white throat and belly of O. bicolor ); and its fin coloration (black vs. pale).

Its other congeners with posterior dorsal-fin origins differ in several characteristics. The new species is similar to O. megalops Asano, 1987 , known from Japan and Taiwan, but differs from megalops in: its preopercular pore condition (2 vs. 3); the slightly more anterior location of its dorsal-fin origin (22 % of TL behind snout tip vs. 24– 26%); and in its body coloration (uniform dark brown vs. light brown with a pale underside, and fins lacking a dark margin). The new species is also similar to Ophichthus retrodorsalis Liu, Tang & Zhang, 2010 , known from the holotype collected from Fuzhou, Fujian Province, China, which is similar in its dorsal-fin origin and its dentition. Hsuan-Ching Ho (McCosker & Ho, 2015: 80) examined and reported upon the holotype of O. retrodorsalis . He “found it to differ from O. bicolor in its coloration (uniformly light brown), pectoral fins (dark), body depth (50 times in TL at gill opening), head length (14.7 times in TL), eye position (posterior margin of orbit above rictus) and in its dentition (triserial throughout)." He was unable to obtain its vertebral counts.

Several fish species captured from station 68 are also dark in color, similar to that of this new eel. We are unable to precisely describe the habitat of these benthic collections, however the station records report them as “coarse silt” ( Krakstad et al. 2016). Included in the trawl were: Benthobatis moresbyi Alcock 1898 (Narcinidae) ; Neoharriotta pinnata Schnakenbeck 1931 (Rhinochimaeridae) ; Bathyclupea hoskynii Alcock, 1891 (Bathyclupeidae) ; Coloconger raniceps Alcock, 1889 (Colocongridae) ; Ateleopus indicus Alcock, 1891 (Ateleopodidae) ; Parascombrops glossodon Schwarzhans & Prokofiev, 2017 (Acropomatidae) ; and Hypopleuron caninum Smith & Radcliffe, 1913 (Ophidiidae) . Other specimens, not identified to species, included: Alepocephalus sp. ( Alepocephalidae ); Photonectes sp. ( Stomiidae ); and Benthodesmus sp. ( Trichiuridae ).

It is also noteworthy that the water masses below 150 m off the Ayeyarwady Delta region are markedly hypoxic with an oxygen level of 0.5 ml/l to 500 m depth ( Krakstad et al. 2016). Some species such as Iago omanensis (Norman, 1939) (Triakidae) have developed special adaptations (such as expanded gill lamellae) to cope with reduced oxygen ( Compagno, 1984). We did not observe such adaptations in the morphology of Ophichthus naga .

Figures

FIGURE 4. Ophichthus naga McCosker & Psomadakis, sp. nov., holotype, USNM 438246, 573 mm TL, female. Photographed soon after capture and before preservation by P. N. Psomadakis. Arrows indicate the origins of the dorsal fin (left) and the anal fin (right).

FIGURE 5. Semi-diagrammatic illustration of left lateral head view of the holotype of Ophichthus naga McCosker & Psomadakis, sp. nov. Drawn by J.E. McCosker.

FIGURE 6. Semi-diagrammatic illustration of the dentition of the holotype of Ophichthus naga McCosker & Psomadakis, sp. nov. Drawn by J.E. McCosker.