Dysomma taiwanensis, Ho, Hsuan-Ching, Smith, David G. & Tighe, Kenneth A., 2015

Ho, Hsuan-Ching, Smith, David G. & Tighe, Kenneth A., 2015, Review of the arrowtooth eel genera Dysomma and Dysommina in Taiwan, with the description of a new species (Anguilliformes: Synaphobranchidae: Ilyophinae), Zootaxa 4060 (1), pp. 86-104 : 87-91

Dysomma taiwanensis sp. nov.

New English name: Taiwanese arrowtooth eel Figs. 1 A–B, 2A–B; Table 1

Holotype. NMMB-P11115 (496 mm TL), Daxi, Yilan, NE Taiwan, northwestern Pacific, bottom trawl, ca. 200– 400 m, 12 May 2006.

Paratypes. NMMB-P 22194 (443), collected with the holotype. USNM 427172 (2, 192–248), Dong-gang, southwestern Taiwan, market, 16 Nov. 2007.

Diagnosis. Pectoral fin present; dorsal-fin origin slightly in front of level of pectoral-fin base; anus well behind tip of pectoral fin; trunk very short; two intermaxillary teeth; 4 compound teeth on vomer; single row of 7–10 large compound teeth on lower jaw; head pores: IO 4, SO 3, M 6, POP 0, AD 1, F 0, ST 0; lateral-line pores: predorsal 4– 5, prepectoral 5–7, preanal 9–12, total 40–48, the last to about half of total length. Vertebrae: predorsal 9–10, preanal 13–17, total 137–139; MVF 9-15-138. Body uniformly brownish, lower part of posterior one-eighth of body darker, with black base and margin on rear part of anal fin and lower part of caudal fin.

Description. Morphometric data of the holotype (in mm): total length 496; head length 63.5; predorsal length 60.1; preanal length 83; trunk length 16.5; tail length 413; depth at gill opening 23.5; width at gill opening 16.7; depth at anus 26.5; width at anus 17.4; eye diameter 3.9; interorbital width 9.4; snout length 14.1; rictus 27.4; postorbital length 47.2; gill opening 6.6; interbranchial width 9.7.

The following values are given for the holotype, followed by that of paratypes in parentheses. Head relatively short, 12.8 (10.5–12.7)% TL; origin of dorsal fin slightly in front of a vertical through gill opening, predorsal length 12.1 (11.3–11.9)% TL; trunk very short, 3.3 (2.8–3.8)% TL, 26.0 (26.2–34.5)% HL; anus at one pectoral-fin length behind tip of fin; origin of anal fin immediately behind anus, preanal length 16.7 (13.3–16.5)% TL; tail long, tail length 83.3 (83.5–85.4)%TL.

Body moderately slender, head and trunk somewhat cylindrical, becoming gradually compressed to rear end; body width at anus 3.5 (2.2–2.8)% TL; body depth relatively uniform, depth at anus 5.3 (2.5–4.2)% TL, narrowing gradually to caudal fin; depth of gill opening 4.7 (3.4–3.7)% TL. Dorsal and anal fins low and fleshy, continuous with a small caudal fin. Pectoral fin well-developed, at upper corner of gill opening.

Head slender in profile; snout blunt anteriorly and broad dorsally, covered by many short papillae, snout length 22.2 (24.4–26.7)% HL; tip of snout projecting slightly beyond lower jaw; eye small, covered by a thick and semitransparent membrane; eye diameter 6.1 (4.7–7.5)% HL; interorbital space broad, slightly elevated, its width 14.8 (13.9–15.1)% HL; postorbital space very wide. Anterior nostrils tubular, directed anteroventrally. Posterior nostril rounded, below anterior margin of eye, opening directed posteroventrally. Lower jaw shorter than upper, its tip reaching first pore of supraorbital series. Mouth gape far behind eye, upper jaw length 43.1 (41.1–42.2)% HL. Tongue well-attached to mouth floor. Gill opening a narrow slit.

Head and lateral-line pores large ( Fig. 2 A). Supraorbital pores 3, all restricted to anterior portion of snout; infraorbital pores 4, 2 pores between nostrils and 2 below eye; mandibular pores 6, all under mouth gape; preopercular pores 0; adnasal 1; supratemporal commissure 0; frontal 0. Lateral line incomplete, extending to midbody, predorsal 5 (4 or 5), prepectoral 7 (4 or 5), preanal 12 (9–12) and total 45 (right)/48 (left) (40–45).

Teeth ( Fig. 2 B) small and pointed. Intermaxillary teeth 2, side-by-side, followed by 4 large compound vomerine teeth, uniserial, the third one largest. Maxillary with 3 to 4 irregular rows of small teeth, those in inner row slightly larger than the rest. Lower jaw with single row of 7 (7–10) large compound teeth; two smaller paratypes with 1–3 much smaller embedded compound teeth behind these exposed teeth, which can only be seen on the x-ray.

* Values in brackets from a single 312 mm specimen (part of NMMB-P 11121).

Mean vertebral formula 9-13-139; predorsal vertebrae 9–10, preanal vertebrae 13–17 and total vertebrae 137– 139.

Coloration. Fresh color unknown. When preserved ( Figs. 1 A–B), uniformly brownish including all fins; lower part of posterior one-eighth of body darker, with black base and margin on rear part of anal fin and lower part of caudal fin. Peritoneum dark grayish. Mouth cavity light brownish.

Distribution. Known from the type specimens collected from off Taiwan at depths around 200–400 meters. Etymology. The specific name is derived from its type locality, Taiwan.

Comparison. Dysomma taiwanensis sp. nov. is closely similar to D. anguillare in its body proportions and dentition. It differs from D. anguillare in having more total vertebrae (137–139 vs. 119–128, 119– 130 in Robins & Robins,1989) and fewer total lateral-line pores (40–48 vs. 57–75).

The coloration of D. taiwanensis is somewhat different from D. anguillare ; the type specimens of D. taiwanensis have uniformly brown coloration whereas D. anguillare usually has a darker dorsal surface and a paler ventral surface to uniformly black. However, many specimens of D. anguillare collected by midwater trawl (e.g. epipelagic zone less than 100 meters) may have paler body coloration.

Although D. taiwanensis co-occurs with D. anguillare in Taiwan, only four specimens were found among hundreds specimens in the collection, which may suggest either that D. taiwanensis is a rare species and its population is far smaller than that of D. anguillare , or that D. taiwanensis is more abundant at other depths and localities.

It is also similar to D. polycatodon but can be distinguished by the dentition on the lower jaw (single row of large compound teeth in D. taiwanensis vs. single row of two large anterior teeth followed by 22–31 small teeth) and by the total lateral-line pores (40–48 vs. 65–78). Moreover, D. taiwanensis has a uniformly brown body, whereas that of D. polycatodon is pale to light brown, and the posterior margin of the anal fin and lower half of the caudal fin are black in D. taiwanensis , whereas they are white in D. polycatodon .

Dysomma taiwanensis can also be distinguished from D. bucephalus Alcock, 1889 by having single row of 7– 10 large compound teeth on the lower jaw (vs. single row of many small teeth) and 137–139 total vertebrae (vs. 107), and from Dysomma fuscoventralis Karrer & Klausewitz, 1982 by having 137–139 total vertebrae (vs. 119– 124), 40–48 total lateral-line pores (vs. 57–63), and 7–13 teeth on the lower jaw (vs. 16–21).

The new species differs from D. brevirostre ( Facciolà, 1887) , D. muciparus ( Alcock, 1891) , D. dolichosomatum and D. tridens Robins, Böhlke & Robins, 1989 by the presence of well-developed pectoral fins (vs. without pectoral fin) and total vertebrae 137–139 (vs. 146–204); from D. longirostrum , D. goslinei , and D. melanurum by the presence of two intermaxillary teeth (vs. without intermaxillary teeth) and a single row of large compound teeth on the lower jaw (vs. multiple series of small teeth); and from D. opisthoproctus by having the anus below the pectoral fin (vs. far behind the tip of pectoral fin) and 137–139 total vertebrae (vs. 120).

Remarks. It is notable that two large specimens (443–496 mm TL) have only 7 compound mandibular teeth, whereas two small paratypes (192–248 mm TL) have 8–10 larger teeth anteriorly and 1–3 much smaller posterior teeth which can only be seen on the x-ray, not with direct observation of the specimens, at least without damaging the rear of the jaws. This variation may be attributed to change with growth.

Figures

FIGURE 1. Dysomma taiwanensis sp. nov. A. Holotype, NMMB-P 11115, 496 mm TL, the black line denotes the joining of the anterior and posterior halves, which were imaged separately and merged for this figure. B. Paratype, NMMB-P 22194, 443 mm TL.

FIGURE 2. Dysomma taiwanensis sp. nov., from the holotype. A. Semi-diagrammatic illustration of lateral view of head. B. Semidiagrammatic illustration of dentition of upper jaw (left) and lower jaw (right). Arrows indicate anteriormost pores of supraorbital series.

Tables

TABLE 1. Morphometric and meristic data of Dysomma taiwanensis sp. nov., Dysomma anguillare and Dysomma dolichosomatum.

	D. taiwanensis sp. nov.	D. anguillare	D. dolichosomatum
	Holotype Paratypes
Total length (mm)	496 192–248 (n=3)	334–477 (n=25)	166–357 (n=25)
% TL	Mean (range)	Mean (range)	Mean (range)
Head length	12.8 11.4 (10.5–12.7)	13.3 (11.0–15.1)	9.9 (8.3–10.9)
Predorsal length*	12.1 11.6 (11.3–11.9)	11.8 (10.7–13.1)	17.8 (16.5–19.1) [23.4]
Preanal length	16.7 14.9 (13.3–16.5)	17.0 (15.2–18.9)	25.8 (23.3–28.6)
Trunk length	3.3 3.5 (2.8–3.8)	3.7 (2.8–5.5)	16.0 (14.3–17.7)
Tail length	83.3 84.1 (83.5–85.4)	82.8 (81.1–84.1)	74.2 (71.4–76.7)
Depth at gill opening	4.7 3.6 (3.4–3.7)	4.5 (3.1–5.8)	4.2 (2.9–6.5)
Depth at anus	5.3 3.4(2.5–4.2)	4.5 (2.4–6.0)	3.8 (1.1–6.1)
Width at anus	3.5 2.5 (2.2–2.8)	3.1 (1.7–4.6)	2.8 (2.0–5.8)
% HL
Snout length	22.2 25.3 (24.4–26.7)	20.2 (17.8–23.5)	20.1 (17.1–23.9)
Eye diameter	6.1 6.5 (4.7–7.5)	6.5 (5.3–9.4)	8.6 (5.9–14.3)
Interorbital width	14.8 14.5 (13.9–15.1)	15.6 (11.5–20.4)	14.8 (11.0–22.2)
Upper jaw length	43.1 41.7 (41.1–42.2)	35.8 (31.9–39.5)	37.8 (31.0–43.9)
Interbranchial width	15.3 9.0 (7.1–10.9)	8.3 (4.4–12.1)	13.2 (8.8–19.4)
Vertebrae	n=3	n=20	n=23
Predorsal*	9 9–10	7–9	15–20 [25]
Preanal	14 13–17	12–16	23–31
Total	137 137–139	119–128	146–156
Lateral-line pores	n=3	n=16	n=26
Preanal	12 9–12	8–12	Not reaching anus
Total	45;48 40–45	57–75	9–11
Head pores	n=3	n=16	n=26
Supraorbital	3 3	3	3
Infraorbital	4 4	4 or 5	4
Adnasal	1 1	1	1
Mandibular	6 6	6	6 (1 with 7)
Preopercular	0 0	0	1
Supratemporal	0 0	0	0
Frontal	0 0	0	0
Vomerine teeth	4 4	4	3
Intermaxillary teeth	2 2	2	2