Psilorhynchus hamiltoni Conway, Dittmer, Jezisek & Ng, 2013, new species
Creators: name: Conway, Kevin W.
name: Dittmer, Drew E.
name: Jezisek, Laci E.
name: Ng, Heok Hee
Publisher: name: Zenodo
Year: 2013
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Actinopterygii
Cypriniformes
Psilorhynchidae
Psilorhynchus
Psilorhynchus hamiltoni

description: Psilorhynchus hamiltoni, new species
Figs. 23, 24
Holotype. UMMZ 249903, 30.6 mm SL; India: West Bengal, Tista River at Tista Barrage, 105 m asl, 26°45'1.0"N 88°35'11.0"E, H. H. Ng et al., 15 April 2004.
Paratypes. UMMZ 249904, 5 (1c&s; 1 SEM stub), 25.8–26.3 mm SL: same data as holotype.
Diagnosis. A member of the P. balitora species group, distinguished from all other members by the following combination of characters: lateral stripe well-developed; 7–11 round to squarish lateral blotches; 6–7 dorsal saddles; dorsal saddles without contact to lateral blotches; lateral line scales 34–35; principal caudal-fin rays 9+8– 9; pectoral-fin rays v–vi.9–11; total vertebrae 36 (18+18); two branchiostegal rays; parietal portion of temporal sensory canal with 2 openings; ventral surface between paired fins with a broad rectangular scaleless patch.
Description. General body shape as in Figure 23. Morphometric data are listed in Table 1 and select meristic characters in Tables 2 and 3. Body elongate, dorsal profile weakly arched. Body depth greatest at point slightly anterior to dorsal-fin origin, narrowest at base of caudal peduncle. Ventral profile straight from lower jaw to analfin origin, weakly concave from anal-fin origin to caudal-fin base.
Head and eye large, pupil elliptical, with longest axis of ellipse orientated along dorsal-ventral body axis. Mouth inferior, relatively wide (Fig. 24). Snout short, rounded anteriorly, its ventral surface bordered by a deep longitudinal groove on each side. Rostral cap and upper lip fused, separated by a shallow groove; posterolateralmost part of rostral cap continuous around corner of mouth, contacting skin fold at posterolateral corner of mouth; posterior margin of rostral cap smooth. Lower lip portion of lower-jaw cushion broadly rectangular, anterior edge straight; superficial layer of lower-jaw cushion covered in low, rounded globular papillae, separated by deep grooves; majority of globular papillae concentrated directly posterior to middle of lower lip portion of lower-jaw cushion, becoming scarcer laterally. Upper lip covered with narrow, dagger-like unculi, ranging in height from ~10–20 μm, becoming increasingly longer towards mouth opening; lower jaw unculi not examined. 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 smooth, globular papillae absent; tubercles present around outer edge of skin fold. Posterior edge of rostral cap, lower lip, skin fold, and globular papillae all densely covered with tastebuds. Gill membranes joined to isthmus.
Following osteological features based on a single c&s specimen. Pre-epiphyseal fontanelle small, triangular. Post-epiphyseal fontanelle present, long, irregularly shaped (pinched along its center; Fig. 13 L), separated from pre-epiphyseal fontanelle by wide expanse of frontal. Five infraorbitals (IO1–5). IO1–3 platelike, IO1 largest of the series, IO4–5 narrow tube-like bones, composed of sensory canal ossification only. Anguloarticular portion of preopercular mandibular canal and parietal portion of supraorbital canal absent. Openings in preopercular portion of preopercular mandibular canal 4. Openings in nasal portion of supraorbital canal 3. Openings in parietal portion of temporal canal 2. Supraoccipital portion of temporal canal open. Fifth ceratobranchial with a single row of four needle-like teeth. Branchiostegal rays 2; anteriormost branchiostegal ray articulating with medial face of anterior ceratohyal absent. Anterior swimbladder chamber surrounded by a thick peritoneal tunic, partially enclosed in a bony capsule formed anteriorly by lateral process of 2nd vertebral centrum and laterally by outer arm of os suspensorium. Posterior swimbladder chamber greatly reduced in size.
Dorsal-fin rays iii.9 (5, including holotype [*]) or iii.8 (1). Anal-fin rays ii.6 (6). Principal caudal-fin rays 9+9 (2*) or 9+8 (4), dorsal procurrent rays 9, ventral procurrent rays 6. Pectoral fin rays vi.9 (1), vi.10 (4*) or vi.11 (1), pelvic fin rays ii.7 (6). Paired fins horizontally placed, pectoral fins larger than pelvic fins. Pectoral fin reaching vertical through dorsal-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 four-five anteriormost pectoral fin rays and two anteriormost pelvic-fin rays. Dorsal fin moderately high, triangular in shape with weakly pointed tip; posterior margin weakly concave. Anal fin small, triangular in shape with weakly pointed tip; posterior margin weakly concave; not reaching caudal-fin base when adpressed. Caudal fin weakly forked, tips of both lobes rounded.
Scales cycloid, large, with several well-developed radii over posterior field of scale body. 34 (1) or 35 (2*) scales along lateral line, plus 2 (3) pored scales on base of caudal fin. Number of transverse scale rows from dorsalfin origin to pelvic-fin origin not obtainable (scales between dorsal fin and lateral line missing in all type material). 10 scale rows around caudal peduncle, 10 (1) or 11 (2*) predorsal scales, 10 (3) or 11 (1*) scales between anus and anal-fin origin. Ventral surface between paired fins without scales. Total number of vertebrae 36, consisting of 18+18 (1) abdominal and caudal vertebrae.
Small conical tubercles with poorly keratinized tip distributed over lateral and dorsal surfaces of head, including snout and anteroventral region of rostral cap. Posteroventral margin of rostral cap with small dagger-like tubercles. Body and fin tuberculation not developed in available material.
Coloration. In alcohol body background color light cream (Fig. 23). Occiput dark brown. Dorsal surface between occiput and dorsal fin with two dark brown saddles, first situated at midpoint between occiput and dorsalfin origin, second situated at dorsal-fin origin. Number of dorsal saddles posterior to dorsal-fin origin variable, with six present in holotype and four present in paratypes. In paratypes, first post-dorsal-fin origin saddle situated below middle of dorsal fin, between insertions of branched dorsal-fin rays 4–6, second situated directly between dorsalfin and anal-fin origin, third situated directly above middle of anal fin, fourth situated at base of caudal fin. In holotype, post-dorsal-fin origin saddles narrower than those of paratypes. First situated at same position as in paratypes, second situated closer to insertion of last dorsal-fin ray than point opposite anal-fin origin, third situated slightly anterior to point opposite anal-fin origin, fourth situated directly above middle of anal fin, fifth situated at center of caudal peduncle, sixth situated at base of caudal fin. Saddle situated at dorsal-fin origin largest of series. Dorsal saddles extending ventrally 1–2 scale rows on body side, without contact with lateral blotches or lateral stripe.
Flank with 7 (2), 8(2), 9(1) or 11(1*) small round to squarish dark brown blotches arranged in a longitudinal row. Size and position of lateral blotches along flank highly variable, excluding first, situated posterodorsal to opercular opening, and last, situated at caudal-fin base. Lateral stripe dark brown, well developed. Scales situated over anterodorsal surface of body, anterior to dorsal fin, with scattering of light brown melanophores along posterior edge, forming weak reticulate pattern. Scales in L+1 row dorsal to lateral line scales 10–25 with small light brown spot at center. Scales in L-1 row ventral to lateral line scales 6–11 with small dark brown spot at center.
Unscaled base of pectoral fin and scales adjacent to pelvic-fin origin peppered with small dark brown melanophores, forming indistinct pectoral-base and pre-pelvic spots, respectively. Lateral surface of snout and skin over opercle densely scattered with dark brown melanophores.
Ventral surface largely devoid of pigment except for a 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 2nd to 5th scale in scale row running between anus and anal-fin origin. Dorsal surface of pectoral- and pelvic-fin rays marked with small melanophores. Dorsal fin with weak stripe across center, formed by aggregations of small dark brown melanophores at fork of branched rays. Anal fin hyaline. Caudal fin with dark brown triangular blotch at base, orientated with posteriormost tip of triangle extending into base of lower lobe. Additional small dark brown markings variably present throughout upper and lower lobes.
Distribution. Psilorhynchus hamiltoni is known currently only from the type locality on the Tista River at the Tista Barrage (Fig. 21). This locality has been described previously by Ng (2005) and Conway & Mayden (2008b).
Etymology. Named in honor of the exceptional Scottish naturalist Francis Hamilton (1762–1829), a pioneer of Indian ichthyology, who spent more than 17 years surveying the freshwater fish fauna of large areas of the Indian subcontinent.
Remarks. The six specimens on which the above description is based were collected together with specimens of P. balitora, and all were subsequently catalogued together (as P. balitora) in UMMZ 244849. During their redescription of P. balitora, Conway & Mayden (2008b) examined only the five largest specimens from UMMZ 244849 (all P. balitora), overlooking the fact that this lot contained individuals belonging to two separate species (Conway, pers. obs.). The mixed nature of this lot did not became apparent until later (in 2010), when one of us (KWC) reinvestigated its contents and noticed that the six smallest specimens do not possess scales along the ventral surface of the body between the paired fins (an area that is invariably scaled in P. balitora; Conway & Mayden, 2008b), which prompted further investigation and ultimately led to the conclusion that they belonged to an undescribed species.
In addition to the absence of scales along the ventral surface between the paired fins, P. hamiltoni can be further distinguished from P. balitora by its higher number of lateral line scales (34–35 vs. 30–32), caudal vertebrae (18 vs. 15–16), and total vertebrae (36 vs. 31–33), a lower number of branchiostegal rays (two vs. three) and openings in the parietal portion of the temporal canal (two vs. three), and by having a large post-epiphyseal fontanelle that is bordered anteriorly by the frontals (vs. post-epiphyseal fontanelle small, restricted to posterodorsalmost portion of neurocranium, bordered anteriorly by the parietals). Features of the color pattern also serve to distinguish P. hamiltoni from P. balitora. In P. hamiltoni the lateral stripe is well developed, whereas in P. balitora (and all other members of the P. balitora species group) the lateral stripe is poorly developed. In P. hamiltoni the saddles along the dorsal surface of the body do not make contact with the lateral blotches along the body side, whereas in P. balitora (and all other members of the P. balitora species group, excluding P. rahmani), the saddles extend far enough ventrally (over the side of the body) to make contact with the lateral blotches.
All available specimens of P. hamiltoni are presumably juveniles or small adults, ranging in size from 25.7– 30.6 mm SL. Despite their small size, these specimens exhibit small round tubercles over the dorsal and lateral surfaces of the head and snout. Tubercles are, however, absent from the body and fins. SEM investigation of the mouthparts of a single paratype (26.3 mm SL) revealed the presence of small (but nevertheless well developed) conical tubercles on the surface of the skin-flap at either corner of the mouth and small dagger-shaped tubercles along the posteroventral edge of the rostral cap (Fig. 24) that could not be observed in the holotype or other paratypes using light microscopy. Dagger-shaped tubercles (with tips oriented caudally) are common along the posteroventral margin of the rostral cap in other species of Psilorhynchus (for example see Fig. 4 in Conway et al., 2012; Fig. 25 herein) and are easily observed with light microscopy in well-preserved specimens. The presence of tubercles over the surface of the skin-flap positioned at the corner of the mouth appears to be less common, as evidenced by their absence in P. s u c a t i o (Fig. 9) and P. nudithoracicus (Fig. 19). In addition to the paratype of P. hamiltoni, we have observed these structures only in the single individual of P. balitora examined using SEM (Fig. 25).
Though once considered to be a relatively minor component of the South Asian ichthyofauna (Hora, 1920), recent investigations have revealed the Psilorhynchidae to be surprisingly diverse, with twelve new species described since 2007 (Conway & Kottelat, 2007, 2010; Arunachalam et al. 2007; Nebeshwar et al. 2007; Arunachalam & Muralidharan 2008; Conway & Mayden 2008a, b; Conway & Britz, 2010). Given the pace at which new species of Psilorhynchus continue to be described, the discovery of an additional species in the Brahmaputra drainage of northeastern India is not surprising and further highlights our incomplete knowledge of the Indian freshwater ichthyofauna. The discovery of P. hamiltoni in a “well-examined” lot of Psilorhynchus was, however, completely unexpected, and its description should serve as a cautionary tale for those that may be tempted to ignore those smaller, harder to reach specimens, at the bottom of the jar.
description: Published as part of Conway, Kevin W., Dittmer, Drew E., Jezisek, Laci E. & Ng, Heok Hee, 2013, On Psilorhynchus sucatio and P. nudithoracicus, with the description of a new species of Psilorhynchus from northeastern India (Ostariophysi: Psilorhynchidae), pp. 201-243 in Zootaxa 3686 (2) on pages 232-236, DOI: 10.11646/zootaxa.3686.2.5, http://zenodo.org/record/284175