Clarias serniosus, Ng, Heok Hee & Kottelat, Maurice, 2014

Ng, Heok Hee & Kottelat, Maurice, 2014, Clarias serniosus, a new walking catfish (Teleostei: Clariidae) from Laos, Zootaxa 3884 (5), pp. 437-444 : 438-440

Clarias serniosus sp. nov.

( Fig. 1 )

Type material. Holotype: MHNG 2745.072 , 141 mm SL; Laos: Champasak Province, Bolavens Plateau , Houay Slarm near Ban Pha Nouandong , a tributary of Houay Xoy , itself a tributary of Xe Pian , 15°02'34"N 106°21'53"E, 985 masl; M. Kottelat & T. Phommavong, 17 January 2013.

Paratype: CMK 23382, 161 mm SL; data as for holotype.

Diagnosis. Clarias serniosus is distinguished from all Southeast Asian congeners, except for C. meladerma , in having a dark gray head and body with black blotches (vs. black blotches absent). C larias serniosus differs from C. meladerma in having a greater distance between the occipital process and the base of the first dorsal-fin ray (8.5% SL vs. 2.8–4.8), a smooth (vs. serrated) anterior edge of the pectoral spine, a longer pectoral fin (8.5–10.1% SL vs. 5.2–8.5) and a deeper caudal peduncle (8.4–8.6% SL vs. 5.3–7.3). The following combination of characters additionally diagnoses C. serniosus from Southeast Asian congeners: occipital process length 15–17% HL, head length 28.2–28.6% SL, head width 18.5–19.2% SL, head depth 13.0–13.7% SL, smooth anterior edge of pectoral spine, 67 dorsal-fin rays, body depth at anus 16.2–16.5% SL, and 57 total vertebrae.

Description. Morphometric data in Table 1 . Head depressed; dorsal profile slightly convex; ventral profile almost straight. Bony elements of dorsal surface of head covered with thick skin; bones and sutures not readily visible. Anterior pair of nostrils tubular, 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 long, narrow (“knifeshaped” of Teugels, 1986); anterior tip reaching to line through anterior orbital margin. 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 (2) branchiostegal rays.

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, smoothly arched along anterior and posterior margins. Premaxillary and dentary teeth viliform; vomerine teeth subgranular. Barbels in four pairs; long, slender, with thick fleshy bases. Maxillary barbel extending to middle of pectoral-fin base. Nasal barbel extending nearly to tip of supraoccipital process. Inner mandibular-barbel origin close to midline, extending to halfway between posterior orbital margin and base of pectoral spine. Outer mandibular barbel originating posterolateral of inner mandibular barbel, extending to base of pectoral spine.

Body terete, becoming compressed towards caudal peduncle. Dorsal profile rising very gently from tip of snout to origin of dorsal fin, thereafter almost horizontal to end of caudal peduncle. Ventral profile slightly convex to middle of head, thereafter almost horizontal to end of caudal peduncle. Skin smooth. Lateral line complete, midlateral in position. Vertebrae 19+38=57 (2).

Dorsal fin with long base, spanning posterior three-quarters of body; with 67 (2) rays covered by thick layer of skin, without spine. Dorsal-fin margin straight, parallel to dorsal profile of body. Pectoral fin with small spine, sharply pointed at tip, and 8* (1) or 9 (1) rays. Anterior spine margin smooth; 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 (2) 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 49* (1) or 54 (1) rays covered by thick layer of skin; margin straight and parallel to ventral edge of body. Caudal fin rounded, with i,7,6,i (2) principal rays.

Coloration. In 70% alcohol: dorsal and lateral surfaces of head and body dark gray, fading to pale gray on ventral surfaces. Dorsal and lateral surfaces of head and body with irregular, small black blotches randomly distributed throughout. Dorsal, anal and caudal fins dark gray. Pectoral-fin rays dark gray, with hyaline interradial membranes. Pelvic fins hyaline. Barbels and pectoral spines dark gray dorsally and light gray ventrally.

In life: Body and fins dark gray, almost black. Blotches black, distinct but not conspicuously contrasted.

Distribution. Clarias serniosus is known only from its type locality in the Xe Pian drainage (a tributary of the Xe Kong, itself a tributary of the Mekong) flowing from the Bolavens Plateau in southern Laos ( Fig. 2 ).

Habitat and ecology. The Bolavens Plateau is a large basalt plateau of southern Laos, about 50 by 80 km ( Sanematsu et al., 2011), 1000-1300 masl, surrounded by the Mekong and Xe Kong floodplains (about 100 masl). It has very steep slopes, especially along its southern and eastern edges. It has a very wet climate with heavy rainfall in all seasons. Most rivers leave the plateau through high and dramatic series of waterfalls and gorges. The Houay Xoy, a tributary of the Xe Pian, itself tributary of the Xe Kong, is one of the few streams without high falls (based on 1:100,000 maps and air photographs; ground survey was not possible).

The type series of C. serniosus was obtained in a small tributary of the Houay Xoy on which a small wooden weir had been constructed in an area recently logged and converted for agriculture. In the sampling area, the stream was about 2–4 m wide, and less than 70 cm deep (at the beginning of the low-water season). The bottom was almost exclusively soft sand, probably the result of erosion and sedimentation. About 100 m of the stream was sampled with electricity. The two individuals of C. serniosus were obtained in a shaded area of very degraded forest. Only three other fish species were present: Devario gibber , Schistura clatrata and Channa gachua . The small number of species is a common situation of disturbed areas and these three species are common species, often observed in degraded habitats with running water, and they are known throughout the plateau as well as downstream. Clarias serniosus is likely to be present also downstream of the plateau in the Xe Kong drainage. On the day of capture, water temperature was estimated to be 10–15°C (air temperature fluctuated between 6°C at night and about 25°C in the afternoon).

Etymology. The specific epithet is the Latin adjective serniosus , meaning covered with an eruption or scabby, in allusion to the color pattern of small, irregular dark patches.

Figures

FIGURE 1. Clarias serniosus, holotype, MHNG 2745.072, 141 mm SL; Laos: Bolavens Plateau, Xe Pian drainage. Dorsal and lateral views.

FIGURE 2. Map showing type locality of Clarias serniosus.

Tables

TABLE 1. Morphometric data for Clarias serniosus (n = 2).

	Holotype MHNG 2745.072	Paratype CMK 23382
Standard length	141	161
In % SL
Predorsal length	36.1	35.9
Preanal length	51.4	50.3
Prepelvic length	42.9	41.9
Prepectoral length	21.9	20.3
Length of dorsal-fin base	64.4	66.8
Anal-fin length	50.1	46.2
Pelvic-fin length	10.7	10.1
Pectoral-fin length	13.9	13.4
Pectoral-spine length	9.7	10.5
Caudal-fin length	14.9	14.8
Distance between occipital process and dorsal fin	8.5	8.5
Caudal peduncle depth	8.4	8.6
Body depth at anus	16.5	16.2
Head length	28.6	28.2
Head width	19.2	18.5
Head depth	13.7	13.0
In % HL
Snout length	30	30
Interorbital distance	45	45
Eye diameter	9	8
Nasal barbel length	59	51
Maxillary barbel length	78	77
Inner mandibular barbel length	60	46
Outer mandibular barbel length	73	70
Front fontanel length	22	21
Front fontanel width	3	5
Occipital fontanel length	19	13
Occipital fontanel width	6	4
Occipital process length	17	15
Occipital process width	28	30