CAPACITANCE DETECTION IN THE WAVE-TYPE ELECTRIC FISH EIGENMANNIA DURING ACTIVE ELECTROLOCATION

Weakly electric fish can detect nearby objects and analyse their elect ric properties during active electrolocation. Four individuals of the South American gymnotiform fish Eigenmannia sp., which emits a continu ous wave-type electric signal, were tested for their ability to detect capacitive properties of objects and discriminate them from resistive properties. For individual fish, capacitive values of objects had to be greater than 0.22-1.7 nF ('lower threshold') and smaller than 120-6 80 nF ('upper threshold') in order to be detected. The capacitive valu es of natural objects fall well within this detection range. All fish trained could discriminate unequivocally between capacitive and resist ive object properties. Thus, fish perceive capacitive properties as a separate object quality. The effects of different types of objects on the locally occurring electric signals which stimulate electroreceptor s during electrolocation were examined. Purely resistive objects alter ed mainly local electric organ discharge (EOD) amplitude, but capaciti ve objects with values between about 0.5 and 600 nF changed the timing of certain EOD parameters (phase-shift) and EOD waveform. A mechanism for capacitance detection in wave-type electric fish based on time me asurements is proposed and compared with the capacitance detection mec hanism in mormyrid pulse-type fish, which is based on waveform measure ments.