From Lavoué et al (2004): Body ovoid, longer than high (2.6 < standard length/body height < 3.4) and compressed. Head length between 3.4 and 4.3 times in standard length. Snout short and round. Mouth relatively narrow (4.1 < head length/mouth width < 6.1), sub-terminal, just under the anterior half of the eye. Teeth small and bicuspid, 8 to 14 in a single row in the upper jaw, 16 to 22 in the lower jaw. Dorsal fin originates in the posterior half of the body (1.5 < standard length / pre-dorsal distance < 1.8). Pre-dorsal distance equal to, or slightly greater than, the pre-anal distance. Scales cover the body, except for the head. Lateral line visible and complete with 36 to 44 pored scales along its length. Caudal peduncle relatively thin (2.0< Caudal peduncle length / Caudal peduncle depth < 3.7, average 2.8). Twelve scales around the caudal peduncle. Skin on head thick, with numerous electroreceptors. Skin turns opaque with formalin fixation. Three rosettes of Knollenorgan electroreceptors are present on the head: the “Nakenrosette” behind the eye and dorsal to the opercular flap, the “Kehlrosette” anterior to the pectoral fin, and the “Augenrosette” anterior and slightly dorsal to the eye.
Body silver, slightly darker dorsally including the head. Sub-dorsal round black spot never present on the side, as some authors have mentioned previously. Fins translucent, except for the first rays of the dorsal fin, which appear black. Moreover, the first external rays of the caudal fin are pigmented black, in a crescent shape centred on the base of the caudal fin.
Petrocephalus simus is distinguished from all other Petrocephalus species from Central Africa by a combination of five characters: a minimum of 19 branched rays on the dorsal fin, very rarely 18 (range: 18-27; median 22); a minimum of 11 scales, rarely 10 (range: 10-16; median 12.4), between the anterior base of the anal fin and the lateral line; a sub-terminal mouth in which the ratio between the head length and the mouth position is between 3.8 and 6.3 (average of 4.7); a moderately wide mouth (width between 4.1 and 6.1 times in the head length, average of 5.0); and absence of a black spot near the base of the dorsal fin on either side of the body.
The electric discharge waveform begins with a head-positive phase (P1) of approximately 26% of peak-to-peak height, followed by a larger, head-negative phase (P2) with an amplitude of 74% of peak to peak height, followed by a third phase (P3) with an amplitude of 5.1%. The overall duration of the pulse is 436 ± 142 µs. The FFT of the EOD peaks at 4995 Hz.
To 105.7 mm SL
The phylogenetic position of Petrocephalus simus is unresolved, it may represent one of the earliest lineages within the Petrocephalus.
Petrocephalus simus is endemic to the Lower Guinea province where it occurs in the entire Ogooué River basin, as well as the basins of the Ntem, the Rembo Nkomi, the Nyanga and the Niari-Kouilou.