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

publication ID

z01663p047

publication LSID

lsid:zoobank.org:pub:B95ED61C-6C3D-4585-B9A1-40E5EA03A724

persistent identifier

http://treatment.plazi.org/id/40280DB3-7AFD-8AF6-742E-07429DA8E892

treatment provided by

Thomas

scientific name

Psilorhynchus robustus
status

new species

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  ZBK  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  ZBK  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  ZBK  , P. microphthalmus  ZBK  , P. pseudecheneis  ZBK  and P. sucatio  in having 9 branched dorsal fin rays (vs. 8). It is further distinguished from P. microphthalmus  ZBK  , P. arunachalensis  , P. homaloptera  ZBK  and P. pseudecheneis  ZBK  by the lower number of unbranched pectoral rays (5 vs. 7 in P. microphthalmus  ZBK  , 8-9 in P. arunachalensis  and P. homaloptera  ZBK  and 9-10 in P. pseudecheneis  ZBK  ) and lateral line scales (32-34 vs. 39-40 in P. microphthalmus  ZBK  , 42-44 in P. arunachalensis  , 43-44 in P. homaloptera  ZBK  and 46-48 in P. pseudecheneis  ZBK  ) and from P. homaloptera  ZBK  and P. pseudecheneis  ZBK  by a greater body depth (19-23 % SL vs. 10-15 in P. homaloptera  ZBK  and 13-16 in P. pseudecheneis  ZBK  ) and head depth (55-58 % HL vs. 41-45.5 in P. homaloptera  ZBK  and 38-42 in P. pseudecheneis  ZBK  ).

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  ZBK  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  ZBK  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  ZBK  and P. sucatio  . Of these three species, P. robustus  is most similar in terms of meristics to P. balitora  and P. gracilis  ZBK  (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  ZBK  , P. sucatio  ), Yazdani et al., (1990) divided the species of Psilorhynchus  into two genera, Psilorhynchus  sensu stricto and Psilorhynchoides  ZBK  for the inclusion of P. homaloptera  (type species) and P. pseudecheneis  ZBK  . This decision was based on a number of differences that they observed between the external appearance of P. homaloptera  and P. pseudecheneis  ZBK  , 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  ZBK  (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  ZBK  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  ZBK  , they only examined the osteology of a few specimens (number unknown) representing only two species, one for each genus ( P. homaloptera  for Psilorhynchoides  ZBK  and P. balitora  for Psilorhynchus  ). Based on observations of a large number of cleared and stained specimens, representing 5 species ( P. balitora  , P. gracilis  ZBK  , P. pseudecheneis  ZBK  , 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  ZBK  are not restricted to P. homaloptera  and P. pseudecheneis  ZBK  but have a mosaic distribution within Psilorhynchus  (summarised in Table 3). Though it is true that P. homaloptera  and P. pseudecheneis  ZBK  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.

ZRC

Singapore, National University of Singapore, Raffles Museum of Biodiversity Research, Zoological Reference Collection

CMK

CMK