Clarias gracilentus, Ng, Heok Hee, Hong, Dang Khanh & Tu, Nguyen Van, 2011

Ng, Heok Hee, Hong, Dang Khanh & Tu, Nguyen Van, 2011, Clarias gracilentus, a new walking catfish (Teleostei: Clariidae) from Vietnam and Cambodia, Zootaxa 2823, pp. 61-68 : 62-65

Clarias gracilentus sp. nov.

( Fig. 1 )

Clarias Nieuhofi (non Bleeker)— Pellegrin & Chevey, 1937: 315. Clarias nieuhofi (non Bleeker)— Rainboth, 1996: 163.

Type material. Holotype: UMMZ 248862, 189.6 mm SL; Vietnam: Phu Quoc Island, swamp draining into Rach Vem, 10°22'N 103°56'E (coordinates approximate); V. T. Nguyen and T. H. Le, 26 February 2010.

Paratypes: BMNH 1937.9.17:39–40 (2), 173.2–207.5 mm SL; Cambodia: Kampot province, Bokor; J. Delacour & W. Lowe, 1928. NLHC 2010.RV1 (1) 251.5 mm SL; NLHC 2010.RV2 (1) 254.5 mm SL; NLHC 2010.RV3 (1) 299.5 mm SL; NLHC 2010.RV4 (1) 317.5 mm SL; NLHC 2010.RV5 (1) 265.5 mm SL; UMMZ 248863 (4), 151.0– 224.1 mm SL; ZRC 52046 (1), 221.4 mm SL; data as for holotype. NLHC 2010.RT1 (1), 281.5 mm SL; NLHC 2010.RT2 (1), 260.5 mm SL; NLHC 2010.RT3 (1), 294.5 mm SL; NLHC 2010.RT4 (1) 305.5 mm SL; Vietnam: Phu Quoc Island, swamp draining into Rach Tram, 10°24'N 103°58'E (coordinates approximate); V. T. Nguyen and T. H. Le, 13 January 2010.

Diagnosis. Clarias gracilentus can be distinguished from all Southeast Asian congeners, except for C. nieuhofii , C. nigricans and C. pseudonieuhofii in having a greatly elongate body with correspondingly long dorsal- and anal-fin ray bases. This elongation is manifested in the higher number of total vertebrae (74–84 vs. 54–71), dorsal- (81–101 vs. 62–80) and anal-fin (68–89 vs. 47–70) rays seen in all four species of the C. nieuhofii species complex compared to all other Southeast Asian Clarias . Clarias gracilentus differs from C. nieuhofii in having a narrower head (11.9–12.9% SL vs. 13.2–15.8), more slender body (8.2–11.7% SL vs. 10.8–14.4) and shorter pelvic fin (4.3–5.5% SL vs. 5.0–6.3), from C. nigricans in having a larger eye (5.4–7.2% SL vs. 4.5–5.6), larger interorbital distance (42.7–48.0% HL vs. 40.3–44.0), shorter distance between the occipital process and the base of the first dorsal-fin ray (5.3–8.4% SL vs. 8.1–9.8) and longer pectoral fin (8.5–10.1% SL vs. 5.2–8.5), and from C. pseudonieuhofii in having a longer occipital process (7.8–14.7% HL vs. 4.6–6.8), shorter pelvic fin (4.3–5.5% SL vs. 6.4–7.5) and longer anal-fin base (60.0–63.9% SL vs. 56.0–58.9).

Description. Biometric data in Table 1 . Head depressed; dorsal profile slightly convex and ventral profile almost straight. Bony elements of dorsal surface of head covered with thick skin; bones not readily visible, but sutures sometimes evident. Anterior pair of nostrils tubular and medial to maxillary barbel base. Posterior pair of nostrils bordered by nasal barbels anteriorly and fleshy membrane posteriorly; posteromedial to maxillary barbel base. Eye ovoid, horizontal axis longest, subcutaneous; located dorsolaterally on head. Anterior fontanel short and squat (“shoe-shaped” of Teugels, 1986); anterior tip reaching to line through middle of orbits. Occipital process rounded. Gill openings narrow, extending from dorsal-most point of pectoral-fin base to isthmus. Gill membranes free from isthmus but united to each other with 8 (n=8) branchiostegal rays. First branchial arch with 3+13* (n=2), 3+14 (n=2) or 4+13 (n=2) gill rakers.

Mouth subterminal, with fleshy, plicate lips. Oral teeth small and in irregular rows on all tooth-bearing surfaces. Premaxillary tooth band rectangular, with median notch on posterior edge. Dentary tooth band much narrower than premaxillary tooth band at symphysis, tapering laterally. Vomerine tooth band unpaired, continuous across midline, crescentic and smoothly arched along anterior and posterior margins. Premaxillary and dentary teeth viliform; vomerine teeth subgranular. Barbels in four pairs; long and slender with thick fleshy bases. Maxillary barbel extending to base of third or fourth dorsal-fin ray. Nasal barbel, extending to middle of pectoral fin. Inner mandibular-barbel origin close to midline; barbel thicker and longer than nasal barbel and extending just beyond base of last pectoral-fin ray. Outer mandibular barbel originating posterolateral of inner mandibular barbel, extending to midway between base of last pectoral-fin ray and base of first pelvic-fin ray.

Holotype Range Mean ±SD

%SL

Body anguilliform and cylindrical, becoming compressed towards caudal peduncle. Dorsal profile rising very gently from tip of snout to origin of dorsal fin and thereafter almost horizontal to end of caudal peduncle. Ventral profile slightly convex to middle of head and thereafter almost horizontal to end of caudal peduncle. Skin smooth. Lateral line complete and midlateral in position. Vertebrae 22+58=80 (n=1), 23+58=81* (n=1), 22+60=82 (n=1), 23+59=82 (n=2), 23+60=83 (n=1), 24+59=83 (n=1) or 22+62=84 (n=1).

Dorsal fin with long base, spanning posterior three-quarters of body; with 96 (n=1), 97 (n=2), 99 (n=1), 100* (n=3) or 101 (n=1) rays covered by thick layer of skin and without spine. Dorsal-fin margin straight, parallel to dorsal edge of body. Pectoral fin with small spine, sharply pointed at tip, and 8,i (n=8) rays. Almost entire length of anterior spine margin with a series of small serrations; posterior spine margin smooth or with uneven asperities. Pectoral-fin margin straight anteriorly, convex posteriorly. Pelvic-fin origin at anterior third of body, with i,5 (n=8) rays and convex margin; tip of adpressed fin reaching base of first two or three anal-fin rays. Anus and urogenital openings located at vertical through middle of adpressed pelvic fin. Anal fin with long base, extending for posterior three-fifths of body, and 84 (n=1), 85* (n=2), 86 (n=1),87 (n=1) or 89 (n=3) rays covered by thick layer of skin; margin straight and parallel to ventral edge of body. Caudal peduncle very short. Caudal fin rounded, with i,6,6,i (n=8) principal rays.

Coloration. In 70% alcohol: dorsal and lateral surfaces of head and body dark gray, fading to pale gray on ventral surfaces. Fifteen to twenty-one vertical rows of two to five white spots present, subtended ventrally by two irregular rows of white spots running below lateral line. Dorsal and caudal fins dark gray with very thin hyaline distal margin. Anal fin light gray, with thin hyaline distal margin. Pectoral-fin rays dark gray, with hyaline interradial membranes. Pelvic fins hyaline. Barbels and pectoral spines dark gray dorsally and light gray ventrally.

Distribution. Clarias gracilentus is known from the Rach Tram and Rach Vem drainages on the northern part of Phu Quoc Island, Vietnam, and from the Kampot River drainage in southern Cambodia ( Fig. 2 ). This species is expected to occur elsewhere on Phu Quoc Island and southern Cambodia, but further surveys are needed to verify this.

Habitat and ecology. On Phu Quoc Island, C. gracilentus inhabits forested swamps and slow-flowing streams. During the dry season, the fish hides in small holes under dead-tree roots. At the type locality, the water was clear, cool (24–27°C), tinted brown, acidic (pH 4.5–5.5) and well oxygenated (DO 4 mgl -1). Other fishes collected at the type locality were Monopterus albus (Synbranchidae) , Channa sp. ( Channidae ), and Betta cf. prima (Osphronemidae) . Gut contents of dissected specimens consisted of smaller fishes (frequently Channa and Betta , and occasionally smaller individuals of C. gracilentus ), freshwater crabs, molluscs, and terrestrial arthropods (insects and spiders).

Etymology. The specific epithet is the Latin adjective gracilentus , meaning slender, in reference to the slender body of this species when compared to C. nieuhofii .

Figures

FIGURE 1. Clarias gracilentus, holotype, UMMZ 248862, 189.6 mm SL; Vietnam: Phu Quoc Island. Dorsal, lateral and ventral views.

FIGURE 2. Map of southeastern Cambodia and Phu Quoc Island, showing collecting localities of the type series of Clarias gracilentus.

Tables

TABLE 1. Biometric data for Clarias gracilentus (n = 11).

Predorsal length Preanal length	25.5 38.8	22.4–26.8 37.4–41.9	25.4±1.23 39.8±1.44
Prepelvic length Prepectoral length Length of dorsal-fin base Anal-fin length	36.5 15.3 76.5 62.1	35.0–38.2 14.0–16.0 72.5–77.6 60.0–63.9	36.3±1.07 14.9±0.63 75.0±1.74 62.0±1.24
Pelvic-fin length Pectoral-fin length	5.2 10.0	4.3–5.5 8.5–10.1	4.9±0.38 9.4±0.52
Pectoral-spine length Caudal-fin length Distance between occipital process and dorsal fin Body depth at anus Caudal peduncle depth Head length	7.0 13.3 8.1 11.2 5.3 18.4	6.1–7.8 10.7–14.6 5.3–8.4 8.2–11.7 3.8–5.4 17.3–19.2	6.9±0.47 12.5±1.36 7.5±0.91 10.3±1.24 4.9±0.55 18.2±0.67
Head width Head depth	12.8 9.8	11.9–12.9 8.4–9.8	12.6±0.33 8.9±0.43
%HL Snout length	29.6	23.0–31.5	28.6±2.70
Interorbital distance Eye diameter	43.1 6.3	42.7–48.0 5.4–7.2	44.4±1.81 6.2±0.54
Frontal fontanelle length Frontal fontanelle width Occipital fontanelle length Occipital fontanelle width	14.4 8.6 11.2 4.9	12.1–18.0 6.5–10.3 8.5–13.0 4.9–7.1	15.8±1.55 8.0±1.37 11.5±1.62 5.8±0.77
Occipital process length Occipital process width	12.6 33.0	7.8–14.7 27.3–33.3	11.0±2.30 30.4±2.21
Nasal barbel length Maxillary barbel length Inner mandibular barbel length Outer mandibular barbel length	71.8 149.4 70.4 108.0	52.8–87.3 103.7–158.4 50.6–86.4 70.8–121.7	71.6±10.38 139.8±15.49 68.2±8.44 101.9±16.43