Psilorhynchus robustus, Kevin W. Conway & Maurice Kottelat, 2007

Kevin W. Conway & Maurice Kottelat, 2007, A new species of Psilorhynchus (Teleostei: Psilorhynchidae) from the Ataran River Basin, Myanmar, with comments on the generic name Psilorhynchoides., Zootaxa 1663, pp. 47-57 : 49-56

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. sucatio 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. 2a). 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.

Discussion

The genus Psilorhynchus was created by McClelland (1839) for the species described by Hamilton (1822) as Cyprinus balitora and C. sucatio . Hora (1920) first revised the genus, though he did not have access to P. sucatio . He later split Psilorhynchus (Hora, 1921a), creating the new genus Parapsilorhynchus for the species P. tentaculatus , and redescribed P. sucatio based on fresh material (Hora, 1921b). Since Hora’s revision several authors have further defined the genus (Mukerji, 1933; Jayaram, 1981; Rainboth, 1983; Yazdani et al, 1990).

The new species, P. robustus , can be assigned to the genus Psilorhynchus (sensu Rainboth, 1983) based on the following combination of characters: back arched, ventral surface flattened; mouth small and inferior; barbels absent; gill-membranes joined broadly to isthmus with aperture extending ventrally to base of pectoral fin; paired fins inserted horizontally; scales large, 32-34 in lateral line; dorsal fin with iii.9 rays, anal fin with iii.6 rays, pectoral with v.11-12 rays, pelvic fin with ii.7 rays; 5th ceratobranchial with 4 pharyngeal teeth, arranged in a single row; pre-epiphysial fontanel present; posterior swimbladder chamber greatly reduced, anterior chamber partially enclosed in a bony capsule formed anteriorly by the lateral process of the 2nd vertebral centrum and laterally by the outer arm of the os suspensorium (= 4th pleural rib of other authors).

Though the interrelationships within the genus Psilorhynchus are currently unknown, the new species appears to be more closely related to those species of Psilorhynchus with a papillated skin fold at the posterolateral most corner of the mouth (Fig. 2), specifically P. balitora , P. gracilis and P. sucatio . Of these three species, P. robustus is most similar in terms of meristics to P. balitora and P. gracilis (Table 2) as both have 9 branched dorsal-fin rays and 3 unbranched anal-fin rays (vs. 8 and 2, respectively).

Based on examination of the four Indian species available to them ( P. balitora , P. homaloptera , P. pseudecheneis , P. sucatio ), Yazdani et al., (1990) divided the species of Psilorhynchus into two genera, Psilorhynchus sensu stricto and Psilorhynchoides for the inclusion of P. homaloptera (type species) and P. pseudecheneis . This decision was based on a number of differences that they observed between the external appearance of P. homaloptera and P. pseudecheneis , and P. balitora and P. sucatio , and in the osteology of P. homaloptera and P. balitora , including: body flattened anteriorly and compressed laterally towards posterior half of the body in Psilorhynchoides (vs. body more or less spindle shaped with distinct convexity of the dorsal profile with peak at dorsal-fin origin in Psilorhynchus ); broad based paired fins, well spread out horizontally (vs. narrow based, not so much spread out horizontally); small eyes (vs. eyes fairly large); scales absent on chest (vs. present); 8-10 unbranched pectoral-fin rays (vs. 4-6); lateral line scales numbering above 40 (vs. 32-33, as counted by Yazdani et al.); skull broad (vs. long and slender); supraethmoid fossa (= post-epiphysial fontanelle) present in P. homaloptera (vs. absent in P. balitora ); ethmoid-frontal fontanelle (= pre-epiphysial fontanelle) long and slender in P. homaloptera (vs. short and broad in P. balitora ); dorsal ribs of second and fourth vertebrae forming a bony capsule enclosing the anterior swim bladder chamber in P. homaloptera (vs. dorsal ribs of second and fourth vertebrae just folded with lateral openings); urohyal thick and catapult shaped with forked anterior tips in P. homaloptera (vs. urohyal a more compact shaft-like structure with anterior tips only a little forked in P. balitora ); lateral foramen present on postero-lateral border of basipterygium in P. homaloptera (vs. absent in P. balitora ).

The placement of P. homaloptera and P. pseudecheneis within a separate genus appears to have been followed only by Nelson (1994, 2006) and Nebeshwar et al. (2007), with other authors retaining them in Psilorhynchus . Though characters of osteology featured prominently in Yazdani et al.’s characterization of Psilorhynchoides , they only examined the osteology of a few specimens (number unknown) representing only two species, one for each genus ( P. homaloptera for Psilorhynchoides and P. balitora for Psilorhynchus ). Based on observations of a large number of cleared and stained specimens, representing 5 species ( P. balitora , P. gracilis , P. pseudecheneis , P. robustus , P. sucatio ), encompassing a greater geographic range than material examined by Yazdani et al. (Nepal, India, Bangladesh & Myanmar vs. India) it is clear that the characters used to define the genus Psilorhynchoides are not restricted to P. homaloptera and P. pseudecheneis but have a mosaic distribution within Psilorhynchus (summarised in Table 3). Though it is true that P. homaloptera and P. pseudecheneis exhibit a more elongate body and a higher number of unbranched pectoral-fin rays and lateral line scales than all other species of Psilorhynchus (Tables 1-3), we prefer to retain these species within Psilorhynchus pending the outcome of a forthcoming phylogenetic analysis (Conway, in prep.).

The earlier eastern extent of the range of Psilorhynchus was the Irrawaddy drainage. The discovery of the genus in the Ataran drainage leaves a gap as there is no published record from the Salween drainage, located between the previous two. The presence of Psilorhynchus is expected in the Salween, and indeed we have information that suggests that the genus is present in its tributary Mae Nam Moei, but this awaits specimen-based confirmation.