Electrocytes
Electrocytes are modified cells present in electric eels, rays, and other fish that allow them to generate an electric field, used to deter prey and disable predators. The organs that electrocytes make up resemble a biological battery, operating under the same principles as mechanical batteries.
Function
Electrocytes are used by saltwater or freshwater fish to incapacitate prey or discourage predators. A strong dose of electricity scrambles the target's nervous system, causing paralysis (including respiratory or cardiovascular), unconsciousness, or death. Some fish may also use electrocytes for electrolocation, in which a fish navigates, finds prey, etc. by sensing disturbances in the low-level field it emits.[1]
Anatomy
Electrocytes arise as modified muscle cells, with a flat disc shape. Each individual electrocyte functions by actively pumping positive Na+ and K+ ions out through the cell membrane, giving the cell an overall negative charge. Several thousand electrocytes are stacked into a striated tissue, allowing charge to flow and forming an electric organ. Depending on the fish, the number of organs can vary - some electric fish may have only one, while electric rays possess two and electric eels possess three.[2]
Saltwater vs. Freshwater
Since electric current is more easily propagated through salt water than fresh water due to the external ions present (e.g. the resistance of saltwater is lower than the resistance of freshwater), saltwater and freshwater fish have differently organized electric organs. Because of the low resistance of saltwater, the organs of saltwater fish have electrocytes configured in a more "parallel" form, with shorter stacks of electrocytes and a higher number of columns. This produces a lower overall voltage and a higher relative current.
In freshwater fish, more voltage must be produced because of the high resistance of freshwater. Thus, the electrocytes are stacked in a "series" form - longer stacks and fewer overall columns. This produces a higher voltage and a lower relative current.[3]
Firing
To "fire" the electric organ, nervous signals are sent to receptors on the electrocytes. These signals open up Na+ channels, flooding the cells with a positive charge and reversing the polarity. This sudden change in electric potential causes an electric field to be created, and the subsequent current flow out of one end of the fish and into the opposite end, forming a circuit. These firing events can generate a large amount of voltage - in electric eels, it can be up to 860 volts and 1 amp for 2 milliseconds, while in rays in can be between 50-200 volts and 30 amps.[4]
Similarity to mechanical batteries
Electrocytes cause a flow of ions that lead to a change in potential difference and an electric field and current being produced. This is identical to the principles behind a mechanical battery, in which ions in the battery flow from one end to the other and force the movement of electrons, also producing an electric field and current. The formation of electric organs also reflects batteries and circuits - electrocytes can be arranged "in parallel" (fewer, longer columns) or "in series" (more, shorter columns), having the same effect as batteries arranged the same way.
Also, the striation of the electric organs is comparable to batteries being connected in series, increasing the overall potential difference generated by the organs and enabling the fish to emit a stronger electric field.
Equations
As electrocytes function as a biological battery, equations relating to potential difference apply:
[math]\displaystyle{ I=qnAuE }[/math]
[math]\displaystyle{ ΔV_{total,series}=\sum_{k=1}^N ΔV_{battery} }[/math]
[math]\displaystyle{ ΔV_{total,parallel}=V_1=V_2=V_3...=V_N }[/math]
[math]\displaystyle{ I_{total,series}=I_1=I_2=I_3=...I_N }[/math]
[math]\displaystyle{ I_{total,parallel}=\sum_{k=1}^N I_{battery} }[/math]
History
Electric eels were mentioned in the writings of Alessandro Volta, who compared the electrocytes of the eels to his voltaic pile. Both functioned in the same way - the electrocytes were analogous to the cells of the pile, combining to force a flow of electrons and an overall current.