Petrocephalus binotatus Pellegrin, 1924
Original Published Description:
Petrocephalus binotatus, described by Pellegrin (1924), is a small sized species within the genus Petrocephalus (maximum standard length observed = 88.4 mm, holotype = 83.2 mm). Body ovoid, body 2.3 to 2.8 longer than high (holotype = 2.4) and laterally compressed. Head length between 3.6 and 4.0 times in standard length (average = 3.9, holotype = 3.6). Snout short and round. Eye large (3.5 ≤ Head length/eye diameter ≤ 4.0, average = 3.6, holotype = 3.6). Mouth small (4.4 ≤ Head length/head width ≤ 5.2, average = 4.7, holotype = 3.2), sub–terminal, opening under the anterior half of the eye. Dorsal fin shorter than the anal fin (anal fin length/dorsal fin length ≥ 1.5, range = 1.5–1.7). Dorsal fin with 20-24 branched rays. Anal fin with 30-33 branched rays. Teeth small and bicuspid, 16 teeth or fewer (range = 8–16, median = 10) in the upper jaw, 24 teeth or fewer (range = 19–24, median = 21) in the lower jaw. Mouth sub–terminal; ratio of head length to mouth position between 3.9 and 6.1. Scales cover the body, except for the head. Lateral line visible and complete with 37 to 38 pored scales along its length. Eleven to 13 scales (median = 12), between the anterior base of the anal fin and the lateral line. Caudal peduncle thin (1.8 ≤ Caudal peduncle length/Caudal peduncle depth ≤ 2.2, average = 2.0, holotype = 2.3). Twelve scales around the caudal peduncle. Skin on head thick, becoming opaque with formalin fixation. Three distinct rosettes of Knollenorgans are present on the head (Augenrosette, Nackenrosette and Kehlrosette).
Body uniformly white-silver, with the presence of three characteristic melanin marks on each side of the body: (1) a distinct, approximately round/oval black mark situated slightly anterior to the dorsal fin, sometimes covering only a few scales; (2) a black spot at the base of the pectoral fin; (3) an 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 translucent.
Petrocephalus binotatus is distinguished from all other Petrocephalus species from Lower Guinea and Congo provinces by the following combination of characteristics. Dorsal fin shorter than the anal fin (1.5 ≤ anal fin length/dorsal fin length, range = 1.5–1.7). Dorsal fin with at least 20 branched rays (range = 20–24). Anal fin with at least 30 branched rays (range = 30–33). Sixteen teeth or fewer (range = 8–16) in the upper jaw, 24 teeth or fewer (range = 19–24) in the lower jaw. Eye relatively large (Head length/eye diameter ≤ 4.0, range = 3.5–4.0,). Mouth sub–terminal; ratio of head length to mouth position between 3.9 and 6.1. Unique pigmentation pattern with the presence of three well defined black patches: (1) a distinct (although sometimes covering only few scales), more or less round/oval, black mark situated slightly anterior to the dorsal fin on each side of the body; (2) a black mark at the base of each pectoral fin; (3) an ovoid black mark centered at the base of the caudal fin that does not extend onto the upper and lower fleshy lobes of this fin. EOD of normal polarity (i.e., first major phase head–positive).
Electric Organ Discharge
Statistics for waveform landmarks and other EOD measurements are provided by Lavoué et al. (2008) for specimens recorded in Odzala National Park. No EOD recordings are available for the holotype or other specimens collected near the type locality (Ikengo, Congo River).
Petrocephalus binotatus males and females produce EODs with overall waveforms that are typical for the genus, being similar to EODs produced by many other congeners: mainly biphasic of short duration. However, this is one of the few Petrocephalus species for which possible EOD sex differences have been detected thus far in field recordings. As with other Petrocephalus species exhibiting possible sex differences, magnitudes of the differences between males and females are small. In Odzala, for example, mean EOD duration (± std. dev.) is 0.330 ± 0.074 msec in obvious adult males and 0.270 ± 0.033 msec in other adult and sub–adult individuals, based on 1.5% voltage deviations from baseline relative to peak-peak amplitude.
Electrocytes are assumed to be of type NPp based on characteristics of the EOD, although electrocyte anatomy has not yet been confirmed histologically
to 88.4 mm SL
This species appears to be gregarious and active at night, with dozens of specimens forming schools in the Lékoli River system (Congo basin)
Ecology and Distribution
Endemic to the central Congo basin. Holotype from the locality Ikengo on the Congo River just below Mbandaka. One of the most abundant Petrocephalus species in the Odzala National Park (Republic of Congo).
Evolution and Systematics
Within the genus Petrocephalus, P. binotatus forms a monophyletic group with two other Central Africa-endemic species, P. balayi and P. odzalaensis.
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