Gymnarchus niloticus Cuvier, 1829
Species overview
Large fish, body deep, pear-shaped in cross section, tapering towards the end which is without a caudal fin, dorsal fin very long, pelvic and anal fins absent: type of a knife-fish; head long, depressed, snout long, upper jaw feebly protruding lower jaw, mouth hence slightly subterminal, rather long. Olive or brownish above, lighter beneath, slightly countershaded. Size observed: May attain 2 m or more.
Taxonomic description
-Body much elongate, cross section pear-shaped, keeled towards the back. body depth 8-14 % SL, greatest depth somewhat behind pectoral fins.
-Head length 9-14 % SL; head depth 40-50 % head length, head dorso-ventrally depressed, wider than deep; upper profile and underside of head concave; back more or less straight, belly slightly curved. Neither pelvic, anal fin nor caudal fin present, body eel-shaped but much more tapering off, dorsal fin ending shortly before to tip of tail. (Tip of tail often lost but regenerates easily as.) Snout long, shaped like a ducks bill, its tip rounded, scarcely protruding lower jaw.
-Mouth slightly subterminal, its edge nearly reaching to a line below the eye, upper jaw on a level with upper edge origin of pectoral fin.
-Chin without appendage, less deep than upper part of snout.
-Teeth on jaws not indicated in description, no teeth on parasphenoid and tongue.
-Nostrils wide separate, about halfway between to end of snout and eye, both oval, their axis running more or less parallel to axis of head.
-Eye small, diameter about 3-4 % head length. (Eye not rudimentary, the structure of the retina corresponds to that in the other Mormyridae or Osteoglossomorpha, respectively; the number of large optical ganglion cells (per 400 µm2) is even higher than in most mormyrids, though less than in other osteoglossomorpha.) Opercular cleft very long, a strong fold of skin connects both sides across the isthmus.
-Dorsal fin very long, 183-230 fin-rays, originating above pectoral fin. It is the only driving means and by undulating it moves the body forth and back with same ease.
-Pectoral fin rounded, very short, looking somewhat rudimentary. Pelvic fin absent. Scales very small, largest ones more or less along the flanks around the lateral line.
Builts floating nests between grasses, eggs are of the size of a pea, yolk-sac larvae and early larve with external gills.
-Coloration »Olive or brown above, whitish beneath, end of tail blackish«.
Type locality
»Nil« Egypt.
Distribution
Nile, »Upper Nile to Bahr al Ghazal (8° 15 ‘ N 20° 50 ‘ E), Lake Rudolf = Lake Turkana (3° 30 ‘ S 36° 00 ‘ E), Niger, Chad, Senegal and Gambia basins«.
Electric organ discharges
Gymnarchus niloticus is the only species of the Mormyriformes with wave-type discharges, all members of the mormyridae are pulse-type discharges. The single pulse of the waves in Gymnarchus is nearly monophasic, head positive (+/– ), the negative phase being rather low. Frequencies between 180 (260 and 340) and more than 400 Hz have been observed, mainly depending on the specimen, minor changes are due to changes in the temperature of the water. Generally the frequency-shift of a particular fish (constant temperature provided) is below 1 % / day.
G. niloticus(EOD)