coral
a scientific introduction Article by Jonathon Stillman, Ph.D. Images by Boun Khamnouane (except as noted)
Image by Greg Rothschild.
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f you have been successful as a reef hobbyist, or if you just like to look at pictures of others’ tanks, you’ve probably noticed that corals have an amazing array of colors. Some corals have solid, uniform colors, while others have color gradients so smooth that they appear to have been painted by some microscopic artist with an airbrush. Just as we can stroll through an art museum with great joy looking at the pretty pictures without understanding the context behind those paintings, we can appreciate and enjoy the beautiful colors of corals without any further thought. However, just like great masterpiece paintings, the colors we see in corals are the result of a complicated interplay of factors. Understanding the rational basis behind what an artist painted enhances a viewer’s appreciation of the art. The same could be said for your corals – understanding what makes corals colorful may help you appreciate them all the more. In this article, I will explain some of what is known about what makes corals colorful. The article is intended to be introductory and written at a basic level. For more advanced information, I have provided additional references (all freely accessible) for those readers who wish to learn more. First, a little physics lesson, as we need to know something about the electromagnetic spectrum to understand color. This is the spectrum of electromagnetic radiation, or wavelengths of energy,
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that can propagate through space. What we perceive as light is only a small fraction of the total electromagnetic spectrum. We can detect other regions of the electromagnetic spectrum, such as the near infrared, as heat. The longest wavelength and lowest energy portions of the electromagnetic spectrum are radio waves, and as energy increases to microwaves and then infrared radiation, the wavelengths get shorter and the energy increases. The range of visible light begins with dark red colors, and that’s why the first visible light that hot objects radiate is red. Most of the radiation hot objects emit is in the infrared, but some is higher energy and we can detect it with our eyes. Visible light then proceeds through orange, yellow, green, blue and violet as energy increases. Ultraviolet radiation has a shorter wavelength and higher energy, followed by x-ray and gamma radiation, whose energy we know to be damaging. The energy in these short wavelength regions of the electromagnetic spectrum can be absorbed by biomolecules such as DNA and proteins, and can damage them by breaking molecular bonds. This can convert the biomolecules into mutated forms that can sometimes have deleterious consequences such as causing diseases like cancer (the details of which are well understood, but I will not go into them here). While our atmosphere absorbs or blocks much of the higher-energy electromagnetic radiation that comes from the Sun and other stars, UV radiation is present on the surface of our planet and can penetrate into shallow water.
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