How Do Sharks Find Prey Hidden in the Sand?
– Sensing "Electricity" Through the Skin, Not the Eyes or Nose
Have you ever seen footage of a shark heading straight for prey that lies still, hidden in dark, murky water or buried in sand? There is no scent drifting in the water, no sound, and nothing to see. So how does the shark know where the prey is?
A flatfish lies completely buried in the sand of a quiet seafloor, perfectly still. There is nothing to see and no scent in the water. It makes no sound.
Yet a shark swimming nearby heads straight for that one spot in the sand, without a hint of hesitation. What was the flatfish giving off without knowing it?
Breathing, gill movements, a beating heart: simply being alive is thought to produce a tiny amount of electricity (bioelectric current) around the body.
The countless tiny pores around a shark's face (the ampullae of Lorenzini) are thought to pick up this extremely small electric signal.
Let's look at "the electricity that living things give off" and "how it is sensed," one step at a time.
Living Things Give Off "Electricity" Even When Perfectly Still
A heart beats, a muscle contracts, a signal travels along a nerve. All of these workings inside the body run on tiny exchanges of electricity. Seawater contains ions and conducts electricity well, so a small fraction of this internal electricity is thought to spread into the seawater around the body as an extremely small voltage difference.
Even a fish that lies motionless under the sand still has to keep its gills moving in order to breathe. However well it hides, the very fact of being alive keeps leaking an electric clue that it cannot switch off.
The Ampullae of Lorenzini: Jelly-Filled "Electric Antennas"
Around the face of sharks and rays, especially the snout (the tip of the nose), are countless small pores called the ampullae of Lorenzini. Each pore opens into a thin canal running down under the skin, and the canal is filled with a jelly-like substance that conducts electricity well. Deep at the far end of each canal, far beneath the skin, sensory cells that detect electrical changes are clustered together.
Because the canal conducts electricity so well, the working idea is that the voltage difference between the skin surface (the pore opening) and the sensory cells deep below is passed along almost without loss. In effect, the canal itself acts as an "electric wire," carrying the tiny voltage difference in the outside seawater to the sensory cells.
According to values often cited in research, some sharks and rays can sense a voltage difference as small as 5 nanovolts (five billionths of a volt) per centimetre. This is thought to be among the most sensitive senses known in any animal.
It is reading the electric trace of "being alive" that the prey cannot erase.
Something You Can Try Yourself
- Prepare a container of salt water and a simple tester (voltmeter).
- Use two metal rods (such as spoons) as electrodes, and compare how easy the tester is to read with the electrodes close together and then far apart.
- See that, for the same strength of electric field, wider electrode spacing makes the voltage difference easier to pick up.
The length of the canals in the ampullae of Lorenzini is thought to serve a similar purpose: a wide spacing that makes a tiny voltage difference easier to detect.
Summary
A shark's ability to find prey hidden in sand is no special superpower. It comes from the fact that a living thing leaks a tiny amount of electricity around it just by being alive, combined with the ampullae of Lorenzini, which carry that electricity through jelly-filled canals to the sensory cells. Together, these two are thought to explain it.
An animal can hide itself, but it cannot hide the very fact that it is alive.
For another animal that finds things by an invisible clue, see the article on bats, which "see" in the dark using reflected sound (echoes). For electricity from living things, the article on the electric eel explains the side that "makes" it.
Our own bodies also give off electricity all the time. For how that works, see Why Can the Heart Keep Beating on Its Own?
For Those Who Want to Know More – Terms, Numbers, and Links to TextbooksFrom middle-school science to topics still under research, each part is labelled with its level
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school "Basic Physics"
- High school+High-school "Physics," or advanced material and sidebars in textbooks
- UniversityUniversity-level specialist content (biophysics) not taught in high school
- ResearchTopics researchers are still investigating, not yet settled enough to be taught even at university
Middle schoolTerms: Words for the Electric Sense
- Bioelectric current: the tiny flow of electricity in a living body that comes with the work of the heart, muscles and nerves.
- Ampullae of Lorenzini: sense organs on the snouts of sharks and rays that detect faint electricity. Named after the anatomist who discovered them.
- Electric field: the region around a charged object where electric force acts.
High schoolChecking with a Formula: How Far Could a 1.5 V Battery Be Sensed, in Theory?
Let's see how extraordinary "5 nanovolts per centimetre" really is by comparing it with an ordinary battery.
Sensing distance ≒ battery voltage ÷ sensitivity (voltage difference per cm)
| Battery voltage | 1.5 V (one ordinary dry-cell battery) |
| Sensitivity | 5 nanovolts per centimetre (5×10⁻⁹ V/cm) |
* This is a rough estimate from a simplified model that assumes the battery's voltage spreads out evenly and thins into the surrounding water. Real spreading in seawater is said to be far more complicated.
| Sensing distance (cm) | 1.5 ÷ (5 × 10⁻⁹) ＝ 3 × 10⁸ |
| Convert to km | 3 × 10⁸ cm ＝ 3 × 10⁶ m ＝ 3,000 km |
| Result | In the simplified model, in theory up to about 3,000 km away |
3,000 km is far longer than the whole length of the Japanese archipelago. Of course, in a real ocean the electricity weakens in complicated ways with distance and there is a lot of noise, so a shark cannot sense anything from that far. Even so, this calculation helps you feel just how absurdly small "5 nanovolts per centimetre" is.
High school+Why Does a Longer Canal Make Sensing Easier?
Even if the strength of the electric field (the voltage difference per centimetre) is the same, the longer the distance between the two points you measure across, the larger the voltage difference you actually get. The canals of the ampullae of Lorenzini run a fairly long way, from the skin surface deep into the body, and this is thought to have the effect of "widening the spacing so that a faint voltage difference is easier to measure." Some suggest that in hammerhead sharks, whose heads stretch far out to each side, this spacing can be even wider, which may be linked to their ability to sense electricity.
UniversityBiophysics: Converting Electric Signals in Sensory Cells
In biophysics, researchers study how the sensory cells of the ampullae of Lorenzini convert changes in voltage difference into electrical signals for the nerves. Special proteins in the cell membrane (ion channels) are thought to open and close in response to changes in voltage, and this is thought to be the origin of the signal sent to the nerves.
ResearchWhat Is Still Unclear
- The detailed molecular mechanism of conversion into electrical signals in the sensory cells is still under study.
- It has also been suggested that the ampullae of Lorenzini might sense not only the bioelectric current of prey but the faint electricity produced when ocean currents cross the Earth's magnetic field, and use it as a clue to direction and position, but the details of how are still debated.
- How much sensitivity differs between species, and how those differences relate to lifestyle, is also still being investigated.
Even one small pore on a shark's snout holds a rich topic, where biology and physics meet and research continues today.
Links to Textbooks (by Level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: how living bodies work / electric current | Bioelectric current, basic terms for the ampullae of Lorenzini |
| High school | Basic Physics: voltage difference | Comparing a battery with the sensitivity figure |
| High school+ | Physics: electric fields (advanced) | Canal length and how easily voltage differences are measured |
| University | Biophysics | Signal conversion by ion channels in sensory cells |
| Research | Sensory physiology and behavioural ecology (under research) | Molecular mechanism of signal conversion, possible use of the geomagnetic field, comparison across species |
- Classic research by Kalmijn, A. J. on electroreception in sharks and rays.
- Explanations of the structure and function of the ampullae of Lorenzini in marine biology textbooks.
- Explanations of bioelectric current and electroreceptor sensitivity in sensory physiology materials.
- Explanations of the link between feeding behaviour and electroreception in sharks in behavioural ecology materials.
* The sensitivity figure (5 nanovolts per centimetre) is a commonly cited rough value. Actual sensitivity is said to vary with species and conditions.
* This article is a science explainer for general readers. The numbers given are rough estimates to help you understand the mechanisms. Actual values are said to vary by species and by study.