Physiology of Finfish and Shellfish (2+1)

3.3.4. Electro-reception

The primary function of the external pit organs of teleosts is the reception of minute electrical currents in the water. These pit organs open to the surrounding water via canals filled with an electrically conductive gel. The marine catfish (Plotosus) has longer canals resembling similar structures found in marine elasmobranchs called ampullae of Lorenzini, which range from 5 mm to 160 mm in length. The longer ducts in the marine species compared with those living in the freshwater environments are related to the electrical conductivity differences between the environments compared with those of the skin and body tissues. In contrast, freshwater fishes have relatively good conducting tissues compared with their environment. However, their skin has high electrical resistance causing large voltage drops compared with the other tissues (Fig).

Kalmijn (1971) demonstrated that predaceous marine elasmobranchs can locate prey by electroreception alone. For eg. The dogfish shark can locate a flounder buried in sediment, even when concealed by an electrically conductive agar plate. The dogfish was not attracted to burried chopped fish or to live flounder covered by an electrically insulating sheet of polyethylene. The shark dug out 2 electrodes emitting a biological strength electrical current, which were buried in sediment.

Some bony fishes in the families Electrophoridae, Gymnotidae, and Mormyridae produce a low-voltage electric current that sets up a field around the fish. Tiny skin organs on the fish detect disruptions in the electric field that are caused by prey or inanimate objects. Electric organs are made up of cells called electrocytes that have evolved from muscle cells. Electrocytes typically are thin and stacked on top of one another. Electroreception is an adaptation for detecting prey and for navigation in murky water. Some other fishes produce stronger electric currents for stunning prey.