Gnathophis in Taiwan: Three new species described

A taxonomic and genetic study has identified three previously undescribed deep-water conger eels in waters under the jurisdiction of Taiwan, increasing the number of confirmed Taiwanese members of the genus Gnathophis from two to five. The new species—Gnathophis kbalanensis, Gnathophis melanurum and Gnathophis nanhaiensis—were distinguished using body proportions, vertebral counts, sensory-pore patterns, coloration and mitochondrial DNA sequences. Most specimens were collected by bottom trawl off Taiwan, while G. nanhaiensis is known from the South China Sea at depths of 509–540 metres.

The investigation also reassessed an earlier Taiwanese record of the Japanese species G. xenica and found that the specimens had been misidentified. Gnathophis is a genus of medium-sized eels in the family Congridae and occurs in tropical and subtropical deep waters around the Indo-Pacific region. Its members have moderately blunt, slightly stiffened tail tips, short caudal fins and projecting snouts with a longitudinal fleshy keel underneath.

The upper jaw extends beyond the lower jaw, and the fleshy part of the snout projects anteriorly beyond the intermaxillary teeth. Species can be divided into two broad groups according to whether the lateral-line pores above the pectoral fin are elevated. G. kbalanensis and G. melanurum have elevated pores, whereas G. nanhaiensis has non-elevated pores, a condition also shared with the previously recorded Taiwanese species G. asanoi.

The researchers combined detailed examination of preserved specimens with DNA barcoding. Measurements and counts followed established procedures for conger taxonomy and covered total length, head length, the predorsal, preanal, trunk and tail regions, jaw dimensions and eye diameter. Vertebral counts were obtained from digital radiographs: predorsal vertebrae were counted to a vertical through the first dorsal-fin ray, preanal vertebrae to a vertical through the first anal-fin ray, and precaudal vertebrae to the vertebra immediately before the first centrum bearing a complete haemal spine.

Sensory pores were counted under a light microscope. For molecular work, a fragment of the mitochondrial cytochrome c oxidase subunit I gene, or COI, was amplified by polymerase chain reaction, sequenced and aligned with 20 additional Gnathophis sequences obtained from GenBank. A maximum-likelihood tree was reconstructed under the HKY+G nucleotide substitution model with support assessed through 1,000 bootstrap replicates, while Kimura two-parameter distances quantified pairwise genetic differences.

Gnathophis kbalanensis was named for Kbalan, an ancient name for the Yilan region. Its type series came from bottom trawls off Daxi in northeastern Taiwan at approximately 200–300 metres. The 472-millimetre holotype was a ripe female with free eggs, and three paratypes measured 363–443 millimetres; three of the four type specimens were ripe females with well-developed ovaries and loosely attached eggs, indicating that they were approaching spawning rather than confirming a completed reproductive event.

The species has elevated lateral-line pores above the pectoral fin, three preopercular pores, 42–43 precaudal vertebrae and 134–137 vertebrae in total. Head length is 16.4–17.6% of total length, trunk length is 21.0–23.1%, and preanal length is 38.3–39.7%. Fresh animals have a brown upper body, a pale yellowish-green snout tip, a black peritoneum and black dorsal, anal and caudal fins.

Gnathophis melanurum, the second new species, takes its name from the Greek words for “black” and “tail,” referring to the conspicuously black caudal fin of fresh specimens. The type material consists of a 300-millimetre holotype and 46 paratypes measuring 162–353 millimetres. Most were collected off northeastern Taiwan, with additional specimens from southwestern waters, generally at about 300 metres.

Its most immediately visible distinction from G. heterognathos is the coloration of the vertical fins: G. melanurum has pale dorsal and anal fins but a black caudal fin, whereas G. heterognathos has pale vertical fins edged in black. It also possesses a black peritoneum, three preopercular pores, 29–32 preanal lateral-line pores, 37–41 precaudal vertebrae and 120–125 vertebrae in total. The holotype and additional type specimens were ripe females with developed ovaries and loose eggs, a condition interpreted by the authors as indicating imminent spawning.

Gnathophis nanhaiensis was named from Nan Hai, the Chinese name for the South China Sea, where its holotype was collected by bottom trawl at 509 metres. Its paratype was taken at 540 metres, although the fishing method and more precise collection information were unavailable. These are the only two specimens currently known of the species, measuring 245 and 275 millimetres.

Like G. asanoi, G. nanhaiensis has lateral-line pores above the pectoral fin that are not elevated, as well as pale vertical fins and a pale peritoneum. It differs from G. asanoi in having a relatively larger head, 16.9–17.9% of total length, a longer predorsal region of 19.7–20.3%, a longer preanal region of 42.0–43.6% and a longer trunk of 24.1–26.7%. It has 40–42 preanal vertebrae, 47–48 precaudal vertebrae and 139–143 vertebrae in total, compared with 34–37, 42–44 and 139–141, respectively, in G. asanoi.

Pairwise COI distances between G. kbalanensis and G. melanurum ranged from 4.4% to 4.9%. G. kbalanensis was 4.7–5.7% distant from G. heterognathos and 4.0–4.2% from G. johnsoni, while G. melanurum was 5.1–6.6% from G. heterognathos and 2.0–2.6% from G. johnsoni. The maximum-likelihood analysis placed the sampled taxa within a phylogenetic framework consistent with the diagnoses based on external anatomy and internal skeletal counts.

The authors used both kinds of evidence to distinguish the new species, but the molecular sampling did not include every recognized Gnathophis species. The tree therefore should not be read as a complete reconstruction of the evolutionary relationships across the genus. The study also examined the two Gnathophis species previously reported from Taiwan, G. asanoi and G. heterognathos.

G. asanoi was represented by three Taiwanese specimens measuring 290–357 millimetres, collected off southwestern Taiwan; the species was originally described from the Sibuyan Sea in the Philippines at depths of about 200–400 metres. G. heterognathos was much better represented by 74 examined specimens from northeastern, western and southwestern Taiwan, ranging from 155 to 483 millimetres. Two were ripe females with developed ovaries and loose eggs.

Taiwanese material overlapped extensively in morphology with Japanese specimens historically called G. nystromi. COI differences between Taiwanese G. heterognathos and the Japanese sequences reached 1.8%, whereas the maximum pairwise difference among the sampled Taiwanese G. heterognathos was 1.4%. The authors retained G. nystromi as a junior synonym of G. heterognathos, while noting that further investigation of geographic variation is required.

All five Gnathophis species now confirmed around Taiwan occupy deep-water habitats, but the available records remain uneven in geographic coverage and specimen number. G. kbalanensis is known only from northeastern Taiwan; G. melanurum has been recorded in both northeastern and southwestern waters; and G. nanhaiensis is represented by only two South China Sea specimens. Much of the new material was obtained through bottom trawling around 200–300 metres, while the deepest records extend to 540 metres.

The study demonstrates how the re-examination of archived fishes can reveal species diversity overlooked by earlier national records. Specimens formerly listed as G. xenica in the Taiwan Fish Database were determined to represent Rhynchoconger ectenurus, Bathycongrus sp., Congriscus maldivensis and one of the newly described Gnathophis species, underscoring the need to check apparent range records against diagnostic anatomy.

Subject of Research: Taxonomic and genetic identification of three new Gnathophis conger eels from waters around Taiwan

Article Title: The congrid eel genus Gnathophis in Taiwan ( Anguilliformes , Congridae ), with descriptions of three new species

Article References: Huang, J.-F., Liao, Y.-C., Chan, T.-Y., Chen, H.-M., & Smith, D. G. (2026). The congrid eel genus Gnathophis in Taiwan (Anguilliformes, Congridae), with descriptions of three new species. ZooKeys, 1287, 11-46. https://doi.org/10.3897/zookeys.1287.183842

Image Credits: AI Generated

DOI: 10.3897/zookeys.1287.183842

Keywords: Gnathophis, conger eels, Taiwan, species discovery, marine taxonomy, deep-water fish, mitochondrial DNA, morphology, vertebral counts, South China Sea, marine biodiversity, congrid