Eirmotus isthmus, Kottelat, 2008

Kottelat, Tan Heok Hui Maurice, 2008, Revision Of The Cyprinid Fish Genus Eirmotus, With Description Of Three New Species From Sumatra And Borneo, Raffles Bulletin of Zoology 56 (2), pp. 423-433 : 426-428

Eirmotus isthmus , new species

( Figs. 4 , 7b , 8a )

Eirmotus octozona (non Schultz, 1959): Kottelat, 1982: 431.

Material examined. – MZB 10720 , holotype , 29.2 mm SL, Sumatra, Jambi: Danau Arang Arang, Muara Kompeh area , brown water lake (1°37'32.0"S 103°47'19.0"E); H. H. Tan & H. H. Ng, 25 Jul.1997 ; ZRC 50991 , 81 paratypes , CMK 17457 , 5 paratypes , MZB 10970 , 5 paratypes , USNM, 5 paratypes , 16.2–29.8 mm SL, same locality data as holotype .

Non-type material: ZRC 50992 , 8 ex., 14.2–32.5 mm SL, Sumatra, Jambi: Pijoan, Danau Souak Padang (1°36'34.4"S 103°26'55.1"E); H. H. Tan et al., 8 Jun.1996 ; CMK 11271 , 5 ex., 17.5–24.6 mm SL, Sumatra: Jambi: Pijoan, from aquarium fish collectors; M. Kottelat, 8 Jun.1994 ; MHNG 2056.27 , 1 ex., Kalimantan Tengah: Mentaya basin, about 250 km northwest of Banjarmasin ; A. Hanrieder, Jun.–Jul.1979 .

Diagnosis. – Eirmotus isthmus is distinguished from all other species of Eirmotus by its subinferior mouth, with a narrow lower jaw whose tip is angulous in ventral view. It is further distinguished by the following combination of characters: a small black mark immediately in front of anus with scattered chromatophores near it, visible in lateral view ( Fig. 7b ); bar 1 absent or very faintly marked; simple dorsal-fin rays black on their entire length, black mark not extending onto first branched ray, rest of fin hyaline ( Fig. 7b ); 22–26 serrae on posterior margin of last simple dorsal-fin ray; bars narrow (width of bar 5 equal to ½ or less lateral row scale).

Description. – See Figure 4 for general body shape and Table 1 for meristics and morphometrics. Size up to 32.5 mm SL. Dorsal with 3 simple and 8½ branched rays; origin above lateral row scale 6–8. Anal fin with 3 simple and 5½ branched rays; origin under lateral row scale 10–12. Pelvic fin with 1 simple and 8 branched rays, origin under lateral row scale 5–6. Lateral row scales 19–21; predorsal scales 7–8; transverse scales ½6–7½ between dorsal-fin and anal-fin origins, ½3½ on caudal peduncle.

Mouth subinferior, upper lip visible in ventral view; lower jaw narrow, tip angulous in ventral view ( Fig. 8a ). No marked angle at articulation of lower jaw.

Colouration. – Preserved: Body yellowish brown; markings dark grey to black, all very narrow (width of bar 5 equal to ½ or less lateral row scale). Scales between bars with a crescent-shaped patch of pigments centred on each scale, some of them forming faint dots along dorsal midline; very distinct on upper half of body but may also be present on flank and on caudal peduncle. Bar 1 absent or very faintly marked. Bars 2–4 not reaching ventral midline. In some specimens, one very poorly defined spot between bars 5 and 6, at posterior extremity of dorsal-fin base. Bar 5 not reaching pelvic-fin base in smaller individuals, reaching it in larger ones. A small and faint dot on ventral midline immediately in front of anus, with scattered chromatophore near it, about halfway between bars 5 and 6. Axial streak distinct between bars 5 and 8. In some specimens, a dot on lateral midline between bars 5 and 6, and 6 and 7.

Fins hyaline. Dorsal fin with simple rays black on their entire length, black mark not extending onto first branched ray, rest of fin hyaline. Anterior rays of anal fin black at base, continuing bar 6. Base of pelvic-fin rays black.

In life, body translucent with slight yellowish tinge, belly silvery, bars black.

Distribution. – Eirmotus isthmus is known from the flood plains and lakes in Pijoan and Muara Kompeh areas in central Jambi (Sumatra) and from the Mentaya drainage (Borneo: Kalimantan Tengah).

Life history notes. – Eirmotus isthmus is found in the water column in lakes and large permanent water bodies in the floodplain. Species caught together in the water column include Pectenocypris cf. korthausae ( Cyprinidae ), Hyalobagrus flavus ( Bagridae ), Pseudeutropius brachypopterus , P. moolenburghae ( Schilbeidae ) and Gymnochanda filamentosa ( Ambassidae ).

Etymology. – From the Latin isthmus , meaning a narrow passage or neck of land, alluding to the narrow black bars on the body. A noun in apposition.

Comparative notes. – Eirmotus isthmus is distinguished from all other species of Eirmotus by its subinferior mouth, with the upper lip entirely distinct in ventral view (vs. terminal, upper lip not entirely distinct); the lower jaw angulous anteriorly (vs. broadly rounded; Fig. 8 ). See under E. octozona for additional characters distinguishing it from E. isthmus . Eirmotus isthmus is distinguished from E. furvus by: the absence of a distinct black mark on the posterior tip of the dorsal fin adjacent to the upper extremity of bar 6 (vs. black mark present, chromatophores scattered on all dorsal-fin rays); no black upper margin on pectoral fin, no black anterior margin on anal fin (vs. both present); more serrae along the posterior edge of the last simple dorsal-fin ray (22–26, vs. 17–21); mode of lateral scales lower (19 vs. 20); bars narrower (width of bar 5 equal to or less than one lateral row scale, vs.1½–2); and slightly greater eye diameter (8.2–9.5 % SL, vs. 7.3–8.6).

Eirmotus isthmus is distinguished from E. insignis by: simple dorsal rays black along their entire length (vs. pigmentation restricted to anterior two rays and proximal quarter of third ray); no black mark along the posterior edge of the dorsal fin and the rest of the dorsal fin hyaline (vs. presence of a distinct black mark along the posterior edge, and a patch of chromatophores in centre of fin); more serrae along the posterior edge of the last simple dorsal-fin ray (22–26, vs. 16–21); fewer modal lateral scales (19, vs. 20); smaller predorsal length (49.1–51.7 % SL, vs. 51.6–53.7; 67.3–76.1 % BL, vs. 76.4–78.8); shorter head (length 29.1–31.4 % SL, vs. 32.3–33.5; 39.7–45.7 % BL, vs. 47.4–50.0); and greater interorbital width (26-31 % HL, vs. 22–25).

Figures

Fig. 4. Eirmotus isthmus, MZB 10720, holotype, 29.2 mm SL, Sumatra: Jambi.

Fig. 7. Flank of Eirmotus species, showing dorsal fin and pre-anus patterns in: a, E. octozona, ZRC 8385, 35.6 mm SL; b, E. isthmus, ZRC 50992, 32.0 mm SL; c, E. furvus, ZRC 39152, 27.3 mm SL; and d, E. insignis, ZRC 38786, 23.2 mm SL.

Fig. 8. Head of Eirmotus species, showing ventral view of snout and lower jaw of: a, E. isthmus, MZB 10720, 29.2 mm SL; and b, E. insignis, ZRC 38786, 23.2 mm SL.