In 1800, Alexander von Humboldt witnessed a swarm of electric eels leaps out of the water to defend themselves against oncoming horses. Most people thought the story so unusual that Humboldt made it up. But fish using electricity is a more common matter. Electric eels are a type of fish. These electrical signals offer ways to communicate, navigate, find, and prey.
According to scientists, it’s fantastic that evolution created an electrical organ in even one group, let alone six. All muscles can produce energy, but certain fish-evolved cells generate higher voltages than others. There are so many specific steps that a cell has to evolve to become electrical. All these fish have the same electric organ. Scientists hope to create electricity for Bionic devices in the human body by understanding these fish.
How do fish make electricity?
About 350 species of fish have specialized anatomical structures. They generate and detect electrical signals. These fish are divided into two groups, depending on how much electricity they produce. Scientists call the first group the weakly electric fish. A structure near its tails is called an electric organ. It produces up to a volt of electricity, about two-thirds as much as a AA battery. How does this work?
The fish’s brain sends signals through its nervous system to the electric organ. It is filled with stacks of hundreds or thousands of disc-shaped cells called electrocytes. Usually, electrolytes pump out sodium and potassium ions to maintain a positive charge outside and a negative. When the nerve signal arrives at the electrocyte, it prompts the ion gates to open. Positively charged ions flow back in.
One face of the electrocyte is negatively charged outside and positively charged inside. But the far side has the opposite charge pattern. These alternating charges can drive a current, turning the electrolyte into a biological battery. The key to these fish’s powers is coordinating nerve signals to arrive at each cell simultaneously. That makes the stacks of electrocytes act like thousands of batteries in series. The tiny charges from each one add up to an electrical field that can travel several meters.
- The electrolytes, or the electrically excitable cells, receive simultaneous signals from the brain to fire.
- When firing, the electrocytes are asymmetrically polarized and work as a series of connected batteries.
- The continuous firing of electrocytes results in the electric organ discharging, discharged in nearby water.
Cells called electroreceptors buried in the skin allow the fish to constantly sense this field and its changes caused by the surroundings or other fish. For example, Peter’s elephant nose fish has an elongated chin called a Schnauzenorgan. That’s riddled with electroreceptors. It allows it:
- To intercept signals from other fish.
- Judge distances.
- Detect the shape and size of nearby objects.
- Determine whether a buried insect is dead or alive.
But the elephant nose and other weakly electric fish don’t produce enough electricity to attack their prey. That ability belongs to the strongly electric fish, of which there are only a handful of species. The electric knife is the most powerful, strongly electric fish, more commonly known as the electric eel. Three electric organs span almost its entire two-meter body.
Like the weakly electric fish, the electric eel uses its signals to navigate and communicate. But it reserves its strongest electric discharges for hunting using a two-phased attack that checks out and then incapacitates its prey.
- First, it emits two or three strong pulses, as much as 600 volts. These stimulate the prey’s muscles, sending it into spasms and generating waves that reveal its hiding place.
- Then, a volley of fast, high-voltage discharges causes even more intense muscle contractions.
The electric eel can also curl up to overlap the electric fields generated at each end of the electric organ. The electrical storm eventually exhausts and immobilizes the prey, and the electric eel can swallow its meal alive. The other two strongly electric fish are the electric catfish. It can unleash 350 volts with an electric organ. That occupies most of its torso. The electric ray, with kidney-shaped electric organs on either side of its head, produces as much as 220 volts.
Types of electric fish
Scientists divide the electric fish into three categories.
- Strongly electric fish.
- Weakly electric fish.
- Fishes that can only sense electricity.
Strongly electric fish: Their electrical discharges are strong enough to be used in an attack or defense. Examples include electric eels, electric catfish, and electric rays. In strongly electric fish, the electric organ is huge and contains a lot of electrolytes. Hence the discharge voltage sometimes can reach as high as 500 to 600 volts.
Weakly electric fish: Their electric discharges are too weak to stun prey but are used for navigation object detection and communication with other fish. Examples are knife fishes and elephant noses.
Fishes that can only sense electricity: They can only sense electricity. Sharks have an organ known as ampullae, using which they can sense electric fields. They can find a small fish buried in sand by weak electricity given off by the prey. In weakly electric fish like the elephant nose fish, the discharge voltage is small, often less than a volt.
The electric fishes produce electricity and sense it with a sensitive sensory organ embedded in the skin. Electric fishes can thus electrically see objects in an environment where vision is less clear. This process is called active electrolocation. Some electric fishes are known to communicate with each other by electric signals. They use the combination of electrogenic and electroreceptive capabilities for this.
Why don’t fish electrocute themselves?
It may be that the size of strongly electric fish allows them to withstand their shocks or that the current passes out of their bodies too quickly. Some scientists think that particular proteins may shield the electric organs. But the truth is, this is one mystery science still hasn’t illuminated.
- How Do The Eels Make Electricity?
- How Do The Sharks Generate Electricity?
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Alves-Gomes, J, “The evolution of electroreception and bioelectrogenesis in teleost fish: a phylogenetic perspective.” Journal of Fish Biology.
Bullock, Theodore H.; Hopkins, Carl D., “Electroreception.” Springer Handbook of Auditory Research.
Albert, J. S.; Crampton, W. G. R. Electroreception and electrogenesis, The Physiology of Fishes (3rd ed.).
Macesic, Laura J.; Kajiura, Stephen M., “Electric organ morphology and function in the lesser electric ray, Narcine brasiliensis.”