Glyptothorax decussatus, Ng, Heok Hee & Kottelat, Maurice, 2016

Ng, Heok Hee & Kottelat, Maurice, 2016, The Glyptothorax of Sundaland: a revisionary study (Teleostei: Sisoridae), Zootaxa 4188 (1), pp. 1-92 : 13-16

Glyptothorax decussatus new species

( Fig. 8 , 9 )

Glyptosternum platypogon (non Valenciennes, 1840)— Popta, 1906: 79.

Type material. Holotype. MZB 17216, 74.5 mm SL; Borneo: Kalimantan Timur, Kayan River drainage, Bahau River sub-drainage, En’ggeng B’io draining into Bahau River , 2°52'35"N 115°49'11"E; H. H. Tan & D. Wowor, 14 November 1999.

Paratypes. RMNH 7565 (3), 45.7–78.3 mm SL; Kalimantan Tengah: Kayan River ; A. W. Nieuwenhuis, September–October 1900.

Diagnosis. Glyptothorax decussatus differs from congeners in Sundaland except for G. amnestus , G. major and G. plectilis in having (vs. lacking) anteromedial striae on the thoracic adhesive apparatus. It is distinguished from G. amnestus in having (vs. lacking) a dark vertical bar at the base of the caudal fin in the shape of an irregular cross, from G. major in having a deeper caudal peduncle (10.3–10.9% SL vs. 7.7–10.1) and a faint, pale band around the caudal peduncle (vs. absent) and from G. plectilis in having non-prominent tubercles of uniform size (vs. with prominent, enlarged tubercles) along the flanks. The following unique combination of characters further distinguishes G. decussatus from Sundaic congeners: premaxillary tooth band approximately half exposed when mouth is closed; eye diameter 8–9% HL; head length 27.2–30.2% SL; head width 19.9–22.3% SL; predorsal length 35.8–40.0% SL; dorsal-fin spine length 16.0–17.3% SL; margin of dorsal fin concave; dorsal-to-adipose distance 18.6–21.2% SL; body depth at anus 17.8–19.7% SL; straight dorsoposterior margin of adipose fin; post-adipose distance 16.2–17.5% SL; caudal peduncle depth 2.0 times in body depth at dorsal-fin origin; absence of both dark vertical bars at adipose-fin base and caudal-fin base and prominent pale midlateral stripe on the body.

Description. Morphometric data in Table 2 . Head depressed; body robust, 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 and urogenital openings located at vertical through middle of adpressed pelvic fin. Skin tuberculate, with tubercles of even size on sides of body. Lateral line complete and midlateral. Vertebrae 17+16=33 (2), 17+17=34 (1) or 18+18=36* (1).

Head depressed and broad, triangular when viewed laterally. Snout prominent. Anterior and posterior nares large and separated only by base of nasal barbel. Gill opening broad, extending from ventral margin of posttemporal to isthmus. First branchial arch with 2+8 (3) rakers. 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 and slender, extending to middle of pectoral-fin base. Nasal barbel slender, extending to midway 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 two-thirds of distance between its base and that of pectoral spine.

Mouth inferior, premaxillary tooth band partially (approximately half) exposed when mouth is closed. Oral teeth small and 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 consisting of keratinized striae in an elongate oblong field extending from isthmus to almost posterior limit of pectoral-fin base ( Fig. 3 b); anterolateral edges of adhesive apparatus almost straight. Anteromedial striae present. Chevron-shaped medial pit on posterior half.

Dorsal fin located above anterior third of body, with I,6 (4) rays; fin margin concave; spine short and straight, smooth on anterior and posterior margins in smaller individuals (up to approximately 60 mm SL) and with 3–6 (holotype =6) serrae on posterior margins of individuals larger than approximately 70 mm SL. Adipose fin with anterior margin straight or slightly concave and posterior margin straight. Caudal fin strongly forked, with lower lobe slightly longer than upper lobe and i,7,8,i (4) principal rays. Procurrent rays symmetrical and extending only slightly anterior to fin base. Anal-fin base vertically opposite adipose-fin base. Anal fin with straight anterior margin and straight or slightly concave posterior margin; with iv,7,i (1) or iv,8,i* (3) rays. Pelvic-fin origin at vertical through posterior limit of dorsal-fin base. Pelvic fin with slightly convex margin and i,5 (4) rays; tip of adpressed fin just reaching anal-fin origin. Pectoral fin with I,7,i (4) rays; posterior fin margin slightly concave; anterior spine margin smooth, posterior margin with 7–11 (holotype =11) serrations.

Coloration. In 70% ethanol: dorsal and lateral surfaces of head and body purplish gray, fading to light gray on ventral surfaces. Sides of body below adipose-fin base with very diffuse, indistinct dark band. Dorsal spine purplish gray. Proximal third of all dorsal-fin rays purplish gray; purplish gray color extending to proximal threequarters of first two dorsal-fin rays. Pectoral and pelvic fins with purplish gray on base of fin rays and hyaline posterior half. Anal fin with purplish gray base, with diffuse purplish gray spot on anterior third of fin. Adipose fin purplish gray with hyaline distal margin. Base of caudal fin with dark vertical marking in shape of irregular cross. Each caudal-fin lobe with irregular, hook-shaped purplish gray blotch spanning outer and middle fin rays, contiguous with dark vertical marking on base of caudal fin; most of inner rays of lobes hyaline. Maxillary and nasal barbels purplish gray dorsally, light gray ventrally. Mandibular barbels light gray. Live coloration similar, but with body and head brown, fading to lighter brown ventrally; base of dorsal-fin spine with small, cream-colored spot ( Fig. 9 ).

Distribution. Glyptothorax decussatus is known from the Kayan River drainage in northeastern Borneo ( Fig. 5 ).

Etymology. The specific epithet comes from the Latin verb decusso, meaning to divide crosswise in the form of an X. The name is used in reference to the dark vertical mark at the base of the caudal fin, which is shaped like an irregular cross.

Comparisons. Besides G. major (which has already been compared with G. decussatus in the diagnosis), there are four congeners known from Borneo: G. exodon , G. nieuwenhuisi , G. pictus and G. stibaros . Glyptothorax decussatus further differs from G. exodon in having the premaxillary tooth band approximately half (vs. almost entirely) exposed when the mouth is closed, a smaller eye (diameter 8–9% HL vs. 10–13), a wider head (19.9– 22.3% SL vs. 16.0–18.1) and a shorter post-adipose distance (16.2–17.5% SL vs. 21.5–24.0), and from G. nieuwenhuisi in having a deeper caudal peduncle (10.3–10.9% SL vs. 7.0–8.4). It is further distinguished from G. pictus in the pelvic fins reaching (vs. not reaching) the base of the first anal-fin ray when adpressed, and from G. stibaros in having a less tapering body, as manifested by the smaller ratio between the body and caudal peduncle depths (caudal peduncle depth 2.0 times in body depth at dorsal-fin origin vs. 3.4–3.8) and a deeper caudal peduncle (10.3–10.9% SL vs. 6.9–8.2).

Among the remaining Sundaic congeners, G. decussatus further differs from G. famelicus in having a greater predorsal length (35.8–40.4% SL vs. 32.1–35.2), a longer dorsal-fin spine (16.0–17.3% SL vs. 8.9–14.0), a deeper body (depth at anus 17.8–19.7% SL vs. 11.4–14.3), a deeper caudal peduncle (10.3–10.9% SL vs. 5.8–7.3), a shorter post-adipose distance (16.2–17.5% SL vs. 19.1–22.0% SL) and lacking (vs. having) a prominent pale midlateral stripe on the body, from G. f u s cu s in having a concave (vs. straight) margin of the dorsal fin and a less tapering body (caudal peduncle depth 2.0 times in body depth at dorsal-fin origin vs. 2.3–2.8), and from G. keluk in having a straight (vs. convex) dorsoposterior margin of the adipose fin, a deeper body (depth at anus 17.8–19.7% SL vs. 14.0–15.8) and a deeper caudal peduncle (10.3–10.9% SL vs. 7.7–8.8),. It is further distinguished from G. ketambe in having a greater predorsal length (35.8–40.0% SL vs. 33.3–35.9% SL), shorter post-adipose distance (16.2–17.5% SL vs. 19.5–21.7) and lacking (vs. having) a prominent pale midlateral stripe on the body, from G. platypogon in having a straight (vs. convex) dorsoposterior margin of the adipose fin, and from G. platypogonides in having a smaller eye (diameter 8–9% HL vs. 11–14), a longer head (27.2–30.2% SL vs. 25.2–26.8), a deeper body (depth at anus 17.8–19.7% SL vs. 12.1–14.8) and a deeper caudal peduncle (10.3–10.9% SL vs. 6.5–7.9). Glyptothorax decussatus further differs from G. prashadi in having a concave (vs. straight) margin of the dorsal fin, a longer post-adipose distance (16.2–17.5% SL vs. 17.9–20.8) and a deeper caudal peduncle (10.3–10.9% SL vs. 8.3–10.3), from G. robustus in having (vs. lacking) a medial pit in the thoracic adhesive apparatus, a deeper caudal peduncle (10.3–10.9% SL vs. 6.6–8.3) and lacking (vs. having) distinct a longitudinal stripe running through each lobe of the caudal fin, and from G. s c h m i d t i in having a longer dorsal-fin spine (16.0–17.3% SL vs. 8.9–14.9), a shorter dorsal-to-adipose distance (18.6–21.2% SL vs. 23.9–28.1), a shorter post-adipose distance (16.2–17.5% SL vs. 18.9–22.0), a deeper caudal peduncle (10.3–10.9% SL vs. 5.8–8.7) and lacking (vs. having) a prominent pale midlateral stripe on the body.

Figures

FIGURE 3. Thoracic adhesive apparatus of: a. Glyptothorax amnestus, ZRC 54231, paratype, 51.2 mm SL; b. G. decussatus, MZB 17216, holotype, 74.5 mm SL; c. G. exodon, BMNH 1982.3. 29.190, paratype, 63.2 mm SL; d. G. famelicus, USNM 193015, holotype, 73.3 mm SL; e. G. f u s c u s, ZRC 42123, 54.3 mm SL; f. G. keluk, MZB 17217, holotype, 52.5 mm SL; g. G. ketambe, MZB 8694, holotype, 100.0 mm SL; h. G. major, ZRC 38761, 59.1 mm SL; i. G. nieuwenhuisi, MZB 16475, 61.0 mm SL; j. G. pictus, MZB 17218, holotype, 54.2 mm SL; k. G. platypogon, ZRC 48376, 56.3 mm SL; l. G. platypogonides, UF 166626, 52.4 mm SL; m. G. plectilis, ZRC 46420, paratype, 44.3 mm SL; n. G. prashadi, CMK 25369, 50.7 mm SL; o. G. robustus, ZRC 48377, 59.9 mm SL; p. G. schmidti, CMK 4536, 74.6 mm SL; q. G. stibaros, MZB 17215, holotype, 55.3 mm SL. Illustrations not to scale.

FIGURE 5. Map showing collection localities of Glyptothorax amnestus, G. decussatus, G. exodon and G. famelicus.

FIGURE 8. Glyptothorax decussatus, MZB 17216, holotype, 74.5 mm SL; Borneo: Kalimantan Timur, En’ggeng B’io. Dorsal, lateral and ventral views.

FIGURE 9. Glyptothorax decussatus, ca. 100 mm SL, Borneo: Kalimantan Utara, Sungai Setulang, 3 ° 25 ' 25.8 " N 116 ° 27 ' 38.8 " E, showing live coloration; specimen not preserved. Photograph by Martin Holland and Frank van Veen.

Tables

TABLE 2. Morphometric data for Glyptothorax decussatus (n = 4).

	Holotype MZB 17216	Range	Mean±SD
Standard length (mm) %SL	74.5	45.7–74.5
Predorsal length Preanal length Prepelvic length	39.6 68.2 54.2	35.8–40.0 66.9–68.3 49.9–54.2	38.8±1.98 67.7±0.68 51.7±1.92
Prepectoral length Length of dorsal-fin base Dorsal-fin spine length	24.3 15.2 17.0	23.5–25.8 14.0–15.2 16.0–17.3	24.6±0.97 14.8±0.56 16.6±0.62
Length of anal-fin base Pelvic-fin length Pectoral-fin length	15.0 17.7 24.6	14.8–16.3 14.9–17.7 23.2–24.6	15.3±0.68 16.2±1.34 23.9±0.73
Pectoral-fin spine length Caudal-fin length Length of adipose-fin base	18.5 28.1 15.0	18.0–19.1 24.8–29.1 13.6–16.6	18.5±0.45 27.2±1.86 15.0±1.24
Dorsal to adipose distance Post-adipose distance Length of caudal peduncle	20.3 17.2 20.4	18.6–21.2 16.2–17.5 18.1–21.1	20.1±1.09 16.9±0.59 20.1±1.36
Depth of caudal peduncle Body depth at anus Body depth at dorsal-fin origin	10.9 19.7 24.7	10.3–10.9 17.8–19.7 21.0–24.7	10.7±0.30 19.1±0.96 22.9±2.62
Head length Head width Head depth	30.2 22.3 18.7	27.2–30.2 19.9–22.3 15.6–18.7	28.7±1.34 20.6±1.17 17.1±1.35
%HL Snout length Interorbital distance	49 25	46–52 25–30	49±2.9 27±1.9
Eye diameter Nasal barbel length Maxillary barbel length	8 22 87	8–9 22–28 85–94	8±0.5 24±2.4 89±4.2
Inner mandibular barbel length Outer mandibular barbel length	28 42	28–39 42–51	33±4.7 45±5.1