Psilorhynchus robustus, new species

Figure 1

Holotype. ZRC 51111, 60.3 mm SL; Myanmar: Kayin [Karen] State: stream “Chon Son” between Kyondaw and Phadaw, about 20km northwest of Payathouzu (at border with Thailand). K. Kubota, December 2002.

Paratypes. CMK 17773, 1, 54.8 mm SL; same data as holotype; ZRC 51113, 10; 53.3–66.1 mm SL; CMK 17941, 32 (3 cleared and stained); 49.1–68.1 mm SL; same data as holotype, March 2003.

Diagnosis: Psilorhynchus robustus is most similar to P. gracilis and P. balitora in overall body shape, meristics and colour pattern. It is distinguished from both species by the presence of a large dark blotch situated posterodorsally to opercle opening, extending along lateral-line scales 3–6 (vs. absence) and in having upper lip separated from the rostral cap by a shallow groove (vs. upper lip separated from rostral cap by a deep groove). It is further distinguished from P. gracilis by a greater mouth width (28–31 % HL vs. 20–25), head width (67–73 % HL vs. 56–61), head depth (55–58 % HL vs. 46–50), pre-pelvic distance (53–56 % SL vs. 47–50), pre-anal distance (78–83 % SL vs. 76–79), and caudal peduncle width (5–6 % SL vs. 3–4). Psilorhynchus robustus is distinguished from P. arunachalensis, P. homaloptera, P. microphthalmus, P. pseudecheneis and P. s u c a t i o in having 9 branched dorsal fin rays (vs. 8). It is further distinguished from P. microphthalmus, P. arunachalensis, P. homaloptera and P. pseudecheneis by the lower number of unbranched pectoral rays (5 vs. 7 in P. microphthalmus, 8–9 in P. arunachalensis and P. homaloptera and 9–10 in P. pseudecheneis) and lateral line scales (32–34 vs. 39–40 in P. microphthalmus, 42–44 in P. arunachalensis, 43–44 in P. homaloptera and 46–48 in P. pseudecheneis) and from P. homaloptera and P. pseudecheneis by a greater body depth (19–23 % SL vs. 10–15 in P. homaloptera and 13–16 in P. pseudecheneis) and head depth (55–58 % HL vs. 41–45.5 in P. homaloptera and 38–42 in P. pseudecheneis).

Description: General body shape as in Figure 1. Morphometric and meristic data are listed in Tables 1–2. Body high, greatest depth at dorsal-fin origin. Dorsal profile arched, rising gradually to dorsal-fin origin, sloping steeply towards caudal peduncle. Ventral profile straight, from lower jaw to caudal-fin base.

Dorsal fin with iii. 9 rays. Anal fin with ii.6 rays. Branched caudal-fin rays 8-9+8-9 (8+ 8 in holotype), dorsal procurrent rays 5 or 6, ventral procurrent rays 6. Pelvic-fin rays ii.7 pectoral-fin rays v.11–12. Total number of vertebrae 34–35, consisting of 18+16(1) or 19+16(1).

Head and eye large, mouth inferior, snout rounded, ventral surface bordered by a deep longitudinal groove on each side. Rostral cap and upper lip fused, separated only by a narrow, shallow groove. Lower jaw covered by a thick squarish 'cushion' that can be folded backwards. 'Cushion' composed of two adnate tissue layers: a deeper layer, the lower lip, smooth, not continuous with upper lip around corner of mouth; and a superficial layer, papillated, thick, continuous with skin of isthmus and connected with rostral cap by a narrow strip of skin around corner of mouth, extended posteriorly and broadened as a flat, slightly papillated skin fold at posterolateral most corner of mouth (Fig. 2 a). Large pre-epiphysial and post-epiphysial fontanelle. Five infraorbital bones (IO1-5); IO1-3 platelike; IO4-5 reduced in width, comprised of sensory canal only. Gill membranes joined to isthmus. Fifth ceratobranchial with 4 needle-like pharyngeal teeth, arranged in a single row. Swimbladder coated by thick peritoneal tunic, posterior chamber greatly reduced. Anterior chamber partially enclosed in a bony capsule formed anteriorly by lateral process of the 2nd vertebral centrum and laterally by the outer arm of the os suspensorium.

Paired fins horizontally placed. Pectoral fin almost reaching horizontal through dorsal-fin orgin. Pelvicfin origin posterior to dorsal-fin origin, insertion opposite 3rd branched dorsal-fin ray. Anus positioned between pelvic fins. Caudal fin emarginated, upper lobe slightly longer than lower lobe. Scales large, 32–34 along lateral line, plus 1–2 on base of caudal fin. 3.5/1/2 transverse scale rows from dorsal-fin origin to pelvicfin origin, 10 around caudal peduncle. Scales absent from ventral surface between pectoral fins.

Coloration: In alcohol body background olive. Scales on flanks and dorsal surface edged with dark pigment. Dark pigment between pores of lateral line, forming an indistinct lateral streak. Occiput and dorsal surface of snout dark. Dorsal surface between occiput and dorsal-fin origin with one or two indistinct saddles. Five prominent dark saddles along dorsal surface between dorsal-fin origin and caudal-fin base, first situated at dorsal-fin origin, second between insertions of branched dorsal-fins rays 6–9, third between dorsal-fin and anal-fin origin, fourth directly above anal fin and fifth situated anterior to caudal-fin base. Large dark blotch on flank, situated posterodorsal to opercle opening, extending across lateral line scales 3–6. Unbranched pectoral-fin rays and base of dorsal-fin rays edged with dark pigment. Caudal fin with irregular dark patterning. Peritoneal lining silvery, speckled with dark melanophores.

Distribution: Definitively known from headwaters of the Ataran basin in Myanmar (Fig. 3). See Kottelat (2003, 2004) for a more detailed description of the basin.

Etymology: From the Latin adjective robustus, meaning strong or robust, in allusion to the overall robust appearance of this species.

...continued.

Table 1 continued.