Diagnosis: Petrocephalus arnegardi is distinguished from all other Petrocephalus species of Central Africa by the following combination of characteristics: pigmentation pattern comprising three well-defined, bilateral black patches, one usually distinct round/ovoid subdorsal black mark situated slightly anterior to dorsal, one black mark at the base of each pectoral fin, and one ovoid black mark centered at the base of caudal fin; dorsal fin at least one third shorter than anal fin; dorsal fin with at least 20 branched rays but no more than 22; anal fin with at least 30 branched rays; 16 teeth or fewer in upper jaw, 25 teeth or fewer in lower jaw; eye relatively large, ration head length and eye-diameter less than 4.7; mouth subterminal; ration of head length to mouth position between 4.2 and 5.6; mouth small, ration head length and mouth width more than 4.1; and EOD of normal polarity, mainly biphasic with sometimes the presence of a small-amplitude positive third phase (Ref. 96601).
Description: Body ovoid, longer than high, its ration between 2.3 and 2.8, and laterally compressed (Ref. 96601). Head length between 3.4 and 4.0 times in standard length (Ref. 96601). Snout short, ration head length and snout length between 6.5 and 9.3, and round (Ref. 96601). Eye large, the ration head length and eye-diameter between 3.5 and 4.4 (Ref. 96601). Mouth small, the ration head length and mouth width between 4.1 and 5.0, subterminal, opening under the anterior half of the eye; teeth small and bicuspid, 8 to 16 in a single row in the upper jaw, 20-24 in the lower jaw (Ref. 96601). Caudal peduncle thin, the ration caudal peduncle depth and caudal peduncle height between 1.8 and 2.5 (Ref. 96601). Dosral and anal fins originate in the posterior half of the body; pre-dorsal distance slightly greater than the pre-anal distance; dorsal fin with 20-22 branched rays; anal fin with 31-34 branched rays (Ref. 96601). Scales cover the body, except for the head; lateral line visible and complete with 34 and 38 pored scales along its length; 10 to 13 scales between the anterior base of the anal fin and the lateral line; 12 scales around the caudal peduncle (Ref. 96601). Skin on head thick (Ref. 96601). The three rosettes of knollenorgans are present on the head (Ref. 96601).
Colouration: Body uniformly silvery white, with three distinct bilateral melanin marks, a distinct, ovoid black mark situated slightly anterior to the dorsal fin, sometimes covering only a few scales, a black spot at the base of the pectoral fin and a somewhat vertically orientated ovoid black mark centered at the base of the caudal fin that does not extend onto the upper and lower parts of the caudal fin; fins hyaline (Ref. 96601). Preserved colouration of body light brown, with head and dorsum slightly darker; thick skin on head opaque; numerous light melanophores on body, slightly larger ventrally from the snout to the anal fin; fins whitish/opaque (Ref. 96601).