Reef-building corals are known for their beautiful colors and shapes. But they are also significant biologically because they are a consortium of species that together can produce structures of biological origin so large they can be seen from space. These structures, of course, are coral reefs, the grandest example of which is the Great Barrier Reef off the east coast of Australia.
The species that come together to form the organism we recognize as reef-building corals are an animal related to sea anemones and sea jellies, and a single-celled alga. Together they live in a symbiotic relationship in which the algae live inside the tissue of the animal, and through photosynthesis provides food to support the animal, while the waste from the animal provides the algae with the fertilizer it needs to grow and reproduce. This intimate relationship in which each member of the partnership gains an advantage is called a mutualism. This combination has resulted in the development of the most diverse ecosystem found in Earth’s oceans – the coral reef.
In today’s photo you see a close-up of a small bit of a patch of pineapple coral, which is found throughout the tropical waters of the Indo-Pacific region. The bright green patches surround the mouth of the individual coral animals, which are separated from each other by the ridges of calcium carbonate (limestone) that the coral secretes to form its own individual cup, and ultimately, the bones of the entire reef. It is tempting to see the green and assume it is the algae the lives inside the coral anemone. But in fact, this is most likely a pigment used to shield the sensitive parts of each coral animal from ultra-violet light. The algae are actually brown in color, and create the color of the living tissue covering the underlying coral skeleton surrounding the center of each animal.
Despite the limestone skeletons and the supercharged biology of this symbiotic relationship, coral reefs worldwide are in danger. Coral exists within a narrow band of water temperature around the globe. A true Goldilocks story, the water temperature must be “just right” to allow corals to grow and thrive. Too hot, or too cold, and they cannot survive. Unfortunately, as the Earth’s oceans warm from heat absorbed from a warming atmosphere, corals are finding a narrowing niche in which to survive.
What can we each do to help reverse this trend?
Today’s photo was taken on Rainbow Reef off the island of Vanu Levu in the Republic of Fiji with a Canon EF100 mm f/2.8 Macro USM lens on a Canon EOS 5D Mk III in an Ikelite underwater housing. Light was provided by an Ikelite DS161 strobe set to eTTL exposure. The exposure was set to 1/60 sec at f/11 and ISO 200.