Vision probably isn't much use if you live in sluggish, muddy waters. So several species of West African fish have opted for an alternative view. They perceive the world through a weak electric field, emitted by an electrical organ near the fish's tail. Jacob Engelmann says that most analyses of the electrical images perceived by these fish have focused on the electric field cast over the body surface, but explains that this does not square with the elephantnose fish's behaviour or physiology. According to Engelmann, the electric fish uses its trunk-like structure, known as the Schnauzenorgan, to probe soft sandy river bottoms and the Schnauzenorgan is covered in electro-sensitive cells. Engelmann suspected that he should measure the electric field near to the Schnauzenorgan to get a better sense of the field from the fish's perspective ().
But measuring electric fields around the fish's head was going to be tricky: elephantnose fish can whip their Schnauzenorgans back and forth at speeds up to 800° s–1 and they have a complex head shape. Engelmann would have to design a precise measuring system, as well as carefully immobilising the animal, to accurately plot the electric field around its head. Teaming up with Roland Pusch and engineer Axel Mickenhagen,Engelmann designed and built an electrode that could simultaneously measure all three components of the electric field at precise positions to glimpse what the elephantnose perceives.
Plotting the electric field in planes around the fish's head, Engelmann quickly realised that it looked almost like a classic dipole field, but elongated along the head. The Schnauzenorgan also appeared to funnel the field along the proboscis's surface, just like his colleague Angel Caputi had suggested. And when the team gently moved the Schnauzenorgan to one side and remeasured the field, they saw that the field followed it. Engelmann admits that he was stunned by this discovery and explains that this allows the fish to distinguish the presence of novel stimuli from distortions, caused by their activity, by using their anatomy rather than colossal amounts of brain power.
Next the team wondered how objects would distort the field around the Schnauzenorgan to produce an electrical image. Placing small metal and PVC cubes or spheres close to the Schnauzenorgan, Pusch and Engelmann painstakingly recorded the distorted electrical fields and found that they were very different from the fields recorded near the fish's body. Electric field measurments at the fish's trunk give a misleading view of the field perceived by the electrosensitive Schnauzenorgan.
Finally Engelmann teamed up with Kirsty Grant, Michael Hollmann and João Bacelo to measure the density of electroreceptors on the proboscis to find out which region of the Schnauzenorgan is most sensitive to electric fields. Sabine Nöbel also tested the sensitivity of the Schnauzenorgan by recording the electric organ's discharge rate when presented with an electric dipole at positions near the Schnauzenorgan, and found that it was most sensitive at the tip. Which was the exact spot where Grant, Hollmann and Bacelo had recorded the highest electroreceptor densities.
