Dysomma opisthoproctus

diagnosis

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]

Review of the arrowtooth eel genera Dysomma and Dysommina in Taiwan, with the description of a new species (Anguilliformes: Synaphobranchidae: Ilyophinae)

discussion

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).

Review of the arrowtooth eel genera Dysomma and Dysommina in Taiwan, with the description of a new species (Anguilliformes: Synaphobranchidae: Ilyophinae)

discussion

Remarks. A rare species, to date known only from the holotype.

Annotated checklist of eels (orders Anguilliformes and Saccopharyngiformes) from Taiwan

Dysomma opisthoproctus Chen & Mok 1995

Dysomma opisthoproctus Chen & Mok, 1995 Dysomma opisthoproctus Chen & Mok 1995: 927, Fig. 1. Holotype: NSYSU 2701. Type locality: Taiwan, Nanfang-ao , 24° 51’ 24” N, 121° 58’ 30” E, 200 m.

Redescription of the eel Ilyophis arx Robins, 1976 (Anguilliformes: Synaphobranchidae: Ilyophinae) with a description of a new species of Ilyophis, and a taxonomic synopsis of the Ilyophinae