Why Doesn't an Electric Eel Shock Itself?
― Electricity Takes the "Path of Least Resistance"
An electric eel is said to be able to release a powerful jolt of several hundred volts in an instant at prey or enemies. That is enough to give a person a severe shock. Yet the eel that makes all this electricity keeps swimming calmly, without being shocked or weakened. Why?
The electric eel is a fish with special organs for generating electricity inside its body. It is said to release a strong jolt from these organs in an instant, stunning prey or driving off enemies.
To us, making so much electricity inside your own body sounds like it would shock you too. Yet the electric eel does this again and again and lives on unharmed.
The key to this puzzle is said to lie in clever body design and in the nature of electricity itself.
Life-critical organs such as the heart are thought to sit far from the electricity-generating organs.
Electric current has a tendency to flow mainly along the easier path (the one with lower resistance). The surrounding water and the prey are said to conduct electricity better than the eel's own body.
Let's look at how this "body design" and this "nature of electricity" work together, step by step.
An electric eel lines up many "tiny batteries" inside its body
Most of an electric eel's body is actually taken up by special cells for generating electricity (electrocytes). Each electrocyte produces only a small voltage. But thousands of them are lined up inside the body, like batteries connected in series, and together they are thought to build a very high voltage.
It keeps vital organs away from where the electricity is made
Most of the eel's life-critical organs, such as the heart, are gathered in a fairly small area near the head. The electricity-generating organs, on the other hand, spread over most of the rest of the body, the long stretch toward the tail. This layout is thought to leave some distance between the place where the strong current is made and the delicate vital organs.
Its body is built so that little strong current flows into the vital parts in the first place.
Electricity flows along the "path of low resistance"
Electric current has a tendency: when there are several paths, more of it flows along the one with lower resistance (how hard it is for current to pass). Compare the inside of an electric eel's body with the water around it. Water (especially the body of prey, which conducts well) is said to carry electricity more easily than the eel's own body. So most of the current from the generating organ is thought to flow preferentially into the outside water and the prey, not through the eel's own body.
Researchers have reported that when an enemy sticks out of the water, the electric eel lifts part of its body out of the water and touches the target directly, sending the current straight into it. This is thought to be because direct contact passes the current on more effectively than sending it through water.
Things you can check yourself
- Get two hoses or tubes of different widths (a wide straw and a thin straw will do).
- Pour the same amount of water into each at the same time.
- Check that more water comes out of the wider one (lower resistance).
- Now imagine that electricity, too, flows preferentially along the "easy path" (the path of low resistance).
With the same idea as water flow, you can picture which path more electricity takes.
Summary
One reason an electric eel is not easily shocked by its own electricity is that its vital organs sit away from where the electricity is generated. The other is that current prefers the path of low resistance, so most of it escapes out of the body into the water or the prey. The two together are thought to explain it.
The eel's electricity is designed in advance to be sent outward, into the world, and not inward, into itself.
The electric eel "makes" electricity. The opposite side, "sensing" the faint electricity given off by prey, is explained in the article on sharks. The same physics also explains why a bird sitting on a power line is not shocked: current flows only where there is a "voltage difference". We work it out in the article on birds and power lines.
For those who want to know more ― terms, numbers, and links to textbooksFrom middle-school science to active research, each item is labelled with its level
- Middle schoolCovered in Japanese middle-school science
- High schoolCovered in high-school "Basic Physics"
- High school+High-school "Physics", or advanced or sidebar material in textbooks
- UniversityUniversity-level specialist material (electrophysiology), not taught in high school
- ResearchTopics researchers are still investigating, not yet taught even at university as settled fact
Middle schoolTerms: words around the electric eel
- Voltage: how strongly electricity is pushed. The unit is the volt (V).
- Current: how much electricity actually flows. The unit is the ampere (A).
- Resistance: how hard it is for electricity to flow. The unit is the ohm (Ω).
High schoolChecking with a formula: how do tiny cells add up to hundreds of volts?
First we get the main subject, the total voltage itself. Then we look at how that voltage splits as it flows.
| In symbols | V ＝ n × v |
| In words | Total voltage ＝ number of electrocytes in series × voltage of one cell |
| Where it comes from | It comes from a circuit rule (Kirchhoff's voltage law): batteries connected in series have their voltages added. |
| Symbol v: voltage of one electrocyte (unit: V) | About 0.15 V, it is said |
| Symbol n: number of electrocytes in series (unit: cells) | About 4000 cells, it is said |
| Symbol R: resistance of the path the current takes (unit: Ω) | As an illustration, take 1000 Ω inside the body and 100 Ω in the water |
| Total voltage (V) | 4000 × 0.15 ＝ 600 |
A battery of only 0.15 V, lined up 4000 times in series, gives 600 V. A voltage many times that of a household outlet comes from simply adding up cells.
| In symbols | I ＝ V ÷ R |
| In words | Current ＝ voltage ÷ resistance |
| Where it comes from | This is Ohm's law. You can use it to compare two paths that have the same voltage across them. |
| Current through the body (A) | 600 ÷ 1000 ＝ 0.6 |
| Current through the water (A) | 600 ÷ 100 ＝ 6 |
| How many times the body's current flows through the water | 6 ÷ 0.6 ＝ 10 |
| Result | The water carries 10 times as much current as the body |
As in this example, more current flows along the path of lower resistance. Thanks to this, the current through the eel's own body is thought to be kept relatively small.
* The resistance values here are illustrative numbers to help explain the mechanism. This article does not cover the exact resistance of a real body or of real water.
High school+How current divides in a parallel circuit
In the study of electric circuits, it is known that when the same voltage is applied across two or more paths (a parallel circuit), the ratio of the currents in the paths is the inverse of the ratio of their resistances. The lower the resistance of a path, the more current it carries. This relationship can also be applied to the two "paths" here: the eel's body and the water around it.
UniversityElectrocytes: the "tiny batteries" inside the body
In electrophysiology, researchers study how an electric eel's electrocytes, triggered by a signal from a nerve, rapidly change their electrical properties between the inside and outside of the cell and produce a voltage in an instant. When many electrocytes act at once, a large voltage is thought to be produced all at once. This rapid change is called an action potential, and it is said to use the same mechanism as in nerve and muscle cells.
📖 Derivations and further reading: Action potential (how cells produce voltage)
ResearchWhat is still unclear
- How far the tissues inside an electric eel's body resist electricity, and the details of that insulation mechanism, are still being studied in electrophysiology.
- The behavior of leaping at a target that sticks out of the water is a research topic reported relatively recently, and the exact conditions and meaning of the behavior are still being investigated.
- Research is also under way on bio-derived batteries and tiny power sources inspired by how the electric eel generates electricity.
The electric eel's body is packed with rich topics, where electrophysiology and physics meet, and research continues today.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: current and voltage | Basic terms: voltage, current, resistance |
| High school | Basic Physics: Ohm's law | Calculating current for different resistances |
| High school+ | Physics: parallel circuits (advanced) | How current divides in a parallel circuit |
| University | Electrophysiology | How electrocytes work |
| Research | Electrophysiology and behavioral science (ongoing research) | Details of the insulation mechanism, leaping attacks, applied technology |
- Explanations of Ohm's law and parallel circuits in physics textbooks.
- Explanations of how electric eel electrocytes work in physiology materials.
- A research review on the electric eel's defensive behavior (leaping attacks) in animal behavior studies.
- A research review on the internal insulation mechanisms of electric fish in electrophysiology.
- Explanations of bio-battery research using biological power generation in applied engineering.
* Values such as voltage and resistance include rough figures and illustrative values used to explain the mechanism. Actual values are said to vary by individual and situation.
* This article is a science explainer for general readers. Never carelessly touch a real electric eel or any other electricity-producing animal, or put your hand into its water.