Hollmann, M., Engelmann, J., and G. von der Emde. 2008. “Distribution, density, and morphology of electroreceptor organs in mormyrid weakly electric fish: anatomical investigations of a receptor mosaic.” Journal of Zoology, 276, 149-158.

The electric fish Gnathonemus petersii uses electroreception instead of vision for orientation and foraging presumably due to environmental restrictions on vision (it lives in murky African streams). It does so through a process called electrolocation, during which the fish produces an electrical field around its body using its electric organ (located in the tail) and detects and analyzes distortions in the field caused by surrounding objects using electroreceptor organs distributed across the surface of its body. Both active and passive electrolocation is used in this system. Gn. petersii [sic] has three kinds of electroreceptors: ampullary organs (which detect low-frequency electric fields, and which are used passively), ‘Knollenorgans’ (which respond to high frequencies and which are used when communicating with conspecifics), and mormyromasts (which respond to the fish’s own electric organ discharges [EODs] and changes to the fish’s electric field caused by nearby objects; used for active electrolocation). The mormyromasts are the most abundant type found in Gn. petersii.

Previously it had been proposed that this species possessed two ‘electrosensory foveae,’ one of which was at the chin area (called the ‘Schnauzenorgan’) and the other of which was between the mouth and nares (called the ‘nasal region’). It was known that these two regions had high electroreceptor densities and were overrepresented in the brain. The authors were interested in illuminating the spatial patterns of mormyromast distribution across the bodies of five different species of mormyrids. They used 22 specimens of Gn. petersii and one specimen each of four other species. Using electron microscopy, the researchers counted the number of mormyromast pores on the Schnauzenorgan, the nasal region, and the back, and measured their inner and outer diameters.

They found that the Schnauzenorgan of Gn. petersii was significantly more mormyromast-dense than other regions, possessing up to 65 receptors per square millimeter at the tip (which also contained many ampullary organs and Knollenorgans). The dorsal side of the Schnauzenorgan had significantly more mormyromasts than the other three sides, which did not differ significantly from each other. They also found that the nasal region was rich in mormyromasts as well, but did not have as many on average (4.6 per square millimeter). The density of these receptors decreased with distance from these two regions, and total number of mormyromasts did not vary much between individuals despite size differences (resulting in decreasing density as body length increases). Though other species had differing numbers of mormyromasts, the spatial distribution of them across their bodies followed the general pattern observed in Gn. petersii.

Additionally, the researchers found that the outer diameters of the mormyromasts on the dorsal side of the fish were significantly wider than those at the Schnauzenorgan, but did not differ significantly nasal region mormyromasts. The inner diameters were also wider on the dorsal side of the fish compared to other regions (with the exception of the lateral sides of the head, where inner pore diameter did not significantly differ from that of the dorsal pores). The authors found a highly significant correlation between outer and inner diameters of the mormyromast pores.

The authors suggest that one cause for the difference in mormyrmast diameters (increasing diameter towards the back of the fish, though density decreases) is increased sensitivity (as a size effect: larger inner and outer diameters correspond to larger-diameter canals and total size of the organ, respectively). The larger overall organs on the dorsal side of the fish may be more sensitive (increased number of receptor cells) than those at the Schnauzenorgan. The authors conclude that their results are in concordance with the hypothesis that two electrical foveae are present in Gn. petersii, one at the Schauzenorgan and one in the nasal region. However, they could not claim that these two foveae were really discrete, as the density of receptor organs decreases continuously from the anterior to posterior end of the fish. They do suggest that, because the Schnauzenorgan is moved from side to side and over objects during foraging or exploration, while the nasal region is held “at a fixed angle while the fish is swimming,” it may be possible that the Schnauzenorgan and nasal region are functionally distinct. They go on to hypothesize that the Schnauzen organ is a fovea for near-field prey detection/identification “equipped with a focusing mechanism (funneling of current and movement of the chin),” while the nasal region is for long-range detection of objects.

This study’s findings are in agreement with information presented in class about weakly electric fish: the high frequencies of the fields they use for electrolocation and communication, the concentration of electroreceptors at the head region, and the use of spatially differing electric field disturbances across the surface of the body to “map” images. Additionally, this study suggests that decreasing mormyrmast density from the anterior to posterior end of the fish is accompanied by increasing sensitivity of each mormyrast in the same direction, the significance of which is not yet clear.