Petrocephalus microphthalmus Pellegrin, 1908

Lavoué, Sébastien, Sullivan, John P. & Arnegard, Matthew E., 2010, African weakly electric fishes of the genus Petrocephalus (Osteoglossomorpha: Mormyridae) of Odzala National Park, Republic of the Congo (Lékoli River, Congo River basin) with description of five new species, Zootaxa 2600, pp. 1-52 : 19-22

Petrocephalus microphthalmus Pellegrin, 1908

Petrocephalus microphthalmus Pellegrin (1908) : 185.

[Odzala field identification: Petrocephalus sp. 5, OTU 5]

Images. Fig. 7 A, photo of a live specimen from Odzala, Fig. 7 B, photo of a preserved specimen from Odzala and Fig. 14 , drawing from Poll (1967) of a specimen collected in Angola. Photo of the holotype (MNHN 1908–211) in Lavoué et al. (2004).

Type material. Holotype, MNHN 1908–211, male, 73.7 mm SL. Gabon, Ogooué basin at Ngomo (Lower Ogooué) [estimated 0.82S, 9.95E], E. Haug coll.

Other specimens. We examined 14 other specimens from Odzala National Park (specimen list provided in the section "additional material examined"). A list of additional specimens examined from Lower Guinea is given in Lavoué et al. (2004).

Diagnosis. We prepared the following diagnosis using all the specimens of P. microphthalmus that we examined, regardless their geographic origins. Petrocephalus microphthalmus is distinguished from all other Petrocephalus species in Central Africa by the following combination of characteristics. Short dorsal fin with only 18 or fewer branched rays (range 15–18). Long anal fin with 23–27 branched rays. Eye small (4.0 ≤ HL/ ED, range = 4.1–4.8). Mouth moderately wide (3.5 ≤ HL/MW ≤ 4.9). Only 9–11 teeth in the upper jaw, 14–20 teeth in the lower jaw. Absence of black pigment patches, except for a characteristic black blotch on the anterior dorsal fin rays near the origin of this fin. Body silvery/purplish, iridescent. Electroreceptors on the head are not clustered into "rosettes" but, instead, appear as isolated receptor pores. EOD of normal polarity with two main phases and, in Odzala, a third minute phase of very low amplitude.

Description. Morphometric ratios and meristic data for the holotype (from Gabon) and the non-type specimens (from Odzala and Lower Guinea) are presented in Table 5 . However, the following description is based only on the Odzala specimens we examined, except where we make separate reference to the holotype. With a maximum observed standard length of 59.1 mm in Odzala National Park, P. microphthalmus is the smallest species in the diverse Odzala assemblage of Petrocephalus . Body ovoid, longer than high (2.6 ≤ SL/ H ≤ 2.8, holotype = 2.7) and laterally compressed. Head length between 3.8 and 4.0 times in standard length (average = 3.9, holotype = 4.0). Snout short (5.2 ≤ HL/SNL ≤ 6.4, average = 5.8, holotype = 4.6) and round. Mouth small (3.6 ≤ HL/MW ≤ 4.2, average = 3.9, holotype = 3.8), opening under the eye. Teeth small and bicuspid, 9–11 (holotype = 10) in a single row in the upper jaw, 14–16 (holotype = 20) in a single row in the lower jaw. Dorsal and anal fins originate in the posterior half of the body (SL/PDD = 1.5 and SL/PAD = 1.7). Pre-dorsal distance slightly greater than pre-anal distance (PDD/PAD ≈ 1.1). Dorsal fin with 16–18 branched rays (holotype = 16). Anal fin with 26–27 branched rays (holotype = 25). Scales cover the entire body, except for the head. Lateral line visible and complete with 34 to 35 pored scales along its length (holotype = 36).

Eight to 10 scales between the anterior base of the anal fin and the lateral line. Twelve scales around the caudal peduncle. Skin on head thick, becoming opaque with formalin fixation, containing numerous Knollenorgan electroreceptors that do not form "rosettes" in their typical positions. Instead, Knollenorgans appear as isolated receptor pores in the skin covering the head, the character state observed in the Mormyrinae .

Holotype Specimens Specimens from

(m)* from Odzala Gabon (Ogooué

(n=3) River) (n=36)*

Live coloration ( Fig. 7 A). Body generally blue-gray, with the dorsum darker than the abdomen. Numerous chromatophores occur below the skin surface. This species can appear metallic blue to violet depending on the angle and intensity of illumination. The color is especially intense on the operculum. The fins are translucent except for the first dorsal fin rays, which are black near their insertion ( Lavoué et al., 2004).

Distribution ( Fig. 1 ). Present in Congo and Lower Guinea provinces. Holotype from Gabon. Abundant in Odzala, where we collected P. microphthalmus in small tributary creeks flowing through forest. Elsewhere in the Congo basin it is present in the Lower Congo River in the vicinity of Brazzaville. In the Lower Guinea province this species is widespread throughout the entire Ogooué and Ntem basins in Gabon, including streams and lakes associated with main river channels (e.g., Lac Zilé). It can also be found along the coastal region from the Sanaga River ( Cameroon) in the north to the more southern Niari-Kouilou River ( Republic of the Congo).

Electric organ discharge ( Fig. 7 C). Petrocephalus microphthalmus produces EODs of short duration, which are typical of the entire genus. No sex differences have yet been reported in any population. Similar EOD durations have been observed in the Odzala population of P. microphthalmus [range = 0.252 – 0.511 msec; Lavoué et al. (2008)] and among conspecifics from Gabon [range = 0.380 – 0.561 msec; Lavoué et al. (2004)]. A relatively long, slow rise characterizes the initial part of the first head-positive phase in EODs recorded from the Odzala population, often resulting in a shoulder early during the waveform’s head-positive rise to P1, the first main peak ( Lavoué et al., 2008). The early head-positive rise and shoulder are very low in amplitude, however, such that they may only be apparent at high amplifier gain. These subtle waveform features appear to be uncommon in EODs of other Petrocephalus species. A similar slow rise (and shoulder) preceding P1 has been recorded among a small number of P. microphthalmus individuals from Gabon, but it seems to be much less common than in Odzala. Additional statistics for waveform landmarks and other EOD measurements for the Odzala population are provided by Lavoué et al. (2008). Based on histological examination, electrocytes are known to be type NPp ( Lavoué et al., 2008; Sullivan et al., 2000).

Remarks. Petrocephalus microphthalmus closely resembles Petrocephalus schoutedeni Poll, 1954 and Petrocephalus catostoma haullevillii Boulenger, 1912 . Petrocephalus microphthalmus is distinguished from P. schoutedeni mainly by the shape of the caudal peduncle: in the Odzala specimens its length is only 2.7 times (holotype = 2.9 times) its depth, versus 3.4–3.7 times in P. schoutedeni . We did not find any unambiguous morphological differences with which to distinguish P. microphthalmus and P. catostoma haullevillii .

Figures

FIGURE 1. Map of Africa (right) showing the entire Congo River and its main tributaries together with the neighboring Lower Guinea province (first inset at center). The boundary between the Congo and Lower Guinea provinces is shown by a dashed line. Filled black circles indicate type localities for all Petrocephalus species that were described from either province and were considered valid just prior to the present study. Exact type localities are unknown for P. b a l a y i and P. sauvagii. Based on our understanding of the chronology of Central African exploration at the time of the descriptions, we estimate these type localities to be the following: the vicinity of Pool Malebo on the Lower Congo River near the modern cities of Brazzaville and Kinshasa [4.25 ° S, 15.29 ° E] for P. sauvagii; and the vicinity of the present-day city Lambaréné on the Lower Ogooué River [0.7 ° S, 10.22 ° E] for P. b a l a y i. Second inset at the far left: detailed map of Odzala National Park, located in the Congo River basin. Within the boundaries of the park, forest is shown in green and savannah in yellow. The red oval indicates our collection area (a segment of the Lékoli drainage system, including the lower parts of two of its tributaries, the Pandaka and Lékénie Rivers).

FIGURE 7. Petrocephalus microphthalmus of the Lékoli River system of Odzala National Park, Republic of the Congo. A. Photograph of a live specimen (no scale). B. Photograph of a preserved specimen (scale bar = 1.0 cm). C. EOD waveforms. One EOD was arbitrarily selected from each recorded individual and superimposed with the other recordings (N = number of individuals). All EODs are scaled to the same peak-to-peak voltage, and they are plotted (head-positive up) on the same time scale as overlays centered on the largest positive peak of each waveform (scale bar = 0.2 msec).

FIGURE 14. Historical illustrations of Petrocephalus species described from Congo or Lower Guinea that occur in Odzala. The drawings were reproduced from the following sources: Petrocephalus binotatus from Pellegrin (1928); Petrocephalus balayi from Boulenger (1909 – 1916); Petrocephalus microphthalmus from Poll (1967); Petrocephalus christyi from Boulenger (1920); Petrocephalus sauvagii from Boulenger (1909 – 1916); and Petrocephalus grandoculis from Boulenger (1920).

Tables

TABLE 5. Principal morphometric ratios and meristic counts for the holotype (MNHN 1908 - 211), three specimens from Odzala (CU 87938, CU 87940, CU 88003) and 36 specimens from Gabon of Petrocephalus microphthalmus Pellegrin, 1908. (Abbreviations: m = male; Std – Dev = standard deviation; Min – Max = minimum – maximum).

	Min–Max	Mean	Std–Dev Min–Max	Mean	Std–Dev
Standard length (mm) 73.7	51.8–59.1	54.7	3.8 52.1–69.9	59.4	4.6
Head length (mm) 18.3	13.4–14.9	14.0	0.8 14.0–16.7	15.1	0.9
Ratio of standard length (SL):
SL/body height (H) 2.7	2.6–2.8	2.7	0.1 2.7–3.2	3.0	0.2
SL/head length (HL) 4.0	3.8–4.0	3.9	0.1 3.6–4.3	3.9	0.2
SL/pre-dorsal distance (PDD) 1.5	1.5–1.5	1.5	0.0 1.5–1.7	1.6	0.1
SL/pre-anal distance (PAD) 1.7	1.7–1.7	1.7	0.0 1.6–1.9	1.8	0.1
SL/anal fin length (AFL) 5.4	5.0–5.3	5.2	0.2 4.9–6.5	5.5	0.4
SL/dorsal fin length (DFL) 3.3	3.3–3.4	3.3	0.0 3.2–3.9	3.5	0.2
SL/caudal peduncle length (CPL) 5.5	5.5–5.9	5.8	0.2 4.4–5.7	5.1	0.3
SL/mouth width (MW) 15.4	14.2–16.3	15.4	1.1 13.7–19.2	16.3	0.8
Ratio of head length (HL):
HL/snout length (SNL) 4.6	5.2–6.4	5.8	0.6 4.1–6.2	4.7	0.4
HL/mouth width (MW) 3.8	3.6–4.2	3.9	0.3 3.5–4.9	4.2	0.4
HL/eye diameter (ED) 4.8	4.0–4.5	4.2	0.2 4.1–4.8	4.5	0.2
HL/interorbital width (IOW) 2.6	2.6–3.1	2.8	0.3 2.0–2.8	2.5	0.2
HL/head width (HW) 1.7	1.6–2.0	1.8	0.2 1.7–2.0	1.8	0.1
HL/mouth position (MP) 3.4	3.6–4.2	3.9	0.3 2.8–3.7	3.2	0.2
Ratio of caudal peduncle length (CPL):
CPL/caudal peduncle depth (CPD) 2.9	2.7–2.7	2.7	0.0 2.6–3.7	3.0	0.4