Why do sardines and ribbonfish shine silver?
― Fish build mirrors without using any metal
When humans make a mirror, we coat the back of a sheet of glass with a metal like silver or aluminium. But fish have no metal film in their bodies at all. A fish's silver sheen is a mirror built from dozens of stacked, transparent crystal "thin films." And that mirror's job is to make the fish disappear in the sea.
Fresh sardines or saury laid out at the fish counter. Their flanks catch the fluorescent light and flash silver.
Bring one home to clean, and a fine silver powder sticks to your fingers. With a ribbonfish, just stroking it can turn your hand silver.
That powder isn't "silver" the way paint is silver. Look at a single grain, and it's almost completely transparent.
Just two reasons
Even a transparent sheet reflects a tiny bit of light at its surface. Stack dozens of films of matched thickness, and the reflected waves' peaks line up, producing a mirror that rivals metal.
Underwater, brightness changes a lot from top to bottom, but is nearly the same from side to side. A vertical mirror on the fish's flank reflects light that matches the brightness of the water behind it, making the fish hard to see.
The first point is "how" the fish turns silver; the second is "why" it evolved that way. Let's look at each in turn.
How do you build a mirror out of transparent film?
Beneath a fish's skin lie rows of extremely thin, plate-shaped crystals of a substance called guanine. These crystals alternate with layers rich in body fluid, stacking up dozens deep. Each crystal is only about a thousandth of the width of a human hair thick.
Light is a wave. A small amount reflects off the front and back of each crystal — only a few percent at each spot. But when the film thickness is just right, the peaks of the waves reflected at many different points line up exactly. When peaks overlap, waves reinforce each other. Dozens of films working together end up reflecting almost all the light.
Try changing the crystal thickness with the slider in Figure 1. The thickness determines which color reflects most strongly. If every film were the same thickness, only one color would reflect — that's the mechanism behind the colors of soap bubbles and jewel beetles.
So why is a sardine silver rather than rainbow-colored? In a sardine's body, crystals of slightly different thicknesses are mixed together. A thin film handles blue, a thick one handles red, and so on, each taking charge of a different color. Add up the reflections of every color, and you get a colorless mirror — silver.
Why bother becoming a mirror at all?
Dive underwater, and straight up looks bright while straight down looks dark. But turn to face sideways, and left and right look almost the same brightness. Light in the sea varies hugely between up and down, but is uniform in every horizontal direction.
Now imagine a fish with a vertical mirror standing on its flank. Someone looking at it from the side sees light reflected from "the sea on the viewer's own side." Since brightness is uniform in the horizontal direction, that reflected light matches the brightness of the sea behind the fish. So the fish's outline melts into the background (Figure 2).
What if the fish's body weren't a mirror? Light from above would be blocked by its body, and seen from the side it would stand out as a shadow darker than its surroundings. For fish that swim in open water with no cover, schooling together, that difference can be a matter of life and death.
How does this compare with human mirrors and other animals?
A human mirror works because electrons moving freely inside the metal reflect light. Even a silver mirror absorbs a few percent of incoming light and turns it into heat. A fish's mirror is made entirely of transparent materials, so if enough films are stacked, almost no light is absorbed. In fact, lasers and camera lenses use the very same "stacked transparent film" mirror trick. Fish arrived at the same answer long before humans started using this technology.
Other animals use the same guanine crystals too. A scallop's eye is reported to form images using a bowl-shaped mirror tiled with square guanine crystals. Chameleons are said to change body color by adjusting the spacing between similar crystal arrays. The same material can become either a "mirror" or a "color," depending on how it's stacked.
The silver powder from fish has been collected by hand for a long time. Powder taken from herring or ribbonfish scales is said to have been coated onto glass beads to make "fake pearls." Even today, guanine from fish sometimes ends up in the sparkle of cosmetics and nail polish.
Sardines and mackerel have dark bluish-black backs. To a bird looking down from above, a fish's back blends with the dark color of the deep sea. Dark on top, mirrored on the side — wherever the fish is viewed from, a different part of its body is hiding it in a different way.
Summary
A fish's silver comes from a mirror built of dozens of transparent guanine crystals stacked together. The light that each layer reflects just a little reinforces the others because the thicknesses match. Mix films of different thicknesses, and every color reflects, producing silver. Because light in the sea is uniform in the horizontal direction, a vertical mirror reflects the background and makes the fish disappear.
Instead of metal, fish built their mirrors from stacked transparent film.
And that mirror exists not to shine, but to vanish.
For more on how thin films create color, see why soap bubbles look rainbow-colored; for an animal that changes color by rearranging its crystals, see the chameleon's color change. For why silver fish swim in schools, see our article on fish schools too.
- Get one sardine or horse mackerel and tilt its flank little by little under a light. Confirm that at a certain angle it suddenly flashes bright, and at another angle it looks dark. Brightness changing with angle is proof that it's a mirror.
- Scrape a little of the silver powder off the flank with the back of a knife. Spread it on black paper and shine light on it from an angle. Individual grains may flicker blue or green.
- Place the same powder on white paper and view it from directly above. It should look less shiny than it did on black paper. This shows the powder itself isn't colored — it shines because of the light it reflects.
Wash your hands and tools well after handling raw fish. In fish that have dried out with age, the crystal arrangement becomes disordered and the shine grows weaker.
For those who want to know more ― terms, formulas, and textbook connectionsWe clearly mark the level, from junior-high science to university specialist courses
- JHSCovered in junior high school science
- HSCovered in high school physics (waves, light interference)
- HS+High school advanced content, or textbook sidebar material
- Univ.Not taught in high school — university specialist content (optics, biophysics)
- ResearchNot yet settled as textbook fact even at university — something researchers are still investigating
JHSTerms: this phenomenon has names
- Reflection and refraction of light: At the boundary between different substances, part of the light bounces back (reflection) and the rest bends as it continues on (refraction). That's why even a transparent sheet gives a faint reflection on its surface.
- Structural color: Color produced not by pigment but by reinforcement of light waves through a "structure" too fine to see. Jewel beetles, soap bubbles, and fish silver all belong to this family.
- Countershading (concealing body coloration): A coloring pattern with a dark back and a pale belly, so the animal blends into the background whether viewed from above or below.
JHSHSCheck with a formula: crystal thickness determines the reflected color
Here we set up the condition for the reflected peaks from the film's front and back to line up, and calculate the color (wavelength) reflected most strongly from a sardine's crystal thickness. We'll also estimate the fraction of light reflected at a single boundary.
| In symbols | λ = 4 × n × d (reflectance at one boundary R = ( (n₁ − n₂) ÷ (n₁ + n₂) )²) |
| In words | Strongly reflected wavelength = 4 × film's refractive index × film thickness |
| Where the formula comes from | The condition where the difference in the round-trip path of light inside the film, combined with a delay of exactly half a wavelength, makes "peak line up with peak" (light interference). When the film's optical thickness equals one quarter of the wavelength, the light reflected off the front and back reinforces. The reflectance formula is the Fresnel equation — describing how much light reflects at a boundary — written for light hitting straight on. |
| What the symbols mean | λ: reflected wavelength (nanometers), n: film's refractive index (no unit), d: film thickness (nanometers), R: reflectance (fraction) |
| Refractive index of guanine crystal n₁ | said to be about 1.83 |
| Refractive index of the gap between crystals (fluid-rich layer) n₂ | about 1.33 (nearly the same as water) |
| Thickness of one crystal d (example) | 70 nanometers (actual values are said to range from tens to about a hundred nanometers) |
| 4 × refractive index | 4 × 1.83 = 7.32 |
| × thickness = strongly reflected wavelength | 7.32 × 70 = 512.4 (nanometers, green) |
| Difference in refractive index | 1.83 − 1.33 = 0.50 |
| Sum of refractive index | 1.83 + 1.33 = 3.16 |
| Difference ÷ sum | 0.50 ÷ 3.16 ≒ 0.158 |
| Squared = reflectance at one boundary | 0.158 × 0.158 ≒ 0.025 (about 2.5%) |
| Reflectance for 10 stacked crystal/gap pairs | calculated under the peak-matching condition, said to reach roughly 99% |
A crystal 70 nanometers thick reflects green light strongly. At a single boundary only 2.5% reflects, but stacking 10 pairs produces a mirror that reflects almost everything. Mixing films from 56 to 94 nanometers thick works out to cover every color from violet to red (the slider's range in Figure 1).
HSHS+Thin-film interference and the quarter-wave multilayer
HSIn the thin-film interference taught in high school physics, only light reflecting off the surface of the material with the higher refractive index shifts phase by half a wavelength. The light reflecting off the front of the crystal (higher refractive index) and the light reflecting off the back pick up this "shift" differently. So when the round-trip path difference equals half a wavelength — that is, when the film's optical thickness is a quarter wavelength — the two reinforce each other.
HS+A structure where both the crystal and the gap are made a quarter-wavelength thick and stacked alternately is called a quarter-wave multilayer. Because light reflected at every boundary combines in the same phase, reflectance approaches 1 as the number of pairs increases. This follows the same logic as Bragg reflection seen in X-ray diffraction of crystals, where regularly spaced layers selectively reflect a particular wavelength.
Univ.Dielectric multilayer mirrors and the trick for canceling polarization
In optics, a mirror built from stacked transparent (dielectric) materials is called a dielectric multilayer mirror, and its reflectance is found by combining the Fresnel equation at each boundary using the transfer matrix method. A structure that mixes films of different thicknesses to reflect a wide range of wavelengths is called a "chirped" or "chaotic" multilayer, and fish silver is said to belong to this type. However, multilayer mirrors have a weakness: reflectance for light hitting at an angle drops depending on polarization (the Brewster's angle effect). A 2012 study reported that fish avoid this weakness by mixing the orientation of two types of crystal with different refractive indices.
📖 For the derivation of the formulas and further detail: Thin-film interference (Japanese Wikipedia) ／ Fresnel equations (Japanese Wikipedia)
ResearchWhat's still not fully understood
- How is the crystal thickness kept so uniform? How living cells grow guanine crystals to a precise nanometer-scale thickness and align their orientation is still being studied.
- Does it work against predators that can see polarized light? Squid and some fish are said to be able to detect the polarization of light. How well a silver mirror hides a fish from such predators is still debated.
- Can the mirror's properties be adjusted on the fly? Some fish are known to change color by adjusting the spacing of their crystals. Whether schooling silver fish adjust their mirrors depending on brightness is not well understood.
In other words, this article too describes things "as currently understood." A fish's mirror remains a subject that optics researchers still study as a model for designing new mirrors.
Textbook connections (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| JHS | 1st-year science, "Reflection and refraction of light" | Even a transparent sheet reflects a little light |
| HS | Physics, "Wave interference," "Thin-film light interference" | Film thickness determines the reflected color (Figure 1 / formula) |
| HS+ | Physics advanced topic, "Bragg reflection" | Dozens of films bringing reflectance close to 100% |
| Univ. | Optics, "Dielectric multilayers," biophysics | Broadband reflection and the trick for canceling polarization |
| Research | Biomineralogy, visual ecology | How crystals are grown, effects on predators |
| ― | Connection to daily life | Silver at the fish market, camera lens coatings, cosmetic sparkle |
- Denton, E. J. (1970). On the organization of reflecting surfaces in some marine animals. Philosophical Transactions of the Royal Society of London B, 258, 285–313.
- Denton, E. J. & Land, M. F. (1971). Mechanism of reflexion in silvery layers of fish and cephalopods. Proceedings of the Royal Society of London B, 178, 43–61.
- Jordan, T. M., Partridge, J. C. & Roberts, N. W. (2012). Non-polarizing broadband multilayer reflectors in fish. Nature Photonics, 6, 759–763.
- Palmer, B. A. et al. (2017). The image-forming mirror in the eye of the scallop. Science, 358, 1172–1175.
- Land, M. F. (1972). The physics and biology of animal reflectors. Progress in Biophysics and Molecular Biology, 24, 75–106.
※This article is a popular-science explanation for a general audience. The figures given are approximations meant to aid understanding of the mechanism. Crystal thickness and refractive index vary by fish species and body part, and values differ between studies.