Introduction
Deep-sea eels of the genus Bathycongrus, found in the tropical and subtropical Indo-Pacific waters, have captivated researchers since Alcock described the first species Congromuraena nasica in the Bay of Bengal (Castle, Reference Castle1995). Now classified under Bathycongrus, these are small to medium-sized eels, having slender tails and prominent upper jaws with intermaxillary teeth located outside the closed mouth (Smith, Reference Smith and Bohlke1989). They belong to the subfamily Congrinae within the Family Congridae, which comprises an impressive 235 species. Notably, 17 new species have been added to Congrinae in the last decade alone (Fricke et al., Reference Fricke, Eschmeyer and der Laan R2024). However, pinpointing distinct species within Bathycongrus has proven challenging due to a need for clear morphological differences (Castle and Smith, Reference Castle and Smith1999). This taxonomic hurdle continues to be a focus for taxonomists.
Bathycongrus nasicus has a historical distribution spanning the northern Indian Ocean, encompassing the Bay of Bengal and Gulf of Aden (Ben-Tuvia, Reference Ben-Tuvia1993). The species was first described based on specimens collected from the Bay of Bengal (15° 04ʹ 07.0″ N, 80° 25ʹ 07.0″ E) during the Indian Marine Survey Ship ‘Investigator’ expeditions (Alcock, Reference Alcock, Commander and Oldham1893). Subsequent collections from the John Murray Expedition in the Arabian Sea confirmed its presence near the Arabian Peninsula (off Yemen and Oman) and Somalia. Records following these initial discoveries are scarce, with recent samples limited to the eastern Bay of Bengal and Andaman Sea waters off Myanmar (Psomadakis et al., Reference Psomadakis, Thein, Russell and Tun2019).
Recently, we collected a single specimen of B. nasicus from the western Indian waters. Here we describe in detail its morphometric and meristic features and the result of its DNA barcoding.
Materials and methods
A single dead specimen of B. nasicus was obtained during a one-day rapid biodiversity assessment along the Kerala coast, targeting diverse fishery resources. The specimen was collected from the Neendakara fishery harbor in southern Kerala, southwest India. This haul originated from offshore waters of Quilon coast (Wadge Bank, Lakshadweep Sea) at a depth of 100–150 meters.
Morphometric and meristic characters of the specimen were studied using Smith’s methods (Reference Smith and Bohlke1989) (See Table 1) . Photographs of the fresh specimen were taken with a Motorola Edge 30 Fusion mobile camera. Cephalic sensory pores and tooth patterns were studied and photographed in detail under a Nikon SMZ 25 stereo microscope. The vertebral column was studied using microCT images taken at Amrita Vishwa Vidyapeetham, Kochi. Measurements of total, trunk, and tail lengths were recorded using a digital Vernier caliper with an accuracy of 0.1 mm. The head length was measured from the snout tip to the posterior dorsal edge of gill opening. Trunk length was measured from the end of the head to the midpoint of the anus, while the maximum body depth was measured without including the median fins (Smith, Reference Smith and Bohlke1989). The specimen was morphometrically compared with the holotype (Alcock, Reference Alcock, Commander and Oldham1893) as well as the lectotypes and paralectotypes (Castle, Reference Castle1995; Castle and Smith, Reference Castle and Smith1999) and the records from the Andaman Sea (Psomadakis et al., Reference Psomadakis, Thein, Russell and Tun2019). The specimen was deposited in the collections of the Marine Biodiversity Museum of Central Marine Fisheries Research Institute (CMFRI-MBM), Kochi, India (Accession Number: GB. 4.4.5.3).
Morphometric and meristic features of B. nasicus
Molecular identification
For molecular identification, a tissue sample was taken from the postero-lateral region of the specimen and was stored in 99% isopropyl alcohol. The total genomic DNA was isolated from the tissue sample using the DNeasy blood and tissue genomic kit (Qiagen, Germany; Catalogue no 69504) as per the manufacturer’s protocol. The PCR amplification of the Cytochrome c oxidaze subunit 1 (COI) gene fragment of the mitochondrial DNA (mtDNA) was carried out using a pair of universal primers (COI-F-5′-TCAACCAACCACAAAGACATTGGCAC-3′ and COI-R-5′-TAGACTTCTGGGTGGCCAAAGAATCA-3ʹ) (Ward et al., Reference Ward, Zemlak, Innes, Last and Hebert2005). The total PCR reaction volume of 25 μL comprised: 1X EmeraldAmp® GT PCR Master Mix (Takara Bio, Japan), 5 pmoles of each primer, and 20 ng of the extracted DNA. A negative control lacking template DNA ensured the absence of contamination. Thermal cycling conditions in a ProFlex™ cycler (Applied Biosystems, USA) included initial denaturation (95°C, 5 min), followed by 35 cycles of denaturation (95°C, 30 sec), annealing (50°C, 30 sec), and extension (72°C, 45 sec). A final extension (72°C, 7 min) ensured complete amplification. The PCR product was analysed by electrophoresis on a 1.5% agarose gel and visualized by Ethidium Bromide staining under ultraviolet light. The PCR product was then directly sequenced by Sanger Dideoxy terminator sequencing chemistry with a commercial sequencing service provider (Genespec Biosciences, India). The obtained sequences were aligned using the Clustal W algorithm (Thompson et al., Reference Thompson, Higgins and Gibson1994) with the aid of BioEdit sequence alignment editor version (7.2.5) (Hall, Reference Hall1999). The percentage identity was calculated by the Clustal Omega Multiple Sequence Alignment tool provided by EMBL-EBI. The newly generated sequences were aligned with other sequences of the genus Bathycongrus obtained from public domains such as the GenBank and BOLD systems. Phylogenetic and molecular evolutionary analyses were performed using MEGA version 11.0 (Kumar et al., Reference Kumar, Stecher, Li, Knyaz and Tamura2018) with the maximum likelihood method under the best-fit Hasegawa–Kishino–Yano model with a proportion of invariable sites (HKY + I), employing 1,000 bootstrap replicates. The sequences of Thunnus albacares and Thunnus orientalis were used as the outgroup for the phylogenetic analysis.
Results
Description
A medium-sized Bathycongrid species (Figure 1A) with a pronounced upper jaw that projects evidently beyond the lower one (hence the species name). The upper half of the cephalic area is uniformly covered with irregularly shaped, minute black spots; their boundaries on the infraorbital (IO) part are well-marked. A very few prominent, comparatively large, dark black spots are scattered on the cephalic region. It has two large vomerine teeth with a few smaller ones nearby, except in the anterior part. The intermaxillary dentition is very prominent with teeth of varying sizes. Two to five of these teeth are almost the same size as the two large vomerines. The trunk and tail are almost uniform pale brown. The fins are distinctly black towards the caudal region. There are four maxillary pores, one supraorbital (SO) pore, and the remaining are IO. There are 31 pre-anal lateral line pores. The total vertebrae are 150.
(A) Specimen of B. nasicus 312 mm TL, GB. 4.4.5.3. (B) Head of B. nasicus showing characteristic beak-like snout, and inter maxillary teeth projecting anteriorly. (C) Teeth pattern of B. nasicus showing inter maxillaries anteriorly, two large vomerines and the smaller maxillary and mandibular tooth in rows.
Body
Rather slender and frail, rounded in cephalic and trunk regions, while highly compressed towards the caudal part; head and trunk almost equal in depth; anus near anterior third of total length (TL); and tail progressively tapering towards the end, not filiform.
Fins
Dorsal fins origin on a straight line over the pectoral base; it is continuous with caudal and anal fins; pectorals well developed; gill opening larger than eye diameter, its upper end almost joins with the dorsal margin of the pectoral base; and interbranchial width greater than eye diameter and branchial opening. Predorsal vertebrae 8; preanal vertebrae 36; and total vertebrae 150 (Figure 2).
MicroCT image of B. nasicus showing vertebrae.
Head
Rather small, measuring 16.2% of TL and somewhat close (46.9%) to the trunk length (50.94%) with respect to Pre Anal Length (PAL) (Table 2); dorsally flat in the interorbital; broadest at the occipital, immediately anterior to which it is sharply narrowed; snout fleshy, pointed in fresh condition with the tip slightly down-curved giving the appearance of a sturdy beak (hence the common name) (Figure 1B); it projects well beyond the lower jaw; smaller than lower law in length; in lateral view the upper margin of the beak can be seen extending backward as a ridge merging into the occipital mid-anteriorly of the eyes; and rictus in line with the posterior margin of the eye. Anterior nares positioned near the snout tip, tubular; posterior ones elliptical and in front of the eye; and upper lip flange prominent, those on lower lip reduced. Tongue narrow and pointed towards the front.
Morphometric measures of B. nasicus in percentage
Sensory pores
Three pairs of SO pores are present; the first one is very small and located ventrally on the snout tip and immediately anterior to the lip; the second is almost double the size of the first and located just anterior to the front nostrils; and the third is very large and located above the anterior nostril and slightly larger. Three IO pores, of almost the same sizes; the distances between the SO pores and the first two IO pores are equal. The last IO pore is separated by a distance twice that between the preceding ones. It is positioned antero-ventrally to the eye. There are ten preoperculomandibular pores. The first one is small, followed closely by a larger one. The remaining eight pores are separated from the neighbouring ones at an almost equal distance. Only a single, median supra-temporal pore is present.
Teeth
Conical and pointed (Figure 1C); intermaxillary almost in three rows on a raised ridge slightly behind the tip of the snout and outside the closed mouth; the number of teeth increases from the exterior row to the interior ones, with the interior most row possessing prominent incurved teeth almost similar in size to the large central vomerine teeth; the larger fang-like vomerine teeth are placed centrally, surrounded on three sides by ten smaller teeth having a size less than half of the central two; two smaller teeth are present distally in the same horizontal line of the larger vomerines and four each on lateral their lateral sides; and a clear blank dented space, into which the lower jaw tip closes, separates the intermaxillary ridge from the vomerine patch. Maxillary and mandibular teeth are in bands and three to four rough rows; teeth of the outer row of both jaws are much larger than the inner ones; each band on the lower jaw has a pair of larger teeth (in the outer row) separated from the adjacent pair by a clear space; this space is bordered interiorly by a single larger tooth (of the first inner row).
Colouration
The specimen appeared fleshy in colour at the time of collection, with a paler ventral surface. The pectoral fin colour matched the body colour (Figure 1A). The posterior half of the dorsal fin margin, the caudal fin, and the entire anal fin margin were black. Numerous irregularly shaped, minute black spots and very few prominent, comparatively large, dark black spots were present on the head. No distinctive colour variation was observed after preservation in alcohol.
Genetic analysis
The sequence of the B. nasicus was submitted in GenBank under the accession number: PQ149996. The molecular analyses were inferred using the HKY + I model based on partial mitochondrial COI gene sequences (695 bp), revealing an interspecific variation among Bathycongrus species. The specimen forms a well-supported monophyletic clade with a bootstrap value of 96% within the Bathycongrus genus (Figure 3). The sequence similarity, as indicated by the Percent Identity Matrix, shows that the identified species shows the most similarity with, Bathycongrus guttulatus (94.57%), Bathycongrus retrotinctus (93.87%), and Bathycongrus wallacei (93.71–93.56%). The data suggest that the specimen is genetically distinct within the phylogenetic tree, potentially indicating a unique lineage or new species within the genus Bathycongrus. Further research, including more extensive genetic and morphological comparisons, is required to clarify its taxonomic status.
Maximum likelihood tree of COI mtDNA gene partial sequence of B. nasicus based on sequences from Genbank.
Discussion
Because of their few peculiar external characteristics, all Bathycongrus Ogilby, 1898 species are generally difficult to identify. Among the two subgroups identified within Bathycongrus, B. nasicus belongs to the group having two enlarged central vomerine teeth surrounded by smaller ones. In the case of vertebral count, B. retrotinctus is having fewer than 150 vertebrae (vs 150 in B. nasicus) while they share a similar number of preanal pores (29–32) (Castle and Smith, Reference Castle and Smith1999). Furthermore, B. nasicus can be distinguishable in having four prominent teeth (vs absent in B. wallacei and several smaller teeth in B. retrotinctus) on either lateral side of the larger two vomers. Also, in B. nasicus, there are only two conspicuously enlarged teeth on the vomerine patch. The prominent beak-like projection of the snout serves as a quick marker of the species. Furthermore, the tiny melanophores scattered all over the head and the dark margins of the dorsal and anal fins stand as clues to the species. Besides the above-mentioned morphomeristic features, the molecular taxonomy of the B. nasicus showed that it is genetically well distinct from its conspecifics. However, the genetic similarity of B. nasicus is more towards B. guttulatus than B. retrotinctus (Figure 3). But, morphologically, B. guttalatus is more distinct to B. nasicus than B. retrotinctus in having a higher number of pre anal pores (36–38) and total vertebrae (>157) (Castle and Smith, Reference Castle and Smith1999). The current specimen shares a similar number of preanal pores (31) and total vertebrae count (150) compared to paralectotypes (29–32 preanal pores and 106–152 vertebrae) (Castle, Reference Castle1995).
New species of Bathycongrid eels are being frequently described from the east Asian region such as Taiwan (Huang et al, Reference Huang, Ho, Chang, Smith and Chen2018, Reference Huang, Ho, Chen and Chan2022; Smith and Ho, Reference Smith and Ho2018). Also, the recent discoveries of several new congrid and ophichthid eels from the southern part of peninsular Indian waters point to a richer Augilliformes diversity of the Indo-Paific region. Such studies are largely throwing light into the continuous distribution of many marine species in the entire Indo-West Pacific as well as the specification across groups in specific provinces within the region.
Acknowledgements
We are thankful to Dr. Grinson George, Director, ICAR-CMFRI, Dr. A. Gopalakrishnan, former Director, ICAR-CMFRI for the support given to the work. We also thank Siddarammana G. Gowd, Amrita Centre for Nanosciences and Molecular Medicine, Amrita Vishwa Vidyapeetham, India for the microCT images of the specimen.
Author contributions
AKR: Conceptualization, Methodology, Investig- ation, Photography, Species identification, and Writing – Original draft preparation. RKR: Investigation, Supervision, Species confirmation, and Writing – review and editing. DV: Molecular study and Writing – Original draft preparation. AA: Investigation, Writing – Original draft preparation. RJN and SKS: Species confirmation and Writing – review and editing.
Funding
This research received no specific grant from any funding agency, commercial or not-for-profit sectors.
Competing interests
The authors have no conflicts of interest to declare relevant to this article’s content.
Ethical Standards
Not applicable.
Data availability
All data underlying the results are available as part of the manuscript. Additional data can be shared on request.
