according to the surroundings, for example different conductances of salt and freshwater. To maximize the power delivered to the surroundings, the impedances of the electric organ and the water must be matched: Strongly electric marine fish give low voltage, high current electric discharges. In salt water, a small voltage can drive a large current limited by the internal resistance of the electric organ. Hence, the electric organ consists of many electrocytes in parallel. Freshwater fish have high voltage, low current discharges. In freshwater, the power is limited by the voltage needed to drive the current through the large resistance of the medium. Hence, these fish have numerous cells in series. Electric organ Anatomy Electric organs vary widely among electric fish groups. They evolved from excitable, electrically-active tissues that make use of action potentials for their function: most derive from muscle tissue, but in some groups the organ derives from nerve tissue. The organ may lie along the body's axis, as in the electric eel and Gymnarchus; it may be in the tail, as in the elephantfishes; or it may be in the head, as in the electric rays and the stargazers. Physiology Electric organs are made up of electrocytes, large, flat cells that create and store electrical energy, awaiting discharge. The anterior ends of these cells react to stimuli from the nervous system and contain sodium channels. The posterior ends contain sodium–potassium pumps. Electrocytes become polar when triggered by a signal from the nervous system. Neurons release the neurotransmitter acetylcholine; this triggers acetylcholine receptors to open and sodium ions to flow into the electrocytes. The influx of positively charged sodium ions causes the cell membrane to depolarize slightly. This in turn causes the gated sodium channels at the anterior end of the cell to open, and a flood of sodium ions enters the cell. Consequently, the anterior end of the electrocyte becomes highly positive, while the posterior end, which continues to pump out sodium ions, remains negative. This sets up a potential difference (a voltage) between the ends of the cell. After the voltage is released, the cell membranes go back to their resting potentials until they are triggered again. Discharge patterns Electric organ discharges (EODs) need to vary with time for electrolocation, whether with pulses, as in the Mormyridae, or with waves, as in the Torpediniformes and Gymnarchus, the African knifefish. Many electric fishes also use EODs for communication,