Diagnosis: Synodontis abditus can be distinguished from all its congeners of the Lower Guinea ichthyological province by the following unique combination of characters: no serrations on anterior surface of the dorsal spine, vs. entirely serrated in S. albolineatus, S. batesii and S. woleuensis; a relatively high number of mandibular teeth, 63-76 vs. lower, 12-39 in S. acanthoperca, S. carineae, S. marmoratus, S. ngouniensis, S. obesus, S. punu, S. rebeli, S. robbianus, S. schall, S. steindachneri and S. tessmanni; and a relatively high number of serrations on posterior side of the pectoral spine, 16-28 for specimens of 72.2-152.7 mm standard length, vs. 16-31 in S. haugi and S. polyodon, with positive allometry (Ref. 126042).

Description: A medium-sized Synodontis species with maximum recorded size of 164.7 mm standard length; head width at base of pectoral fins larger than head depth at base of pectoral fins; elongate species with more or less straight dorsal and ventral surfaces, when viewed laterally; maximum body depth near origin of dorsal fin, slightly decreasing towards level of anal-fin origin; dorsal profile of head slightly convex from tip of snout to base of dorsal spine; skull strongly ossified, with numerous fleshy tubercles on snout; head broad with pointed snout; skin villous (Ref. 126042). Lateral line complete and situated along lateral midline of body, curved slightly upward at level of humeral process (Ref. 126042). Gill opening situated laterally above pectoral fin, not extending beyond base of pectoral fin; operculum without spine; gill rakers long and smooth, 16-20 on ceratobranchial of first gill arch (Ref. 126042). Three pairs of barbels; maxillary barbel long, fine, unbranched and without crenulations, reaching slightly beyond base of pectoral spine with minute basal membrane, i.e. towards its base, much narrower than barbel thread; outer mandibular not reaching pectoral-spine base, with long, slender ramifications; secondary branches very short; inner mandibular barbel reaching beyond anterior border of eye, with short, often paired ramifications; numerous short secondary branches present (Ref. 126042). Eye in posterolateral position and of moderate size; orbit with free margin; interorbital area flat; mouth inferior; lips curved with prominent fleshy lateral lobes; lobes covered with rounded papillae; upper lip with fringed rostral flap; two irregular rows of unicuspid teeth on ventral shelf of premaxillary toothplate; long and slender, 63-76, mandibular teeth, arranged in band as large as eye diameter; distal tip of teeth clearly curved upwards (Ref. 126042). Dorsal spine striated, slightly curved near its tip; entirely smooth along anterior margin, but with 8-19 serrations on posterior margin; dorsal-fin base rather short with distal edge of fin slightly convex; adipose fin moderately deep and with rounded outline, well separated from dorsal fin, situated right above anal fin and twice longer than it; pectoral fin with strong spine, first branched ray produced into short filament; pectoral spine covered with 25-46, outwardly direct serrations on entire anterior surface of spine; pectoral spine covered with 16-28, inwardly directed serrations on outer 3/4 up to 5/6 of posterior surface of spine; serrations larger distally and more numerous with increasing body size; anal fin with convex distal margin; pelvic fin not reaching level of anal fin origin; caudal fin forked with pointed lobes; upper lobe only slightly longer than lower (Ref. 126042). Humeral process triangular, flat, granulated above and with ridge; lower margin of humeral process convex, upper margin variable; no slit-like axillary pore just ventral to humeral process (Ref. 126042).

Colouration: Life colour pattern unknown (Ref. 126042). Preserved specimens with dark brown flanks marbled with light brown in smaller specimens of less than 148 mm standard length, becoming less conspicuous in larger specimens; head brown; abdomen and ventral part of head whitish to light brown; adipose fin sometimes marbled or with large brown dots; pectoral and pelvic fins dark brown; anal fin dark brown with white base; caudal fin with conspicuous dark brown spots, sometimes forming vertical lines; dorsal and pectoral spines whitish; maxillary barbels dark brown and mandibular barbels cream coloured; lips white (Ref. 126042).