Auchenoglanis biscutatus (Geoffroy St. Hilaire, 1809)

Retzer, Michael E., 2010, Taxonomy of Auchenoglanis Günther 1865 (Siluriformes: Auchenoglanididae), Zootaxa 2655 (1), pp. 25-51 : 40-44

Auchenoglanis biscutatus (Geoffroy St. Hilaire, 1809)

( Figs. 4 , 7b , 8b ; Table 3)

Pimelodus biscutatus Geoffroy Saint-Hilaire, 1809: 320 . Type locality: Nil [ Nile River]. Holotype: MNHN A–8969.

Material examined: Egypt, governorate unknown: Nile River , MNHN A–8969, 1, (SL not measured, holotype) . Ethiopia, Gambella: Alwero River , at Abobo, CU 94646, 1, (257.0 mm SL) . Sudan, Bahr el Gabel: Nile River, Gondkoro, locality unknown, ZMB 16556, 1 , (258.9 mm SL); Jonglei: Atem River, southern Sudd swamps, BMNH 1984.9.27:203, 1, (243.7 mm SL); White Nile River, Fashoda (=Kodok), USNM 86638 , 1 , (226.5 mm SL) and ZMB 16554, 1 , (150.7 mm SL) .

Diagnosis: Auchenoglanis biscutatus can be distinguished from all other Auchenoglanis except A. sacchii (the pigmentation of A. sacchii is only vaguely known from its original description) by the adult body and adipose fin being light brown or grey and having small distinct spots on the rayed fins that do not coalesce to form bands ( Fig. 8b ). Other species of Auchenoglanis lack a plain body color and the very small and noncoalescing spots on the rayed fins (the spots on the fins of A. senegali are larger).

Adult A. biscutatus can be further distinguished from all other Auchenoglanis by having the anterior margins of the nuchal plates strongly curved rather than straight to slightly curved ( Fig. 1b ). Auchenoglanis biscutatus can be further distinguished from A. senegali , A. occidentalis , and A. tchadiensis by having a longer maxillary-barbel length relative to the head length (MxBL to HL ratio 0.50–0.76 in A. biscutatus versus less than 0.50 in A. senegali , A. occidentalis , and A. tchadiensis ). In addition, A. biscutatus can be distinguished from A. senegali and A. occidentalis by having a longer maxillary barbel length relative to the preorbital-head length (MxBL to PrOL ratio 0.9–1.4 in A. biscutatus versus less than 0.9 in A. senegali and A. occidentalis ). Auchenoglanis biscutatus can be further distinguished from A. senegali , A. occidentalis , A. wittei , A. tanganicanus , and A. acuticeps by having a shorter preorbital-head length relative to the interorbital distance (PrOL to IO ratio 0.9–1.4 A. biscutatus versus greater than 1.5 in A. senegali , A. occidentalis , A. wittei , A. tanganicanus , and A. acuticeps ).

Adults of A. biscutatus can be further distinguished from the other adult Auchenoglanis species except A. senegali by the anterior edge of the adipose fin that rises steeply and reaches its maximum height at a point approximately one half of the length of the adipose fin versus having the anterior edge of the adipose fin that rises gradually to its maximum height at the posterior end of the fin, well behind the mid-point of the adipose fin. The origin of the adipose fin is before the anal-fin origin in A. biscutatus versus over it in A. sacchii . Auchenoglanis biscutatus can be distinguished from A. tchadiensis , A. occidentalis , and A. senegali by its black external and internal mandibular barbels versus white or grey barbels. The round premaxillary tooth patches ( Fig. 2b ) of A. biscutatus separate it from the oblong-oval, square, tear-shaped tooth patches or triangular tooth patches of all the other species of Auchenoglanis .

Description: Morphometrics are presented in Table 3. The fins ray counts are as follows: dorsal fin II,7 (5); pectoral fin I,7 (3), I,9 (2); pelvic fin i,5 (5). The head is large with the dorsal edge in the lateral profile rising steeply from the tip of the snout to the orbit then less steeply to the dorsal-fin origin; the ventral edge concave to straight from tip of snout to the lower posterior margin of the opercle. Body depth is greatest at the dorsal-fin origin. The dorsal-lateral profile descends less steeply from the dorsal-fin origin to the caudal peduncle with a slight rise and descent under the adipose fin. The minimum caudal-peduncle height is at the posterior edge of the base of the adipose fin. The ventral-lateral profile from the rear of the head to the anal fin is flat and slightly concave under the caudal peduncle. In adults the leading lateral edges of the nuchal plates are strongly curved. The leading lateral edges do not converge to form a point at the anterior end of the first nuchal plate and that plate does not form a notch in the rear margin of the supraoccipital.

The middle rays of the dorsal fin extend somewhat beyond the spine and the first and last rays. The dorsal spine is stout and nonserrated on the posterior edge. The distal edge of the pectoral fin is straight. The pectoral spine is very stout and serrated on the posterior edge. The distal edge of the pelvic fin is convex with the second branched ray being the longest ray. The rays of the anal fin are much longer than the base and the second or third branched ray of the anal fin is longest. The distal edge of the caudal fin is straight to slightly emarginate.

The pectoral fin is placed approximately on the anterior one quarter of the body, well ahead of the dorsalfin origin. The pelvic fin is placed behind the dorsal-fin origin. The anal fin is placed approximately on the posterior one third of the body.

The maxillary barbel is approximately equal to the length of the external mandibular barbel. The internal mandibular barbel is much shorter than the other barbels. The maxillary and external mandibular barbels are slender, but the internal mandibular barbel is thicker particularly at the base. The lower lip is thin.

The mouth is subterminal and narrow relative to the head width. The premaxillary tooth patches are round in shape.

The color is based on examination of specimens preserved in alcohol. The body of large specimens is uniformly brown on the sides of the body and dorsally and white ventrally. The other fins have distinct small spots that do not fuse to form bands. Vague dark spots are on the sides of the body and adipose fin of juveniles. The barbels are dark. The sides and top of the head are brown and without spots. The lower side of the head is white including the lower lip.

Range: Auchenoglanis biscutatus is restricted to the Nile basin ( Fig. 4 ).

Figures

FIGURE 1. Diagrams of nuchal plate patterns of Auchenoglanis. a) leading edge straight, b) leading edge curved. The arrow indicates the anterior end of the plates. The numbers indicate individual plates.

FIGURE 2. Diagrams of the five types of premaxillary tooth patches of Auchenoglanis. a) oblong-oval, b) round, c) square, d) tear, and e) triangle. The arrow indicates the direction of the anterior end of the patches.

FIGURE 4. Collection localities of Auchenoglanis acuticeps (black squares), A. biscutatus (black triangles), A. sacchii (open circle), and A. tchadiensis (black circles). Symbols may represent multiple collections.

FIGURE 7. Lateral views of a) Auchenoglanis tchadiensis MRAC P–154047, 205 mm SL, b) A. biscutatus modified from Boulenger (1907), and c) A. occidentalis MRAC P–73005.2810, 262.0 mm SL.

FIGURE 8. Spot patterns of the caudal and anal fins of a) A. senegali (MRAC P–79002.211) and b) A. biscutatus (ZMB 16556).