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Nile Knifefish

A fisherman wading in Africa’s Nile River should toss the line with caution. The adult Nile knifefish (Gymnarchus niloticus) will aggressively bite anything that moves, even if it’s a knee-high rubber boot. But this freshwater fish is not known for its chomp. It’s what comes out the other end that counts—electricity.

The Nile knifefish, the only species in the family Gymnarchidae, resembles an electric eel with its dowdy coloring and long, tapering body. Both fishes have specialized organs that generate electric fields. While the eel’s electrical discharge can kill a horse, the knifefish’s wavelike signal is too faint to even stun prey. Instead the pulses enable the knifefish to plot its course, detect food or foes a few feet away, and exchange such information as species and mating or warning signs. These are optimal responses to the Nile’s muddy unpredictability.

While most fishes wiggle, the Nile knifefish keeps a relatively straight spine whether swimming forward and backward or turning. The only thing that ripples is the fin along its back. Scientists think this rigidity makes it easier to interpret electrical fields. Though the knifefish has poor vision, it can navigate the silted waters with acute precision.

Just as each radio station has its own spot on the dial, each knifefish has its own frequency of discharge. But if two individuals swim too close to one another, their frequencies jam up, deteriorating their ability to sense objects. Each fish shifts its frequencies, with what’s called a jamming avoidance response, until they are disparate enough to move on.

Electric fishes may seem beyond our ordinary understanding. To escape your learning gap, turn the dial to “WNIF” or head down to Shedd Aquarium to see a live broadcast by the Nile knifefish in the Waters of the World galleries.

 

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