Gnathophis arabicus, Kodeeswaran & Karmovskaya, 2025

Kodeeswaran, Paramasivam & Karmovskaya, Emma S., 2025, Three new species of the congrid eel genus Gnathophis (Anguilliformes: Congridae) from the Arabian Sea, Zootaxa 5725 (1), pp. 89-105 : 98-102

Gnathophis arabicus sp. nov.

( Figures 3C , 4C , 7 , 8 , Table 1)

Holotype. IO /OV/ ANG/00056 , 255 mm TL, off Kollam , India, Arabian Sea ( 9° 40’ N; 70° 40’ E), 600 m depth, bottom trawl, P. Kodeeswaran, 16 January 2024.

Paratypes. Five specimens, 190–245 mm TL: IO /OV/ ANG/00059 , 240–245 mm TL , 2 specimens; ZSI/ EBRC / F18781 , 211– 240 mm TL , 2 specimens, collected together with the holotype; ZSI/ EBRC / F 18782, 245 mm TL, 1

specimen, off Kollam , India, Arabian Sea ( 8° 57’ N; 75° 33’ E), 450 m depth, bottom trawl, P. Kodeeswaran, 16 January 2024 .

Diagnosis. A slender species of the genus Gnathophis differs from its congeners in having the following combination of characters: lateral-line pores above pectoral fin not elevated, lateral-line pores before anus 36–41; SO pores 5, SO 5 absent, SO 6 present; total vertebrae 145 (143–146); vertebrae formula: 9–10/39–40/44–46/143– 146; stomach pale and black posteriorly.

Description. Measurements in percentage of TL and HL, and counts data are given in Table 1. Body moderately elongate, anterior portion cylindrical in cross section, posterior part more compressed; tail tip somewhat attenuate; anus just anterior to mid-body length, length 2.7 (2.6–2.8) times in TL. Dorsal-fin origin at level of mid adpressed pectoral fin length, confluent with caudal and anal fins. Anal-fin origin immediately behind anus. Pectoral-fin well-developed with narrow base and pointed distally. Gill opening much smaller than eye diameter, its upper portion reaching upper half of pectoral-fin base. Interbranchial smaller than eye and broader than gill opening.

Head large, in comparison with less slender body, deepest at gill opening; snout elongate and pointed, length 1.2 (1.0–1.4) times eye diameter, protruding significantly beyond lower jaw; lower jaw longer than snout; fleshy part of tip of snout projects forward beyond intermaxillary tooth patch; rictus just before posterior eye margin. Tubular anterior nostril at tip of snout, directed ventro-laterally. Elliptical posterior nostril in front of mid-eye level, with slightly raised rim.

Lateral-line pores complete, evident before anus, and anterior part of lateral-line canal convex dorsally and curves smoothly downwards over the base of the pectoral fin, and pores above pectoral fin not elevated.

Head pores small; SO pores 5, the first (ethmoidal) and the second pores on ventral side tip of snout, the third on dorsal of snout, the fourth slightly large just behind third, the fifth absent (SO 5), the sixth above and at posterior margin of eye. IO pores 7, first 4 along upper lip, the fifth behind rictus; 2 behind eye. Mandibular pores 7, six before rictus, and one after rictus; PO pores 2. ST pores 3.

Teeth small, pointed to blunt and curved backwards. Intermaxillary in about 4 transverse rows with backward-curved teeth, connected to maxillary, anterior portion slightly exposed when mouth closed. Maxillary teeth pointed and in bands, anterior with 3–5 rows of pointed teeth, middle portion with 2 rows, inner teeth blunt and outer teeth pointed, posterior portion narrower with single row. Vomerine teeth forming long elongated patch, in about 4 pointer rows anteriorly, irregular biserial in mid-portion and followed by 8 uniserial blunt teeth posteriorly. Mandibular teeth in bands, anterior portion with 3–4 rows, and narrow posterior portion.

Swimbladder ends before anus and stomach reaching up to and beyond anus.

Colouration. Live colouration. Body pale brownish to grey, belly silvery to pale, pale line run along lateral line, belly milky white. Ventral side of body without melanophore pigmentation. Doral and anal fin milky white with dark base. Caudal fin and tail dark. Pectoral fin translucent with dark tinge and base dark. Snout tip whitish, darker followingly. Supratemporal canal, interorbital and preopercular portion dark. Anterior mandibular pores (first three) rims with melanophore pigmentations. Stomach pale and black posteriorly, peritoneum silvery. Gill-opening white. Eyes bright with dark pupil surrounded by whitish iris. Colouration after preservation. Body becomes paler or beige than fresh. Pectoral fin beige.

Size. The largest specimen examined is 255 mm TL, the holotype.

Distribution. Indian Ocean: off Kerala, Arabian Sea. Known only from type specimens collected at the depths of 450– 600 m.

Etymology. The species name arabicus for the area in which the specimens were collected, the Arabian Sea.

Remarks. Gnathophis arabicus differs from its most of the congeners in having fewer SO and PO pores (5 vs. 6 and 2 vs. 3 in most of species other than mentioned below). This species differs from all the congeners mentioned above but it almost same as Indian water congener Gnathophis anilmohapatrai at first sight but differs in having more total vertebrae (143–146 vs. 133–138), fewer SO and IO pores (5 vs. 6 and 7 vs. 8), interbranchial smaller than eye, 14.6–18.8% HL (vs. interbranchial broader than eye, 17.9–25.5% HL). Gnathophis arabicus has similar characters to G. parini , such as a total number of vertebrae, colour of stomach, and cephalic pores counts but readily differs in having anterior dorsal-fin origin (at mid of adpressed pectoral vs. posterior to pectoral tip), fewer precaudal vertebrae (44–46 vs. 47–49), colouration of stomach (pale and black posteriorly vs. the whole stomach dark), stomach long reaching up to and beyond anus (vs. stomach not reaching anus) ( Karmovskaya 1990). It differs from G. umbrellabius , and G. neocaledoniensis in having fewer SO and PO pores (5 vs. 6 and 2 vs. 3 in species mentioned above). Gnathophis arabicus shares similar cephalic pore counts with G. tritos but readily differs in having more total vertebrae (143–146 vs. 136–138). Further, it differs from G. leptosomatus in having fewer total vertebrae (143–146 vs. 156), longer head and trunk length (14.1–16.0% TL vs. 13.5% TL and 19.7–24.1% TL vs. 15.8% TL), shorter predorsal length (16.5–19.4% TL vs. 22.6% TL) and colouration of stomach (pale and black posteriorly vs. pale).

It differs from species such as G. ajithi , G. andriashevi , G. bathytopos , G. castlei , G. codoniphorus , G. grahami , G. heterognathos , G. heterolinea , G. johnsoni , G. macroporis , G. melanocoelus , G. microps , G. mystax , G. nasutus , and G. smithi by lateral-line pores above pectoral-fin not elevated (vs. lateral-line pores above pectoral-fin elevated in mentioned above species). Further, it differs from G. longicauda , G. habenatus , G. ginanago , G. asanoi , G. cinctus , G. bracheotopos , in having more total vertebrae (143–146 vs. 118–123 in G. longicauda , 122–124 in G. habenatus , 126–134 in G. ginanago , 139–140 in G. asanoi , 130–132 in G. cinctus , 125–130 in G. bracheotopos ) and from G. xenica in having fewer total vertebrae (143–146 vs. 151–157 in G. xenica ) and from G. mystax in having more precaudal vertebrae (44–46 vs. 33–34 in G. mystax ).

Molecular analysis. Based on the molecular analysis using mitochondrial Cytochrome c Oxidase subunit 1 gene, the species Gnathophis ajithi is closely related to G. grahami with 2.5% genetic distance and followed by Pacific Ocean species G. heterognathos with 2.9%, N-West Atlantic species G. bathytopos with 6.5%, G. johnsoni with 6.5%, N-East Atlantic species G. mystax with 7.1%, S-East Indian species G. cinctus with 7.3%, G. melanocoelus and G. umbrellabius with 7.8%, S-East Atlantic species G. capensis with 9.2%. Gnathophis anilmohapatrai is genetically closely related to Gnathophis cinctus having a genetic distance 13.1% followed by Gnathophis johnsoni which has a genetic distance 13.4%, furthermore this species shows a distance of 11.6% with sequences submitted as Gnathophis sp. ( MT323556 & HM389414) and the species Gnathophis arabicus exhibits 11.7% with the sequence (HM389406) followed by 13.3–14.5% with the sequences ( MW306760 , MW306761 , MT323465 , MT323588 , MT323628 , MT323556 ).

The threshold value of intraspecific divergence within G. ajithi was 1%, while the minimum interspecific pairwise distances were 2.5% (17 substitutions) for G. grahami and 2.9% (20 substitutions) for G. heterognathos . These values exceed the intraspecific threshold, supporting the recognition of G. ajithi as a distinct species. Likewise, G. johnsoni showing a well-supported clade with G. cinctus , with a divergence of 1.81% (10 substitutions). Divergences among more distantly related taxa, such as G. bathytopos and G. mystax (0.72%), illustrate the range of interspecific variation within the genus. The divergences for G. ajithi (2.5% to 2.9%) fall within this range and are closer to the values separating G. bathytopos / mystax and G. johnsoni / cinctus . These findings confirm that G. ajithi is genetically distinct and supports its recognition as a separate species, consistent with the molecular thresholds and species-level separations in the genus.

The Median-joining haplotype network ( Figure 9 ) clearly shows the genetic distinctions among Gnathophis species.It reveals well-defined, separate clusters for G. arabicus , G. ajithi , G.grahami , G.capensis , G. heterognathos , G. umbrellabius , G. melanocoelus , and G. longicauda , highlighting substantial species-level differences. However, sequences submitted as Gnathophis sp. formed two distinct cluster, one cluster (Hap_11, Hap_7, & Hap_12) showed variable mutational steps with the species G. anilmohapatrai whereas another (Hap_19, Hap_20, & Hap_21) showed closer lineage with the cluster of G. bathytopos . In contrast, some taxa, such as G. johnsoni and G. cinctus , as well as G. bathytopos and G. mystax , have haplotypes linked by only a few mutational steps. This close genetic connection indicates either recent divergence from a common ancestor, the persistence of ancestral haplotypes, or incomplete lineage sorting. The presence of shared or intermediate haplotypes (e.g., Hap_31) further supports the possibility of ancestral polymorphism. However, G. bathytopos and G. mystax showed overlapping cluster mainly Hap_49, Hap_47 ( Italy), Hap_13, Hap_26 and Hap_25 (Atlantic), that depicts there is a possibility of lineage connectivity. These patterns are typical of recently diverged taxa and can complicate clear taxonomic distinctions.

This analysis confirms that Gnathophis is a genetically diverse genus, with most species displaying unique genetic signatures. However, some species have overlapping or closely related haplogroups that warrant further investigation. Based on both the haplotype network tree and the Maximum likelihood tree ( Figure 10 ), all three new species form separate and distinct clades from their congeners’ sequences available at the public domain. This supports the status of all three species as new.

Figures

FIGURE 3. Lateral view of head, showing sensory pores. A. Gnathophis ajithi; B. Gnathophis anilmohapatrai; Gnathophis arabicus. SO, supraorbital pores; IO, infraorbital pores; M, mandibular pores; PO, preoperculomandibular pores; ST, supratemporal pores.

FIGURE 4. Tooth pattern on upper jaw. A. Gnathophis ajithi; B. Gnathophis anilmohapatrai; C. Gnathophis arabicus.

FIGURE 7. Holotype of Gnathophis arabicus sp. nov., IO/OV/ANG/00056, holotype, 255 mm TL, off Kerala, Arabian Sea, India.Arrow indicates dorsal-fin origin.

FIGURE 8. Head of Gnathophis arabicus sp. nov., IO/OV/ANG/00056, holotype. A. Lateral view; B. Dorsal view; C. ventral view.

FIGURE 9. Median-joining haplotype network of Gnathophis species based on mitochondrial CO1 sequences. Each circle represents a unique haplotype, with circle size proportional to the number of individuals sharing that haplotype. Small black dots indicate missing or unsampled intermediate haplotypes. Lines between haplotypes represent mutational steps (each dash = one nucleotide substitution). Coloured ellipses highlight species-specific clusters. Most species (G. arabicus, G. ajithi, G. grahami, G. heterognathus, G. capensis, G. umbrellabius, G. longicauda, G. melanocoelus) form distinct haplogroups, whereas others (G. johnsoni, G. cinctus, G. bathytopos, G. mystax) show close connections, reflecting recent divergence or incomplete lineage sorting.

FIGURE 10. Maximum likelihood tree of Gnathophis from CO1 gene sequences with 20000 bootstrap replicates following TPM2u+F+G4 model generated by IQ-TREE 2.