Distribution, density and morphology of electroreceptor organs in mormyrid weakly electric fish: anatomical investigations of a receptor mosaic

Gnathonemus petersii and other African weakly electric fish (Mormyridae) produce electric signals in order to sense their environment, a process called active electrolocation. During active electrolocation electrical images of objects in the environment are cast onto a mosaic of electroreceptors, which are embedded in the animal's skin. In Gn. petersii and four other species of mormyrid fish, the densities of electroreceptor organs (mormyromasts and ampullary organs) were determined at various skin regions, including the chin, the nasal region and the back. In all species investigated, the highest mormyromast densities were found at the chin, followed by the nasal region. In addition to this comparative approach, we investigated the electroreceptive epidermis of Gn. petersii in more detail. At the moveable chin appendix, the Schnauzenorgan, scanning electron microscopy analysis helped to elucidate the mosaic‐like distribution and the morphology of the electroreceptor organs and their embedding in the epidermis. Because the number of mormyromasts increased very little when fish got bigger, receptor organ densities decreased with standard length. Mormyromast density at the Schnauzenorgan, especially at its tip, highly exceeded that at all other skin regions. Along the Schnauzenorgan, receptor densities decreased exponentially from the tip towards the base. At the nasal region, mormyromast densities were more than three times higher compared to the rest of the fish's head and trunk. A similar distribution to that of the mormyromasts was found for the ampullary electroreceptor organs, which are used for passive electroreception . Our findings support the hypothesis that at least in Gn. petersii the Schnauzenorgan and the nasal region are electroreceptive foveae.