Colors – Where Do the Brilliant Colors of Coral Reefs Come From?

One of the most memorable experiences on a tropical reef is its extraordinary diversity of colors. Brightly colored corals, shimmering reef fish, giant clams, and sea slugs create an underwater landscape unlike any other ecosystem on Earth.

But where do these colors actually come from?

The answer lies in the interaction between light, chemistry, and biology. Understanding reef colors therefore begins with understanding light itself.

Seeing Color

Whenever we perceive a color, three components are involved:

• a light source, such as the Sun or a dive torch,

• an object that modifies the light, for example a coral, fish, or sponge,

• and a detector, namely the photoreceptor cells in the retina of our eyes.

Light leaves its source, travels through a medium such as seawater, interacts with the object, and finally reaches our eyes. During this journey it may be absorbed, scattered, reflected, refracted, or even emitted by fluorescent molecules. The combination of these processes determines the colors we ultimately perceive.

The same principle is used by a scientific spectrometer. Instead of the human eye, an optical detector records the light after it has interacted with a sample, allowing its spectral composition to be measured with great precision.

What Is Light?

From a physical perspective, visible light is electromagnetic radiation. Although this definition is straightforward, the way light creates the enormous variety of colors we observe is considerably more fascinating.

White sunlight can be separated into its individual wavelengths with a prism, producing the familiar rainbow. Each wavelength corresponds to a different perceived color.

Human vision is sensitive to wavelengths of approximately 380 to 750 nanometres (nm), although the exact limits vary slightly between individuals. Short wavelengths appear violet (V) and blue (B), while long wavelengths appear orange (O) and red (R) (picture from Wikipedia).

Beyond the visible range lie ultraviolet (UV) radiation at shorter wavelengths and infrared (IR) radiation at longer wavelengths. Humans cannot normally perceive either, but many animals can detect parts of these regions of the electromagnetic spectrum. Numerous reef organisms, for example, are sensitive to ultraviolet light, which plays important roles in communication, camouflage, and photoprotection.

Another property of light is polarization. While humans are largely insensitive to polarized light, several marine organisms are believed to use polarization for navigation, prey detection, or communication. This fascinating topic deserves its own discussion and will not be covered here.

Describing Light

To understand reef colors scientifically, it is useful to describe light by its measurable properties.

The two most important are:

• Wavelength, which determines the perceived color.

• Intensity, which describes how much light is present at each wavelength.

Light consisting of only one wavelength is called monochromatic. In nature, however, most light sources are mixtures of many wavelengths and are therefore polychromatic. Sunlight, for example, contains a continuous range of wavelengths across the visible spectrum.

The most informative way to describe such a mixture is with a spectrum. A spectrum plots light intensity against wavelength and reveals exactly which wavelengths are present and how strong they are. The picture shows a triangular prism dispersing a beam of white light (picture from Wikipedia).

Interestingly, very different spectra can sometimes appear identical to our eyes. This is because human color vision is based on only three types of cone photoreceptors, which compress the rich physical information contained in a spectrum into the sensation of a single perceived color.

Understanding spectra is therefore the key to understanding why corals fluoresce, why reef fish display such vivid patterns, and why colors change with increasing water depth. These topics will be explored in the following chapters.