Ariosoma tamilicum, Kodeeswaran & Acharya & Mohapatra & Ajith Kumar, 2025

Kodeeswaran, Paramasivam, Acharya, Smrutirekha, Mohapatra, Anil & Ajith Kumar, T. T., 2025, Ariosoma tamilicum (Anguilliformes: Congridae): a new eel species from the Indian waters, Zootaxa (J. Fish Biol.) 5604 (1), pp. 69-76 : 70-72

Ariosoma tamilicum sp. nov.

Tamil short-tail conger urn:lsid:zoobank.org:act:AD969AAB-A3A9-4D2A-A0DA-75EB6B2A00BF

( Figures 1–3 ; Table 1 )

Holotype: NBFGR / CONATAM (244 mm TL, female), Thoothukudi fish landing centre, off Thoothukudi, Bay of Bengal, 12 Sep. 2022, coll. by P. Kodeeswaran.

Paratypes: NBFGR / CONADOL.5–11 (7: 239–308 mm), EBRC/ZSI/F12903 (2: 265–281 mm) from deep-sea trawl by-catch, Kalamukku Fishing Harbour, off Kerala coast, Arabian Sea , 9º 59’ 2.2482” N, 76º 14’ 7.6704’’ E, coll. P. Kodeeswaran 23 & 29 Jan. 2021 [All these specimens designated as paratypes were earlier registered as Ariosoma dolichopterum ].

Diagnosis. A stout eel species of Ariosoma is diagnosed by having: anus located fairly before mid of total length, preanal length 43.8–47.1% of TL; origin of dorsal-fin slightly in front of pectoral-fin base; dorsal surface of head with single whitish band across anterior eye margin. Ventral portion of lower jaw with tiny dark pigmentation patches, forming a dark line along the isthmus; long vomerine teeth patch, reaching half-length of maxillary; intermaxillary teeth continuous with maxillary, concealed when mouth closed; PDV 7–8; PAV 45–49; PCV 59–60; TV120–129; myorhabdoi present.

Description. Body stout, anterior portion cylindrical, and more compressed from sides in caudal portion; caudal fin tip blunt and stiff; anus before middle of total length, preanal length 43.8–47.1% of TL; origin of dorsal-fin slightly before pectoral-fin base, above sixth to eight lateral-line pores; continuous with caudal and anal fins. Anal fin beginning just behind anus. Well-developed pectoral fin, with narrow base and conical distally. Gill openings large, somewhat larger than eye diameter, its dorsal origin extending above dorsal half of pectoral-fin insertion; interbranchial width smaller or equal to gill opening and eye diameter.

Head relatively short, 5.4–6.0 times in TL, deepest at half-length between snout tip and gill opening and tapering anteriorly to tip of snout; snout long, anteriorly blunt or point in dorsal view, its length 1.0–1.4 times eye diameter, moderately projecting over lower jaw; snout length shorter than lower jaw; fleshy part of snout wide, projecting before intermaxillary tooth patch anterior end; rictus located about middle length of eye. Anterior nostril tubular and large, positioned at tip of snout and posterior nostril moderately a larger pore elliptical in shape, located before mid-eye orbit diameter. Jaws with flange. Tongue quite long and narrow; anterior part free from mouth with a nonblunt tip. Lateral line almost complete. Cephalic pores relatively large with some pores small. SO with 6 pores, 1 st (ethmoidal) pore minute at anterior portion of snout tip, 2 nd pore medium ahead of snout tip, 3 rd pore large, on dorsal surface of snout just above and behind anterior nostril; 4 th pore round and moderate-sized, 5 th pore at anterior margin of interorbital, 6 th pore at posterior margin of interorbital. IO canal with 8 (4+4) pores, POM pores 10, ST pores 3.

Teeth small, pointed and conical or blunt. Continuous intermaxillary teeth with 5 to 7 transverse rows, concealed when mouth close. Teeth on maxillary and mandibular continuous in band; anterior portion of maxillary teeth broader with 4 to 6 rows, mid portion with two or three unequal rows, innermost blunt and outermost teeth conical, and two rows posteriorly. Teeth on vomer form a long patch, reaches half the length of maxillary, roughly five or six rows of sharp teeth in anterior portion, followed by two or three rows of blunt teeth and two to four uniserial blunt teeth posteriorly ( Figure. 3 ). Mandibular teeth wider anteriorly and narrower posteriorly.

Swim bladder reaches anal opening. Stomach short, reaches ½ of distance from gill-opening to anus.

Colour (in fresh specimens): Body almost bicolour, dorsally darker and ventrally paler. Median fins darker with thin dark margin; whitish on caudal fin, upper & lower margins and caudal tip thick black. Head darker than body, dorsal surface of head with single whitish band across anterior eye margin ( Figures. 2A & B ). The ventral part of lower jaw with tiny dark pigmentation patches forming a dark line along isthmus ( Figures. 2C ). Eyes bright with dark pupil; pectoral fin almost dark with minute pigmentation at the base. Colour in formalin darker than fresh; dark above the lateral-line pores and beige below, pectoral fin translucent.

Size. The largest specimen examined is 308 mm TL.

Distribution. Indian Ocean: Arabian Sea and Bay of Bengal, and seems to be widespread in the shallow waters of entire Indian coast.

Etymology. The species is named “ tamilicum ” with reference to the language “தமிழ்”—“Tamil”, one of the oldest languages of the world. Since the genus Ariosoma is a neuter genus, so the specific name is written tamilicum .

Comparisons. Ariosoma tamilicum differs from all the congeners except Ariosoma emmae from Taiwan waters and Ariosoma megalops Fowler, 1938 from China, Vietnam, Philippines, Taiwan waters by having similar or overlapping total vertebrae and predorsal, preanal and total pore counts. Ariosoma tamilicum differs from these congeners by having: preanal length 48.4 (43.8–47.1% TL) vs. 47.9 (46.9–49.6% TL) in A. emmae , 44.4–49.1% TL in A. megalops , Taiwan and 45.1–48.1% TL, Vietnam; trunk length 29.2 (26.5–30.6% TL) vs. 31.0 (28.9–32.7% TL) in A. emmae , 27.7–33.2% TL in A. megalops ; larger gill opening width 14.2 (13.5–21.9% HL) vs. 12.0 (10.4–12.8% HL) in A. emmae , 8.2–15.9% HL in A. megalops , Taiwan); smaller eye 18.9 (15.2–20.3% HL) vs. 20.1–25.3% HL in A. megalops , Taiwan); in addition, the new species has a short, blunt snout, while A. emmae has a long, pointed snout ( Smith et al., 2018). The new species has similar morphometrics and meristic characters to species Ariosoma dolichopterum but is easily distinguished by its shorter pectoral fin 31.0 (30.7–37.7% HL) vs. 48.0 (39.4–46.7) % HL in types from Vietnam, and 40.0–56.7% HL in specimens from Taiwan; swim bladder reaches anal opening (vs. far behind anal opening) ( Karmovskaya, 2015; Smith et al., 2018).

The new species differs from the Indian water congener, Ariosoma kannani Kodeeswaran et al. (2024a) by having more total vertebrae 120–129 vs. 116–118 in A. kannani and differs from other congeners mentioned in comparative materials section by having fewer total vertebrae 120–129 vs. 136–164 in others.

Molecular analyses. The new species exhibits 13.4% pairwise K2P genetic divergence with its morphologically close relatives A. emmae ( MG702269 ) and 15.6% A. dolichopterum (PP991378 & TOU-AE7833). Based on the molecular tree ( Figure 4 ), the sequences of Ariosoma tamilicum (PQ282605-PQ282613) forms sister clade to sequences of A. dolichopterum , Ariosoma anago , Ariosoma shiroanago , Ariosoma kapala , and Ariosoma emmae . Further, the new species exhibits 11.2% with A. shiroanago , 11.8% with A. anago , 12.5% with A. kapala , 14.6% with Ariosoma bowersi , 16.9% with Ariosoma scheelei . The new species exhibits highest genetic divergence with the sequences of A. melanospilos , A. albimaculatum , A. maurostigma , A. indicum , and A. thoothukudiense recently described from Indian waters.

Hitherto, 11 species have been recorded in Indian waters ( Kodeeswaran et al. 2021, 2022a, 2022b, 2023, 2024a, 2024b; Ray et al. 2022; Roy et al. 2021; Talwar & Mukherjee, 1977) including the species described in the present study. The new species, Ariosoma tamilicum appears to be most common species of Ariosoma in the entire coast of Indian waters. It is possible that earlier existence of Ariosoma anago in the above-mentioned Indian sources may be a misidentification of the new species described in the present study.

Figures

FIGURE 1. Ariosoma tamilicum sp. nov. holotype (244 mm TL), collected from Thoothukudi fishing harbour, Bay of Bengal. Scale bar: 20 mm.

FIGURE 2. Lateral (A); Dorsal (B); Ventral (C) view of anterior portion of head, Ariosoma tamilicum sp. nov., holotype (244 mm TL), Scale bar: 10 mm.

FIGURE 3. Dentition of upper jaws of Ariosoma tamilicum sp. nov. collected from Indian waters. (A) holotype; (B-D) paratypes.

FIGURE 4. Maximum Likelihood phylogeny tree of the genus Ariosoma from analysis of cytochrome c oxidase subunit 1 gene, including new species, Ariosoma tamilicum from the south Indian coast based on the IQ-Tree.