Psilorhynchus tysoni, Conway, Kevin W. & Pinion, Amanda K., 2016

Conway, Kevin W. & Pinion, Amanda K., 2016, Psilorhynchus tysoni, a new species of torrent minnow from the Salween River drainage, western Thailand (Teleostei: Psilorhynchidae), Zootaxa 4136 (3), pp. 580-592 : 582-588

Psilorhynchus tysoni , new species

( Figures 1–3 )

Holotype. CAS 237342, 27.7 mm SL; Thailand: Tak Province: Mae Nam Moei at Hathe Rapids, ~ 130 km north of Mae Sot on highway 105, 17°30'35"N, 97°58'40"E; Tyson R. Roberts, 30 April 1991.

Paratypes. All Thailand (Tak Province): CAS 237345, 2, 23.3–29.9 mm SL; rapids in Mae Nam Moei, ~ 129 km north of Mae Sot on highway 105, 17°29'33"N, 98°0'16"E; Tyson R. Roberts, 29 April 1991.— CAS 241430, 5, 25.1–27.3 mm SL; same locality as holotype.— TCWC 17185.01, 1 C&S, 45.0 mm SL; TCWC 17185.02, 1, 53.8 mm SL; Hue Mae Song, 600 meters upstream from confluence with Mae Nam Moei at highway 105 road crossing, 17°23'43''N, 98°05'44''E; Jens Kühne, 0 1 June 2004.

Diagnosis. A member of the Psilorhynchus nudithoracicus group, distinguished from all other members by the following combination of characters: 6–7 dorsal saddles along dorsal midline, including 1 or 2 between occiput and dorsal-fin origin and 5 or 6 from dorsal-fin origin to caudal-fin origin; posteriormost dorsal saddle at caudal-fin origin small but prominent and represented by black or dark brown spot; L+1 and L-1 rows restricted to center of body side, not extending posterior to vertical through anal-fin origin; L+2 row absent; dorsal-fin with poorly developed dark brown or black markings along anterior margin; two widely separate dark brown or black markings on both upper and lower caudal fin lobes; anguloarticular portion of preoperculo-mandibular canal present; principal caudal-fin rays 10+9; total vertebrae 34 (17+17); a shallow body (14–18% SL); a short caudal peduncle (11–14% SL); a short snout (35–43% HL); a narrow mouth (mouth width 21-26% HL).

Description. General body shape as in Figure 1 . Morphometric characters listed in Table 2 . Body elongate, dorsal profile arched, rising moderately to dorsal-fin origin, sloping gently towards caudal peduncle. Body depth greatest at dorsal-fin origin, narrowest at base of caudal peduncle. Ventral profile moderately straight from lower jaw to anal-fin origin, weakly concave from anal-fin origin to caudal-fin base.

Head and eye large, pupil weakly elliptical with longest axis of ellipse orientated along dorsal-ventral body axis. Mouth inferior, wide ( Figure 2 ). Snout moderately long, less than half of head length; rounded anteriorly, its ventral surface bordered by a deep longitudinal groove on each side. Rostral cap and upper lip separated by a shallow groove; posterolateralmost part of rostral cap continuous around corner of mouth, contacting skin fold at posterolateral corner of mouth ( Figure 2 ). Lower lip portion of lower-jaw cushion broadly rectangular, anterior edge weakly rounded; superficial layer of lower-jaw cushion covered in large, globular papillae. Skin fold around posterolateral corner of mouth large and flap-like, continuous anteriorly with posterolateralmost part of upper lip and rostral cap; outer edge of skin fold weakly papilliated with small globular papillae; tubercles absent from surface of skin fold. Gill membranes joined to isthmus.

Pre-epiphyseal and post-epiphyseal fontanelle long and rectangular, separated by a narrow strut of frontal, situated dorsal to the epiphyseal bar. Five infraorbitals (IO1–5). IO1–3 plate like, IO1 largest of the series. IO4–5 narrow tube-like bones, composed of lateral line canal ossification only. Cephalic lateral line canal system well developed. Openings in anguloarticular portion of preoperculo-mandibular canal 2. Openings in preopercular portion of preoperculo-mandibular canal 6. Openings in nasal portion of supraorbital canal 3. Parietal portion of supraorbital canal open. Openings in parietal portion of temporal canal 2. Fifth ceratobranchial with a single row of four needle-like teeth. Gill-raker ossifications absent. Hyoid bar with 3 branchiostegal rays of similar length and shape. Basihyal elongate, rod-shaped. Anterior swimbladder chamber surrounded by a thick peritoneal tunic, partially enclosed in a bony capsule formed anteriorly by lateral process of 2nd vertebra and laterally by outer arm of os suspensorium. Posterior swimbladder chamber of single large male (TCWC 17185.01) singular (without internal compartmentalization sensu Conway et al., 2014); approximately 1/3 of size of anterior chamber. Posterior swimbladder chamber of females not assessed.

Dorsal-fin rays iii.9. Anal-fin rays ii.6. Principal caudal-fin rays 10+9, dorsal procurrent rays 8 (1), ventral procurrent rays 5 (1). Pectoral fin rays v.12 * (4), v.13 (1), vi.11 (2), vi.12 (1) or vi.13 (1), pelvic fin rays ii.7 (5) or ii.8 * (4). Paired fins horizontally placed, pectoral fins larger than pelvic fins. Pectoral fin not reaching vertical through dorsal-fin origin, reaching two to three scale rows anterior to pelvic-fin origin when adpressed. Pelvic-fin origin posterior to dorsal-fin origin, insertion opposite second or third branched dorsal-fin ray. Well-developed unculiferous paired-fin pads present along ventral surface of five to six anteriormost pectoral-fin rays and two anteriormost pelvic-fin rays. Dorsal fin high, triangular in shape with weakly pointed tip; posterior fin margin weakly concave. Anal fin small, triangular in shape with weakly pointed tip; not reaching caudal-fin base when adpressed; posterior fin margin weakly concave. Caudal fin moderately forked, tips of upper and lower lobes weakly rounded.

Scales cycloid, large, with 6–8 well-developed radii over posterior field of scale body. 34 (3) or 35 (3) scales along lateral line, plus 1 (1) or 2 (5) on base of caudal fin. 3.5/1/2 transverse scale rows from dorsal-fin origin to pelvic-fin origin, 10 scale rows around caudal peduncle, 10 (1), 11* (6) or 12 (1) predorsal scale rows, 9 scale rows between anus and anal-fin origin. Ventral surface between paired fins without scales. Total number of vertebrae 34, consisting of 17+17 (1) abdominal and caudal vertebrae.

Type series comprised of small (23–30 mm SL), presumably juvenile individuals (including holotype) with poorly developed tubercles or adults (45–54 mm SL) with damaged tubercles. Information on tubercles based on smaller individuals only. Small conical tubercles with hard keratinized tip distributed over lateral, dorsal and ventral surfaces of head, including snout and anteroventral region of rostral cap ( Figure 2 ). Tubercles on posteroventral region of rostral cap dagger-like, larger than those on other surfaces of head.

Coloration. In alcohol body background colour light cream ( Figure 1 ). Occiput light to dark brown. Dorsal surface between occiput and dorsal fin with one (4) or two* (4) indistinct dark brown saddles. In specimens with one, saddle situated anterior to midpoint between occiput and dorsal-fin origin. In specimens with two, second saddle located between saddle situated anterior to midpoint between occiput and dorsal-fin origin and dorsal-fin origin. A well-developed dark brown saddle at origin of dorsal fin. Four* (7) or five (1) dark brown saddles posterior to dorsal-fin origin. In specimens with four, first situated below middle of dorsal fin, second situated midway between posteriormost point of dorsal fin and vertical through anal-fin origin, third situated above posteriormost point of anal fin and fourth (posteriormost saddle) situated at base of caudal fin. In single specimen with five, additional saddle located between saddles homologous to first and second saddles in specimens with four. Posteriormost saddle reduced to a small but prominent dark brown to black spot. Dorsal saddles, excluding two* or three posterior to saddle located at dorsal-fin origin, without contact to lateral blotches, extending ventrally one or two scale rows on body side. Two or three dorsal saddles posterior to saddle located at dorsal-fin origin extending ventrally two or three scale rows on body side and in direct contact with lateral blotches.

Flank with 6* (4), 7 (3) or 8 (1) dark brown lateral blotches arranged in a longitudinal row. First lateral blotch situated on flank at midpoint between opercular opening and vertical through dorsal-fin origin, and last lateral blotch situated at caudal-fin base. Size and position of other lateral blotches along flank highly variable. Lateral stripe distinct in two largest individuals examined; indistinct in smaller individuals. Scales situated over dorsal surface of body bordered posteriorly with dark brown pigment, forming distinct reticulate pattern over dorsal surface, strongest anterodorsally. Scales in lateral line scale row (L1) with dark blotch posteriorly, punctuated centrally by unpigmented lateral line canal (giving appearance of double dash line along lateral-line scale row). L+1 and L-1 pigment rows poorly developed or absent in juvenile individuals; represented by 12 well-developed dark brown spots along center of body (dorsal or ventral to scales 5–17 in lateral-line scale row) in largest individual examined. L+2 row absent. Scaleless base of pectoral fin and scales adjacent to pelvic-fin origin peppered with small dark brown melanophores, forming distinct pectoral-base and indistinct pre-pelvic spots, respectively. Lateral surface of snout, region rimming ventral margin of orbit and skin covering opercle densely scattered with dark brown melanophores.

Ventral surface largely devoid of pigment except for melanophores along anterior edge of rostral cap, small patch of brown melanophores situated beneath scales at anal-fin origin and a short line of melanophores posterior to anus, running along ventral midline from anus to 5th scale in scale row running between anus and anal-fin origin. Dorsal surface of anterior pectoral- and pelvic-fin rays marked with small brown melanophores. Dorsal fin with single, large, irregular, dark brown or black marking or multiple small dark brown or black markings along anterior margin, formed by melanophores scattered along fin membrane between last unbranched ray and anteriormost branched ray. Anal fin hyaline. Caudal fin with dark brown or black blotch over base of central caudal-fin rays and two well-separated dark brown markings on upper and lower lobes. In several specimens (including holotype) anteriormost blotch along lower lobe and blotch over base of central caudal-fin rays confluent, forming irregular dark brown or black marking.

In life ( Figure 3 ), body background colour pale golden brown. Dorsal saddles and lateral stripe chocolate brown. Lateral blotches and pigmentation on fins dark brown or black. Scales in lateral-line scale row with a silvery iridescent spot at center, forming distinct iridescent dotted line along side of body. Rays of pectoral fin and unbranched rays of pelvic fin pale yellow.

Distribution and habitat. Psilorhynchus tysoni is known currently only from a few localities on the Mae Nam Moei and its tributary the Hue Mae Song, Tak Province, Thailand ( Figure 4 ). At a locality on the Hue Mae Song ( Figure 5 ), 600 meters upstream from the confluence with the Mae Nam Moei, P. tysoni was collected in swift flowing water of shallow to moderate (50–100 cm) depth over a substrate of sand and gravel. Water parameters measured at this locality at time of collection include: 21C, pH 7.5, KH 3, GH 0.

Etymology. Named for Tyson R. Roberts who collected the majority of the specimens in the type series and made them available for study. A noun in genitive.

Comparisons. Psilorhynchus tysoni can be distinguished from P. t e n ur a by a higher number of principal rays in the caudal fin (10+9 vs. 9+9), a higher number of pores in the preopercular portion of the preoperculomandibular canal (6 vs. 4), a slightly greater interorbital width (35–43% HL vs. 32–35) and a slightly shorter caudal peduncle (caudal peduncle length 11–14% SL vs. 14–16). Psilorhynchus tysoni can be further distinguished from P. t e n ur a by the presence (vs. absence) of a short stretch of sensory canal along the anguloarticular (representing the mandibular portion of the preoperculo-mandibular canal), the presence (vs. absence) of a short hiatus between the base of hypural 1 and the compound centrum, and by having fewer vertebrae (total vertebrae 34 vs. 36). Psilorhynchus tysoni can be distinguished from P. melissa by the absence (vs. presence) of black pigment along the distal margin of the dorsal fin, by having two widely separate black or dark brown blotches along the upper and lower lobes of the caudal fin (vs. irregular black markings along much of the outer margin of the upper and lower lobes of the caudal fin), and by a having a weak (vs. intense) scattering of dark brown or black melanophores along the anterior margins of the paired fins. Psilorhynchus tysoni can be distinguished from P. robustus by a slightly shallower body (body depth 14–18% SL vs. 19–23) and head (head depth through orbit 44– 52% HL vs. 55–58%), and a narrower mouth (mouth width 21–26% HL vs. 28–31%). Psilorhynchus tysoni can be further distinguished from P. robustus by features of body and fin pigmentation, including the absence (vs. presence) of the L+2 row, black blotches at base of upper and lower lobes of the caudal fin separate (vs. connected by an irregular black vertical bar), and by having the posteriormost dorsal saddle represented by a small, but prominent, black or dark brown spot (vs. saddle-like). Psilorhynchus tysoni is distinguished from P. nudithoracicus by relatively few characters, including the absence (vs. presence) of the L+2 row, and by a slightly wider caudal peduncle (caudal peduncle width 3–4% SL vs. 1–3% SL).

Remarks. The available material of P. t y s on i is comprised of small (23–30 mm SL), presumably juvenile individuals, and two larger (45–54 mm SL) individuals. The smaller specimens (CAS material) are in relatively good condition, though some individuals have scales missing from the caudal peduncle. The larger specimens are in poor condition (i.e., fins damaged, skin on head and mouthparts sloughed) and because of this we have opted to designate a smaller individual (CAS 237342, 27.7 mm SL) as the holotype.

Figures

FIGURE 1. Psilorhynchus tysoni, CAS 237342, holotype, 27.7 mm SL; Thailand: Mae Nam Moei.

FIGURE 2. Ventral surface of head (a) and mouthparts (b) of Psilorhynchus tysoni, CAS 237342, holotype, 27.7 mm SL; Thailand: Mae Nam Moei. Abbreviations: LL, lower lip; Pap, globular papillae; RC, rostral cap; SF, skin fold; UL, upper lip.

FIGURE 3. Psilorhynchus tysoni, photographed alive; Thailand: Hue Mae Song. (a) TCWC 17185.01, 45.0 mm SL, lateral view. (b) TCWC 17185.02, 53.8 mm SL, dorsal view. Photographs by Jens Kühne.

FIGURE 4. Distribution of Psilorhychus tysoni. Open symbol represents type locality.

FIGURE 5. Hue Mae Song, 600 meters upstream from confluence with Mae Nam Moei at highway 105 road crossing, Tak Province, Thailand. Known locality of Psilorhynchus tysoni. Photographs by Jens Kühne.

Tables

TABLE 2. Morphometric characters for holotype and eight paratypes (CAS 237345, CAS 241430 and TCWC 17185.02) of Psilorhynchus tysoni.

	Holotype	Range	Mean	St.Dev.
Standard length (mm)	27.7	23.3–53.8	-	-
In percent of standard length
Body depth	17.7	14.2–18.4	16.4	1.5
Head length	22.0	19.4–24.2	22.0	1.2
Pre-dorsal fin length	45.1	44.6–49.4	47.1	1.6
Pre-pectoral fin length	20.6	19.4–20.7	20.7	1.0
Pre-pelvic fin length	52.7	51.3–53.5	52.6	0.8
Pre-anal fin length	75.8	75.8–80.6	78.9	1.5
Snout-anus	59.9	58.9–62.9	60.4	1.3
Anus-anal fin	18.1	18.0–20.4	19.0	0.8
Caudal peduncle length	13.0	11.1–13.9	12.8	0.8
Caudal peduncle depth	7.6	6.9–8.1	7.4	0.4
Caudal peduncle width	4.0	3.4–5.1	3.9	0.5
Pectoral fin length	23.5	22.3–25.6	24.0	1.0
Pelvic fin length	18.8	16.7–18.7	17.8	0.7
Length of last unbranched anal-fin ray	14.8	12.8–15.0	13.9	0.8
Length of last unbranched dorsal-fin ray	19.5	18.9–22.3	20.5	1.1
In percent of head length
Head width	65	60–71	63	3.4
Head depth	49	44–52	47	2.2
Eye diameter	33	30–36	33	1.7
Snout length	39	35–43	40	2.3
Interorbital width	40	35–43	38	2.5
Mouth width	23	21–26	23	1.5