Petrocephalus balayi Sauvage, 1883

Mormyrus catostoma – Günther (1867): 116 (non Mormyrus catostoma Günther, 1866). Petrocephalus balayi Sauvage (1883): 159.

Mormyrus ballayi – Sauvage (1884): 195.

Petrocephalus ballayi – Pellegrin (1908): 185. - Boulenger (1909 –1916): 52. - Gosse (1984): 108. Mormyrus amblystoma Günther (1896): 281. - Boulenger (1909 –1916): 52. [Odzala field identification: Petrocephalus sp. 4, OTU 4]

Images. Fig. 6 A, photo of a live specimen from Odzala, Fig. 6 B, photo of a preserved specimen from Odzala and Fig. 14, drawing of the holotype of Mormyrus amblystoma from Boulenger (1909 –1916). Photos of the preserved holotype of Petrocephalus balayi in Lavoué et al (2004) and Harder (2000).

Type material. Holotype, MNHN A 6297, sex undet., 85.5 mm SL. Gabon, Ogooué River, without more precision, Expedition Savorgnan de Brazza, Noël Ballay coll.

Other specimens. We examined two other specimens from Odzala National Park (see specimen list provided in the section "additional material examined"). We also examined other specimens from the Lower Guinea province [listed in Lavoué et al. (2004)].

Diagnosis. The following diagnosis is based on all examined specimens of P. balayi, regardless their geographic origins. Petrocephalus balayi is distinguished from all other Petrocephalus species in Central Africa by the following combination of characteristics. Dorsal fin with 20–22 branched rays. Anal fin with 26 or 27 branched rays. Eye small (4.5 ≤ HL/ED, range = 4.5–4.9). Mouth very wide (HL/MW ≤ 3.9, range = 2.7–3.9), associated with a very characteristic head shape when viewed from below. Fourteen teeth or more in the upper jaw (range = 14–21). Twenty-eight teeth or more in the lower jaw (range = 28–38). Melanin pattern consisting of the following: (1) a distinct black round mark on each side of the body below dorsal fin origin; (2) an ovoid mark on each side at the base of the caudal peduncle, not extending onto the upper and lower parts of the caudal fin; (3) a black mark, sometimes diffuse but always present, at the base of the pectoral fins. The EOD is of normal polarity.

Description. Table 4 presents morphometric ratios and meristic data for the holotype (from the Ogooué River in Gabon) and for non-type specimens (from Gabon or Odzala) separately. Data given in the following description (e.g., ranges) correspond to the two Odzala specimens we examined, except where explicit reference is made to the holotype. Petrocephalus balayi, described by Sauvage (1883), is a large, robust species within the genus Petrocephalus (maximum SL observed in Odzala = 95.6 mm SL, holotype = 85.5 mm SL). Body ovoid, longer than high (SL/H = 2.5–2.7, holotype = 2.8) and laterally compressed. Head length between 3.3 and 3.4 times (holotype = 3.3) in standard length. Head width 1.9 times (holotype = 2.2) in head length. Snout short (8.4 ≤ HL/SNL ≤ 9.3, holotype = 8.1), wide and square-shaped. Mouth wide (2.7 ≤ HL/MW ≤ 3.1, holotype = 3.4), sub-terminal, opening under the anterior half of the eye. Teeth small and bicuspid, 30 in a single row in the lower jaw and 20–21 in the upper jaw. Eye small (4.5 ≤ HL/ED ≤ 4.8, holotype = 4.6). Dorsal and anal fins originate in the posterior half of the body (SL/PDD = 1.6 and SL/PAD = 1.6), with pre-dorsal distance equal to pre-anal distance. Pre-dorsal distance slightly exceeds pre-anal distance in the holotype (1.0 ≤ PDD/PAD ≤ 1.1). Dorsal fin with 22 branched rays (holotype = 21). Anal fin with 27 branched rays (holotype = 26). Scales cover the entire body, except for the head. Lateral line visible and complete with 36 pored scales. Caudal peduncle relatively thick (1.7 ≤ CPL/CPD ≤ 1.8, holotype = 2.3). Twelve circumpeduncular scales. Skin on head thick, becoming opaque with formalin fixation. Knollenorgans organized into the three rosettes named by Harder (1968).

Holotype Specimens from Specimens from

(o)* Odzala (n=2) Gabon (Ogooué

basin) (n=7)*

Min–Max Min–Max Mean Std–Dev Live coloration (Fig. 6 A). Body gray/silver, slightly darker dorsally. The head is also slightly darker than the rest of the body. Iridescent pigment along side of body sometimes visible with correct orientation of light. Pigmentation pattern with three black patches: (1) a distinct round black mark on each side of the body below the dorsal fin origin; (2) an ovoid black mark on each side at the base of the caudal peduncle that does not extend onto the upper and lower parts of the caudal fin; (3) a black mark, sometimes diffuse in larger individuals but always present, at the base of the pectoral fins. The fins themselves are translucent.

Distribution (Fig. 1). Petrocephalus balayi occurs in the southern part of the Lower Guinea province and in the Congo River basin, including Odzala and the Lower Congo River (David & Poll, 1937; Lavoué et al., 2004; Poll, 1939; Sauvage, 1883). In Odzala, Petrocephalus balayi seems to prefer the small tributary creeks flowing through forest. In Gabon (Lower Guinea province), P. b a l a y i occurs in the lower courses of the Ogooué River and numerous associated lakes (and their tributary streams), as well as in small coastal creeks from south of the Ogooué to the Congo River basin. Records from the upper part of the Lower Guinea province and elsewhere appear to be misidentifications (pers. obs.).

Electric organ discharge (Fig. 6 C). EOD recordings are only available for a small number of individuals: one specimen from the coastal river Doumvou at Doumvou, Gabon [03.33S – 10.73E] (S. Lavoué & V. Mamonekene coll., 24 July 2001) (Lavoué et al., 2004); and two specimens from Odzala National Park, Republic of the Congo (Lavoué et al., 2008). Among these few recordings, the overall waveform of EODs produced by P. b a l a y i seems to be typical for the genus, similar to those produced by several other Petrocephalus species. EOD duration = 0.324 – 0.340 msec. Statistics for waveform landmarks and other EOD measurements are provided by Lavoué et al. (2008), who confirmed histologically that electrocytes in P. balayi are of type NPp.

Remarks. Almost all measurements and meristic counts of Odzala specimens fall within the range of values defined by the holotype and other specimens from Gabon. The EOD waveforms of the Odzala specimens and those from Gabon are similar (Lavoué et al., 2004).