Coral, often mistaken for a colorful rock or a plant, is actually a marine invertebrate that builds massive reefs in tropical oceans. The diverse and striking colors of a healthy reef are not simply the animal’s own pigmentation. A combination of the coral’s transparent body, a symbiotic partner, and specialized light-manipulating proteins all contribute to the final appearance. Understanding the source of these colors reveals much about the health of the coral and its environment.
The Inherent Color of the Coral Polyp
The fundamental building material of a stony coral colony is its skeletal structure, which is composed of calcium carbonate. This mineral structure, which the coral polyp secretes, is naturally white, similar to chalk or limestone. The coral animal, or polyp, has a soft, tubular body that sits within a cup-like depression in this hard skeleton.
The tissue layers of the coral polyp itself are generally translucent or transparent. If examined in isolation, the polyp appears mostly clear, allowing the white skeleton beneath to be seen through its tissues. This clear appearance is the baseline color of the animal before other biological factors come into play.
Symbiosis: The Primary Source of Coral Color
The most common colors on a healthy reef—browns, muted greens, and yellowish tones—come from a partnership with microscopic algae, known as Symbiodiniaceae. These tiny, photosynthetic cells live within the coral polyp’s tissues in a mutually beneficial arrangement. The algae use sunlight to produce energy and share the products of this process with the coral host.
This energy transfer is substantial, providing the coral with up to 90% of its required nutrients for growth and survival. The algae contain chlorophyll and other pigments like carotenoids, which create the range of golden-brown and olive hues seen across many reef species. A rich, earthy color signifies a dense, thriving population of these symbiotic algae within the coral’s tissues.
Fluorescence: Coral’s Own Color Pigments
While symbiotic algae provide muted colors, coral polyps also produce their own vivid pigments, resulting in bright blues, purples, and vibrant reds. These intense colors are due to a family of fluorescent proteins created by the coral animal itself. Fluorescence occurs when the proteins absorb light at one wavelength, such as blue light, and immediately re-emit it at a lower-energy, different color.
These fluorescent proteins serve important biological functions for the organism. In shallow, sunlit waters, the pigments act as natural sunscreen, shielding the symbiotic algae from damaging ultraviolet light. In deeper, low-light environments, they may help optimize the available blue light for the algae’s photosynthesis or act as a lure to attract small prey.
The White Coral: Understanding Bleaching
The stark white appearance of a stressed coral is known as bleaching, which occurs when the coral expels its symbiotic algae. When corals face stress, primarily from elevated water temperatures, the algae begin to produce toxic compounds. In response, the coral host is forced to eject its partner from its tissues.
The loss of the algae removes the primary source of the coral’s color, leaving the transparent polyp tissue stretched over the underlying white calcium carbonate skeleton. A temperature increase of only 1 to 2 degrees Celsius above the normal summer maximum can trigger this stress response. While a bleached coral is not immediately dead, it is severely stressed and vulnerable, having lost its main food source. Prolonged bleaching will inevitably lead to starvation and the eventual death of the coral colony.