Dysomma opisthoproctus Chen & Mok, 1995

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 : 98-99

Dysomma opisthoproctus Chen & Mok, 1995

Table 3

Dysomma opisthoproctus Chen & Mok, 1995:930 (type locality: Nan-fang-ao, NE Taiwan, 200 m). Chen & Mok, 2001:79.

Diagnosis. Pectoral fin present; dorsal-fin origin in front of level of pectoral-fin base, predorsal length 10.6% TL; anus posterior, far behind tip of pectoral fin, preanal length 35.1% TL; trunk very long, trunk length 20.2% TL; 2 intermaxillary teeth; 4 compound teeth on vomer; multiple rows of small teeth on upper jaw; single row of 7 or 8 large compound teeth on lower jaw; head pores: IO 4, SO 3, M 7, POP 0, AD 1; lateral-line pores: prepectoral 6, preanal 24, lateral line runs to more than 2 head length before caudal fin; preanal vertebrae 35, total vertebrae 120. Body brownish dorsally and paler ventrally with lower part of posterior margin of anal fin and caudal fin black. [Data adapted from Chen & Mok, 1995]

Remarks. The holotype and only known specimen was not available for our study because its whereabouts is unknown. Dysomma opisthoproctus can be separated from all Taiwanese congeners except D. dolichosomatum and D. longirostrum by the posterior position of the anus. It differs from D. dolichosomatum by having a pectoral fin (vs. absent) and 7 or 8 larger compound teeth on lower jaw (vs. 4 or 5 small compound teeth followed by 1 or 2 irregular rows of small blunt teeth on lower jaw) and 120 total vertebrae (vs. 147–156). It differs from D. longirostrum by having 2 intermaxillary teeth (vs. 0), a relatively short snout (22.0% vs. 32.1% HL), 7 or 8 large compound teeth (vs. small multiserial teeth) on lower jaw, 120 (vs. 130) total vertebrae, and lacking POP pores (vs. 2 POP pores). It is also similar to Dysommina rugosa in the posterior position of its anus and a similar external appearance, but differs by having lateral-line pores (vs. absent), a single row of compound teeth (vs. small multiserial teeth) on the lower jaw, 120 (vs. 127–134) total vertebrae, presence of intermaxillary teeth (vs. absent), and well-developed papillae on both jaws (vs. slightly developed).

Tables

TABLE 3. Morphometric and meristic data of Dysomma opisthoproctus, Dysomma polycatodon and Dysommina rugosa. Data of D. opisthoproctus are adapted from Chen & Mok (1995).

	D. opisthoproctus	D. polycatodon	Dysommina rugosa
	Holotype
Total length (mm)	421	344–600 (n=10)	238–413 (n=4)
% TL		Mean (range)	Mean (range)
Head length	12.5	13.6 (12.7–14.3)	12.8 (12.3–14.0)
Predorsal length	10.6	11.5 (10.0–12.9)	15.6 (14.0–17.1)
Preanal length	35.1	16.9 (15.5–18.2)	27.6 (26.8–28.8)
Trunk length	20.2	3.3 (2.4–3.9)	14.9 (12.8–16.4)
Tail length	64.9	83.1 (81.8–84.5)	72.4 (71.2–73.2)
Depth at gill opening	–	5.7 (4.6–6.6)	5.0 (4.7–5.5)
Depth at anus	5.9	5.6 (4.9–6.6)	5.0 (4.0–6.0)
Width at anus	–	3.4 (2.6–4.7)	3.4 (2.2–4.2)
%HL
Snout length	22.0	21.4 (18.8–23.9)	27.5 (25.1–30.9)
Eye diameter	3.2	6.6 (6.1–7.8)	10.0 (8.0–12.9)
Interorbital width	–	15.3 (13.0–18.7)	16.2 (13.4–19.8)
Upper jaw length	36.9	41.3 (37.5 –47.9)	46.5 (42.3–49.5)
Interbranchial width	–	9.5 (6.5–15.6)	13.4 (9.9–18.2)
Vertebrae		n=12	n=4
Predorsal		7–11	10–12
Preanal	35	12–15	26–32
Total	120	128–133 [1 with 120]	133–141
Lateral-line pores		n=12	n=4
Preanal	24	8–11	absent
Total	–	65–78	absent
Head pores		n=12	n=4
Supraorbital	3	3	3
Infraorbital	4	4	4
Adnasal	1	1 (1 with 0)	1
Mandibular	7	6	6
Preopercular	0	0 (1 with 1)	0
Supratemporal	0?	0	0
Frontal	0?	0	0
Vomerine teeth	4	3	4
Intermaxillary teeth	2	2	0