A number of aquatic species have evolved the capability to produce sizable electric fields. For some species this helps them to detect other organisms, either predator or prey. Other species, such as the electric eel, use this capability to stun or kill a predator or prey.
The organism generates the electric field in an "electric organ" which can take up most of the body cavity. The following description refers to a hypothetical electric organ, which includes many features by which these organs operate.
Electrocytes are flattened cells (like disks) which are stacked in a series, as shown in the figure below. During the equilibrium state, the cells actively exclude sodium ions (Na+), creating a potential difference between the inside of the cell and the outside of the cell. The cell membrane is permeable to potassium ions (K+), so the interior of the cell becomes enriched with K+ ions, partially but not fully compensating for the potential difference due to the imbalance of Na+ ions. The magnitude of the potential difference between the inside and outside is of the cell is about 0.1 V.
During the activated state, the posterior side of the cells becomes permeable to sodium ions, so they rush in through the posterior face due to the potential difference. Potassium ions rush in the same direction through the anterior face of the cell. The result is a potential difference across the whole organ, which acts, in effect, as a battery. (See the figure below.) The circuit is completed in the surrounding water, as shown in the second figure below.
During the activated state, the current across the anterior and posterior faces of a cell in this hypothetical electric organ is 30 milliamps. The duration of a pulse is 2 milliseconds.
The charge on an electron is –1.6 x 10–19 C.
Which is a better conductor of electricity, fresh water or sea water?