Petrocephalus okavangensis Kramer & Bills & Skelton & Wink 2012, sp. nov.

(Figure 2, no. 9; online Figure 10)

A brief overview of records of Churchills from the Okavango is given in Skelton et al. (1985: 5), including a review of the discussions about their identity. Based on their own ‘preliminary observations’, these authors retained the name P. catostoma, ‘pending further analysis of the problem’.

Type specimens. Holotype: SAIAB 030046 (specimen R22) from Botswana, Okavango, Thoage River, Nguma (Guma) Lagoon. Paratypes: SAIAB 186062 (41 specimens).

Type locality. Botswana, Okavango, Thoage River, Nguma (Guma) Lagoon, 18 ◦ 57 ′ 43.2 ′′ S, 22 ◦ 22 ′ 26.1 ′′ E.

Diagnosis. Length of anal fin, LA, mean 0.264 (range 0.247 –0.286) of SL; number of anal fin rays, nA, median 30 (range 27–32); head length, HL, mean 0.25 (range 0.232 –0.274) of SL; depth of caudal peduncle, CPD, mean 0.311 (range 0.264 –0.349) of CPL, length of caudal peduncle; preanal length, PAL, mean 0.576 (range 0.538 –0.619) of SL; dorsal fin length, LD, mean 0.17 (range 0.145 –0.195) of SL; number of scales around caudal peduncle, SPc, median 12 (range 12–12); body depth, BD, mean 0.291 (range 0.261 –0.348) of SL; number of scales in lateral line row, SLS, median 37 (range 37–38). EOD characteristics at 25 ◦ C and “2% threshold criterion” (see Material and methods) for n = 2: peak amplitude of P2 phase, P2amp < 0.13 of P1amp, peak amplitude of P1 phase; duration of P1 phase, P1dur, range 161–202 µs; duration of N phase, Ndur, range 65–83 µs; duration of P2 phase, P2dur, range 294–343 µs.

Description. Body oval shape (Figure 2, no. 9). Head with distinctly pointed nose in most specimens, with a small ventrally positioned subterminal mouth, situated ventral to the eye; head and body dorsolaterally compressed. Dorsal fin (a) origin situated about two-thirds of standard length from snout, (b) obliquely orientated, anteriorly higher and posteriorly lower, (c) distal margin crescentic with anterior two or three rays longer than posterior rays, and (d) number of rays 20 (n = 3), 21 (n = 13), 22 (n = 15), 23 (n = 8), 24 (n = 6). Anal fin (a) longer than dorsal fin, (b) opposite dorsal fin with slightly more anterior origin, (c) obliquely orientated, anteriorly lower and posteriorly higher, (d) anterior 10 or so rays longer than posterior ones, especially in males where they also appear stronger, (e) margin broadly rounded, (f) rays posterior to first 10 with distal margin straight, (g) number of rays 27 (n = 1), 28 (n = 1), 29 (n = 6), 30 (n = 21), 31 (n = 12), 32 (n = 4). Forked tail fin with rounded lobes. Scales cycloid with reticulate striae, scales extending anteriorly to operculum and pectoral fins (beyond pelvic fins). Scales in lateral series, 37 (n = 2), 38 (n = 1). Scales on caudal peduncle circumference, 12 (n = 45). Caudal peduncle slender, subcylindrical entire length, usually 20.7% (18.2–23%) of SL (Table 1). Electric organ discharge a triphasic pulse with strong head-positive phase P1 followed by head-negative main phase N, and very weak head-positive P2 phase (Figure 10). Pulse duration mean 574 (520–628) µs in females (n = 2); 25 ◦ C, 2% threshold criterion). Males with kink in anal fin base which is absent in juveniles and females where the anal fin base is straight.

Colour in preservation. Between ochre and light brown. Some preserved specimens’ dark spot below dorsal fin origin seems to have faded. Present at least in some individuals in all samples referable to P. okavangensis sp. nov., even when sampled in 1983.

Colour in life. Grey-silver or brownish-silver, then with gold-olive hue, anal fin base emphasized by black line, less distinct for dorsal fin base, first few rays of unpaired fins darker, irregular shaped dark spot under dorsal fin origin, sometimes visible only on one body side, throat and belly very fair, back dark, paired fins light and transparent.

Ecology. Guma Lagoon is a large (approximately 1 km 2) lagoon at the edge of the floodplain at the lower Okavango panhandle. It is on the Thoage channel, which is a thin, deep meandering channel off the main Okavango River. Although the lagoon has an inflow channel there is no obvious ouflow point with water seeping out along the southeastern edge. Water flows within the lagoon are effectively nil. The majority of the lagoon is fringed with papyrus and varied emergent vegetation. Petrocephalus were collected under papyrus root stocks. According to Skelton et al. (1985) “ P. catostoma (or a form thereof)” is a fairly hardy and eurytopic species, common in both swamp and river.

Distribution. The species is presently known from Popa Falls in Caprivi / Namibia, and from the point where the Okavango River fans out to form the delta (Botswana), and also from the northeastern sections of the delta, such as Gadikwe Lagoon and Xakanixa (Xakanaxa). According to Skelton et al. (1985), this species (or forms thereof) ranges much more widely (from north to south): it is present in Angolan headwaters of the Okavango, in the Namibian Okavango, in riverine floodplains of the Okavango (Botswana), in its permanent swamps, in the delta’s southern drainage rivers, in Lake Ngami in the southwest of the delta, and, following the course of the Botleti River, even in the distant southeast of the delta, Lake Xau / Mopipi Dam (Makgadikgadi Salt Pan region).

Etymology. The species name okavangensis refers to the Okavango River.

Remarks. Petrocephalus okavangensis sp. nov. in comparison with the other species is characterized by a very long LA, high nA, short HL, low CPD, shorter PAL and LD than P. longicapitis sp. nov., low SPc. EOD with weak P2amp (< 13% of P1amp), long P1dur (161–202 µs) and Ndur (65–83 µs), all different from P. longicapitis sp. nov. and P. tanensis, and more similar to P. wesselsi. If present, dark spot below dorsal fin origin seems to be distinctive with regard to P. magnitrunci sp. nov. that lacks a spot.