Electrocytes

Electrocytes are modified cells present in electric eels, rays, and other saltwater 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 "ordinary" batteries.

History

Put this idea in historical context. Give the reader the Who, What, When, Where, and Why. Volta compared his battery to these guys

Function

Aggression

Electrocytes are used by saltwater or brackish fish to incapacitate prey or discourage predators.

Sensing

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, 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.

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. 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, but a higher current.

This is because, due to the lower resistance of saltwater, a less intense voltage is needed and the tissues can organize into a lower voltage, higher current form.

In freshwater fish, more voltage must be produced because of the high resistance of freshwater. Thus, the electrocytes are stacked in a "series" form -

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 loop. 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.

Similarity to mechanical batteries

The flow of ions into and out of electrocytes is utilized to produce a current that flows from one end of the organ and the animal to the other, very similar to how batteries create a potential difference. 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{ ΔV=IR }[/math]

References

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