What specific structures are in electroreceptors?

Electroreception is limited to aquatic environments because on here is the resistivity of the medium is low enough for electric currents to be generated as the result of electric fields of biological origin. In air, the resistivity of the environment is so high that electric fields from biological sources do not generate a significant electric current. Electroreceptor are found in a number of species of fish, and in at least one species of mammal, the Duck-Billed platypus.