against artificially-darkened water fleas, in both cases with or without light. These fish create a potential usually smaller than one volt (1 V). Weakly electric fish can discriminate between objects with different resistance and capacitance values, which may help in identifying objects. Active electroreception typically has a range of about one body length, though objects with an electrical impedance similar to that of the surrounding water are nearly undetectable. Active electrolocation relies upon tuberous electroreceptors which are sensitive to high frequency (20-20,000 Hz) stimuli. These receptors have a loose plug of epithelial cells which capacitively couples the sensory receptor cells to the external environment. Elephantfish (Mormyridae) from Africa have tuberous electroreceptors known as Knollenorgans and Mormyromasts in their skin. Elephantfish emit short pulses to locate their prey. Capacitative and resistive objects affect the electric field differently, enabling the fish to locate objects of different types within a distance of about a body length. Resistive objects increase the amplitude of the pulse; capacitative objects introduce distortions. The Gymnotiformes, including the glass knifefish (Sternopygidae) and the electric eel (Gymnotidae), differ from the Mormyridae in emitting a continuous wave, approximating a sine wave, from their electric organ. As in the Mormyridae, the generated electric field enables them to discriminate accurately between capacitative and resistive objects. Electrocommunication Weakly electric fish can communicate by modulating the electrical waveform they generate. They may use this to attract mates and in territorial displays. Electric catfish frequently use their electric discharges to ward off other species from their shelter sites, whereas with their own species they have ritualized fights with open-mouth displays and sometimes bites, but rarely use electric organ discharges. When two glass knifefishes (Sternopygidae) come close together, both individuals shift their discharge frequencies in a jamming avoidance response. In bluntnose knifefishes, Brachyhypopomus, the electric discharge pattern is similar to the low voltage electrolocative discharge of the electric eel, Electrophorus. This is hypothesized to be Batesian mimicry of the powerfully-protected electric eel. Brachyhypopomus males produce a continuous electric "hum" to attract females; this consumes 11–22% of their total energy budget, whereas female electrocommunication consumes only 3%. Large males produced signals of larger amplitude, and these are preferred by the females. The cost to males is reduced by a circadian rhythm, with more activity coinciding with night-time courtship and spawning, and less at other times. Fish that prey on electrolocating fish may "eavesdrop" on the discharges of their