Sheltering under rafts of camalote leaves floating along South American rivers, Gymnotus omarorum fish hunt their prey amongst the lilies' roots. However, these predators do not rely on vision when trapping their victims. Gymnotus omarorum sense the natural weak electric field produced by other creatures, in addition to distortions in their own pulsed electric field, to locate lunch. But how far-reaching is the fish's electric sense? Is it analogous to vision or more akin to touch? Explaining that electric images become blurrier as objects become more remote, and that electric fields weaken rapidly away from the source while the electric image of an object expands with distance, Carolina Pereira, Pedro Aguilera and Angel Caputi, from the IIBCE, Uruguay, add that long-range electric field image interpretation is challenging. ‘The detection range of edible prey using electrolocation appears to be very short. Moreover, theoretical predictions suggest that the range for prey location is even shorter’, the team explains. Intrigued by the exotic sense, the trio decided to find out just how extensive the fish's electric sense is (p. 3266).

Measuring the strength of a fish's personal electric field close to the its skin while moving steel and glass spheres of different diameters through the electric field, the team mapped the impact that each sphere had on the field at the fish's surface. Then they placed a cylinder in the fish's electric field, suddenly lowered the object's resistance from 2.5 MΩ to 1 kΩ and monitored the fish's reaction to the change by recording how rapidly the fish pulsed its electric field. The team also measured the limit of the fish's ability to electrolocate by measuring their responses to objects placed at various locations along five lines that extended through its electric field.

Having discovered that objects located near the fish dramatically distorted the distribution of the electric field across the fish's surface, the team was also surprised to see that much of the electric field images varied depending on the size and shape of the object: small objects produced ovoid-shaped fields, while larger objects increased the field volume. In addition, Caputi says, ‘We found an electroreception range smaller than predicted.’ Each fish's personal electric field was only effective over a range of about 10 mm.

‘Electroreception was traditionally compared with vision’, Caputi explains. However, he is now reconsidering this perspective. ‘In this paper we show that active electroreception is a short-range modality, which fits with our hypothesis that this sensory modality is part of a haptic sense, more similar to active touch than vision in humans’, he concludes.

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The active electrosensory range of Gymnotus omarorum
J. Exp. Biol.