Apternotus albifrons , a fish that continuously emits electricity at a frequency of about 1,000 hertz can be used as a detector of pollutants, say scientists at the International Water Centre in Nancy, France. The electric signals given out by the fish are very stable under normal conditions, but vary when a toxic substance is present in the water. For instance, the fish can detect potassium cyanide in the water within less than 10 minutes of the toxin's presence even if the concentration is extremely low ( Global Technoscan , November 1996).