Glyptothorax sulculus, Ng, 2025

Glyptothorax sulculus , new species urn:lsid:zoobank.org:act:

Figs. 1 View FIGURE 1 , 2a View FIGURE 2 , 3

Holotype. UMMZ 232576 View Materials , 51.3 mm SL; Cambodia: Stung Treng Province, Mekong drainage, O Pon Moang, small stream 13 km downstream along Mekong River from Stung Treng, 13°25'N 105°57'E; W. J. Rainboth & C. Rotha, 16 February 1995. GoogleMaps

Diagnosis. G lyptothorax sulculus is distinguished from all Mekong drainage congeners except G. carnatus , G. cf. platypogonides and G. rhadinus by its color pattern of two indistinct dark saddles on body connected by an indistinct dark midlateral stripe, forming an elongate pale ovoid patch on the dorsolateral surface of body, and the absence of a distinctly pale nuchal shield (vs. two indistinct dark saddles often accompanied by irregular dark spots and blotches on body and distinctly pale nuchal shield in G. filicatus , G. horai and G. macromaculatus or uniformly dark-colored body with or without pale midlateral and mid-dorsal longitudinal stripes in G. deqinensis , G. forabilis , G. irroratus , G. laosensis , G. longinema and G. porrectus ), a longer dorsal spine (18.7% SL vs. 11.8–16.9) and a larger eye (14% HL vs. 8–12). It differs from G. carnatus , G. cf. platypogonides and G. rhadinus in having the area between the anterior and posterior fontanels on the neurocranium sharply delineated by the absence of tubercles on the dorsal surface of the head (vs. this area indistinctly delineated; Fig. 2 View FIGURE 2 ), and broader lateral processes of the nuchal plates set at about 90° to the median axis (vs. narrower lateral processes set about 100–110° to the median axis; Fig. 2 View FIGURE 2 ). The following combination of characters further distinguishes G. sulculus from Mekong congeners: TAA with its width 1.7 times in its length, lacking both anteromedial striae and a medial pit; dorsal to adipose distance 15.6% SL (vs 17.2–28.5, except in G. rhadinus ); caudal peduncle length 17.3% SL (vs 19.1–25.8 in G. forabilis , G. laosensis and G. porrectus ); caudal peduncle depth 2.9 (vs 1.9–2.7, except in G. carnatus ) times in body depth at dorsal origin.

Description. Morphometric data in Table 1. Head depressed; body subcylindrical. Dorsal profile rising evenly from tip of snout to origin of dorsal fin, then sloping gently ventrally from origin of dorsal fin to end of caudal peduncle. Ventral profile straight to anal-fin base, then sloping gently dorsally from anal-fin base to end of caudal peduncle. Anus located at vertical through posterior third of adpressed pelvic fin; urogenital openings located at three-quarters of distance between bases of last pelvic-fin ray and first anal-fin ray. Skin on sides of body and caudal peduncle tuberculate. Lateral line complete, midlateral.

Head depressed, broad, triangular when viewed laterally. Snout prominent. Anterior and posterior nares large, separated only by base of nasal barbel. Gill opening broad, extending from ventral margin of post-temporal to isthmus. Bony elements of dorsal surface of head covered with thick, tuberculate skin. Eye ovoid, horizontal axis longest; located entirely in dorsal half of head.

Barbels in four pairs. Maxillary barbel long, slender, extending to base of pectoral-fin spine. Nasal barbel slender, extending to just over halfway between its base and anterior orbital margin. Inner mandibular-barbel extending to midway between its base and that of pectoral spine. Outer mandibular barbel extending to three quarters of distance between its base and that of pectoral spine.

Mouth inferior, premaxillary tooth band partially (approximately half) exposed with mouth closed. Oral teeth small, villiform, in irregular rows on all tooth-bearing surfaces. Premaxillary teeth appearing in single broad semilunate band. Dentary teeth in a single crescentic band, consisting of two separate halves tightly bound at midline.

Thoracic adhesive apparatus (TAA) consisting of striae in a slightly obovate field (Fig. 3) extending from isthmus to slightly beyond posterior limit of pectoral-fin base; anterolateral edges of adhesive apparatus straight. Thoracic adhesive apparatus lacking distinct medial pit, its width 1.7 times in its length.

Dorsal fin located above anterior two-fifths of body, with I,6 rays; fin margin concave; spine slightly curved, posterior margin with 4 weak serrations. Adipose fin with anterior and posterior margins nearly straight. Caudal fin strongly forked, with lobes of nearly equal length and i,7,8,i principal rays. Procurrent rays symmetrical, extending only slightly anterior to fin base. Anal-fin base vertically opposite adipose-fin base. Anal fin with nearly straight anterior and concave posterior margin; with iv,9,i rays. Pelvic-fin origin very slightly anterior to vertical through posterior extremity of dorsal-fin base. Pelvic fin with straight posterior margin and i,5 rays; tip of adpressed fin just reaching anal-fin origin. Pectoral fin with I,6,i rays; posterior fin margin concave; anterior spine margin smooth, posterior margin with 11 serrations.

Colour: Dorsal and lateral surfaces of head, and body brown, fading to beige on ventral surfaces. Darker colour on dorsal surface of interdorsal region, with dark, saddle-shaped markings on lateral surface of body below dorsal and adipose fins. Dark, diffuse midlateral stripe running along lateral line, coalescing with dark markings on lateral surfaces of body below dorsal and adipose fins to form pale, elongate oval above lateral line and between dorsal and adipose fins. Dorsal and lateral surfaces of body with scattered dark brown spots. Dorsal fin hyaline, with brown base and irregular brown brand on middle third of fin. Pectoral fin hyaline, with brown base and brown melanophores forming irregular transverse band subdistally. Pelvic fin hyaline, with brown melanophores forming diffuse brown band in middle third of fin. Anal fin hyaline, with brown base in anterior third of fin and brown melanophores forming diffuse brown band in middle third of fin. Adipose fin brown, with scattered dark spots and with hyaline distal margin. Caudal fin lobes mostly brown, faintly mottled with dark spots; tips and much of lobes and median fin-rays hyaline. Maxillary barbel brown dorsally, beige ventrally.

Habitat. The type locality is a stream about 15 m wide, with moderate current and clear water. The substrate consisted of boulders, sand and gravel.Other fish species occurring syntopically include: Esomus metallicus , Puntigrus partipentazona , Rasbora borapetensis , R. dusonensis , R. paviana , Balitoropsis zollingeri , Homalopteroides smithi , Pseudohomaloptera leonardi , Syncrossus helodes , Clarias aff. batrachus , Dermogenys siamensis and Chaudhuria cf. caudata .

Distribution. Glyptothorax sulculus is known only from the type locality in the Mekong River drainage of nothern Cambodia ( Fig. 4 View FIGURE 4 ).

Etymology. The species epithet is the diminutive form of the Latin noun sulcus, meaning a furrow. The name refers to the anterior and posterior fontanels on the neurocranium being sharply delineated by the absence of tubercles, a diagnostic character of this species.