The electric organ discharges of the gymnotiform fishes: II. Eigenmannia

We present detailed measurements of the electric organ discharge of the weakly electric fish, Eigenmannia sp. These maps illuminate, with high resolution in both space and time, the electric organ discharge potential and electric field patterns in the water about the fish and on the skin surface itself. The results demonstrate that the electric organ discharge of Eigenmannia approximates a simple oscillating dipole, which confirms previous descriptions and assumptions, but is in contrast to the electric organ discharges of several other gymnotiform species. Over each cycle of Eigenmannia's electric organ discharge, the electric field amplitude measured at any point near the fish oscillates from positive to negative, but the field vector remains nearly constant in direction. This electric organ discharge charge pattern is correlated with known anatomical and physiological features of the fish's electric organ, and confirms that the activation of electrocytes comprising the organ is well synchronized. As a result, the relatively simple electric organ discharge leads to a fairly uniform pattern of electrosensory stimuli along the body surface, which may facilitate central processing of electrosensory images. Electric organ discharge maps and animations resulting from this series of studies are available via the Internet (http:// www.bbb.caltech.edu/ElectricFish, or www.fiu.edu/ ~stoddard/electricfish.html).