Thursday, January 15, 2015

The Eel's in Charge

By: Paul Carvalho

Before Jack Cover of NASA developed the Taser stun gun, evolution and natural selection whipped up these bad boys.

The electric eel (Electrophorus electricus) is capable of discharging electricity for navigation, defense, and attack purposes.  They shock their prey into a stupor until the helpless fish twitches uncontrollably. Once the eel feels a slight movement in the water from the twitch, they attack and the fish is a goner. Recently, researchers at Vanderbilt University in Nashville, Tennessee have discovered that eels can also use their shocking ability to remotely control prey items. 

Electric eels (endemic to South America), and many other electrogenic fish in the order Gymnotiformes have evolved an insane cell known as the electrocyte. They are derived from muscle cells and have lost the ability to contract, however, these cells have developed into natural batteries within the eels.  Most electrogenic fish have one electric organ made up of these cells, but this gangster fish contains THREE electric organs (Sachs, Main, and Hunter’s) running along the whole body. All three add up to 6,000 electrocytes allowing this beast to generate 600 volts- that is five times that of a U.S. wall outlet!

Electric eels have three forms of emitting electricity from their organs. The first is used for navigation and consists of low-voltage pulses.  The second is a high-voltage pulse, released in pairs or triplets for hunting, typically used in a complex environment with a lot of hiding places for tasty snacks. The third is a series of high-voltage pulses used for freezing free-swimming prey animals in their tracks. The third form can also be used as a defense mechanism against haters, like the caiman featured in the video below. 

Be aware of the creepy clown laugh in the beginning and end of this video!

Although these forms of electrical discharge have been known for quite some time, little was known about the impact of the discharge on prey. Kenneth Catania from Vanderbilt University designed an experiment to explore how the electrical charge affects prey during an attack. To examine the effects of high-voltage discharge on prey, Catania set up a tank with a gel barrier permeable to electrical current. A scramble-brained fish fixed to a device that measured muscle tension was lowered into one side of the tank with a hungry eel on the other side. Worms were dropped into the eel’s side of the tank in order to initiate an attack response. The fish’s muscles contracted during this test, indicating that the muscles of the prey are FORCED to contract during an attack (A and B in the following figure).

Catania suspected that muscle contractions in the fish may be directly caused by the electrical charge stimulating muscle cells or by stimulating motor neurons in the prey. To explore these options, two fish were placed side by side in the gel-divided tank. One fish was injected with an acetylcholine antagonist (basically a shot disrupting the interaction between the motor neurons and muscle cells) and the other fish was injected with a control. The results seen in the above figure (C and D), show that the tension in the acetylcholine antagonist-treated fish declines to nothing after injection. This reveals that fish motor neuron activity is necessary for the eel’s electrical discharge to affect muscle contraction in prey.

Now let’s talk about that second form of electrical discharge mentioned above. Eels will sometimes use high-voltage pulses in doublets or triplets while hunting. Catania noticed that eels would release an isolated doublet and wait for prey to tweak out. Once the eel detected the prey’s movement in the water, the eel releases the attack response, much like we were just talking about. This attack stuns the prey so it can move in for the kill. To test his theory, Catania put a fish in a plastic bag inside some gel and hooked it up to a machine that the experimenter can use to stimulate a twitch in the fish. The fish was also protected from the electrical doublet sent out from the eel, so unless the researcher induces the twitch, the eel has no clue anything is there.

When the eel released its doublet, the researcher stimulated the fish to twitch. In response, the eel used a high voltage pulse and tried to attack the fish. This happened every time the experimenter simulated a twitch in the fish. When a doublet was released and the researcher did not simulate a twitch, the eels did not gear into attack mode- it was just business as usual.

Now this is truly amazing! At least I think so. These eels can swim around cracks and holes and not detect a prey item but with this “doublet hunting” method they can remotely control the muscle contraction of some poor unfortunate soul giving away their position and leading to their demise.

Now if you ever go to South America looking for some electric eels, learn a thing or two from the following video before you embark on that journey.

Catania, K. 2014. The shocking predatory strike of the electric eel. Science 346:1231-1234.

de Santana, C.D., R.P. Vari, and W.B. Wosiacki. 2013. The untold story of the caudal skeleton in the electric eel (Ostariophysi: Gymnotiformes: Electrophus). PLoS ONE 10:1371.

Keynes, R.D. and H. Martins-Ferreira. 1953. Membrane potentials in the electroplates of the electric eel. The Journal of Physiology 119:315-351.

Weber, B. 2009. Jack Cover, 88, physicist who invented the taser stun gun, dies. The New York Times.


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