The elephantnose fish Gnathonemus petersii is a freshwater fish that lives in slow-moving and muddy rivers in Africa. In these murky, dark waters, using vision to sense its surroundings is virtually impossible. This presents a challenge for avoiding predators and finding prey.
Elephantnose fish solve this problem by navigating their environment with an electric sense. Modified muscle cells near the fish’s tail act as an electric organ that emits electric signals and generates a weak electric field around the fish. When objects nearby modify that electric field, special receptor cells all over the fish’s body can sense these changes in the electric signals. By actively generating and sensing an electric field, the fish can map out its surroundings through a process called electrolocation (similar to the way a bat uses sound to detect objects and prey).
The elephantnose fish’s special elongated chin, which gives the fish its name, is especially loaded with electroreceptors. The fish uses its chin or ‘Schnauzenorgan’ much like a human might use a handheld metal detector at the beach. While foraging, the fish swims forward and sweeps its chin back and forth until it finds an object of interest, which may be a tasty worm or insect buried in the mud. It then approaches that object and uses its chin to explore more closely. If it’s a prey item, the fish may use its chin to dig into the mud. If the object is larger or unknown, the fish can swim all around the object and “scan” it with its chin and the other electroreceptors on its body.
The vast number of electroreceptors enables the fish to determine not only the shape of an object, but its volume, size, material, and maybe even which direction it’s facing. The elephantnose fish can use the images formed through electrolocation to navigate its environment, avoiding hitting objects as it swims while hunting smaller prey. These images aren’t perfectly clear, but are like someone near-sighted seeing a blurry world when they don’t have their glasses on. This doesn’t affect the fish too much, as basic shapes and patterns are all that are needed to survive and thrive.
This summary was contributed by Thomas McAuley-Biasi.
“Weakly electric fishes generate electrical fields around their bodies by emitting electric signals (electric organ discharges, EODs) with a specialized electric organ.” (von der Emde and Fetz 2007:3082)
“If an object is present near the fish, it causes distortions of the electrical field lines, which change the voltage patterns on the skin of the animal opposite the object. The changed pattern is detected by electroreceptors located all over the fish’s skin.” (von der Emde and Fetz 2007:3082)
“Because there is no focusing mechanism, electrical images are always blurred, or “out of focus” and, in this respect, are fundamentally different optical images that are projected onto the retina of a vertebrate eye.” (von der Emde and Fetz 2007:3082-3083)
“Our experiments showed that during active electrolocation, weakly electric fish can do much more than just detect objects and determine their electrical resistance (Lissmann and Machin, 1958). Instead, even in complete darkness, they can perceive parameters such as the volume, size, 3-D shape, contour, material and possibly the orientation of an object.” (von der Emde and Fetz 2007:3090)
“In addition, we studied the receptor mosaic of the sensory epithelium, which in the case of G. petersii constitutes almost the entire body surface…”(von der Emde et al., 2008:280)
“Prereceptor mechanisms also ensure that the moveable skin appendix of G. petersii, the ‘Schnauzenorgan’, receives an optimal sensory signal during all stages of its movement.” (von der Emde et al., 2008:279)