Distribution of electroreceptors, bioelectric field patterns, and skin resistance in the catfish,Ictalurus nebulosus LeS

Three aspects of electroreception in catfish were investigated and discussed. 1) The distribution of the electroreceptive small pit organs over the skin was mapped (Fig. 2). Small pit organs occur all over the body with the exception of the barbels. The total number of small pit organs per fish has to be estimated at several thousands. 2) Some examples of the potential pattern of the bioelectric field ofIctalurus are given (Fig. 4, 5). It was found that the opening or closing of the mouth, and contractions of the anal sphincter result in variations in this bioelectricfield, that might be of significance in behaviour. 3) The skin resistance of catfish proved to be constant in electric fields from d.c. to 10 Hz, and to behave frequency dependent from 10 Hz upwards. The resistive behaviour of the fish's body varies with the direction of the body axes. In dorso-ventral direction the insulating properties of the body are greatest, which suggest a greater electro-sensitivity in this direction than in others (Fig. 6, 7). The relatively small deformation of external homogeneous a.c. fields, caused by the insulating character of the swim bladder, suggests that a subdermal conductive layer screens the inner structures and might function as a potential reference.