Author: Lydia Vavoula

As you snorkel over a coral reef you see thousands of living organisms. Sponges, sea squirts, anemones, soft corals, hard corals, crinoids, algae. The list is endless. Each wacky life form begs the question, what are you? Animal or plant? Individual or a group? What do you eat? How do you reproduce? When it comes to coral, the answers are truly mind boggling. So What is Coral? At first glance corals look like plants but they are, in fact, animals. Most of the corals we see on a reef consist of hundreds, often thousands, of coral polyps that together form a coral colony. The individual polyp is often microscopic, but if we zoom in close we can see that polyps resemble tiny upside-down jellyfish. A coral polyp has a central cavity (its mouth) surrounded by tentacles, which it uses to capture plankton drifting through the water.  So how can such small animals, feeding on even smaller prey, build habitats visible from space? Millions of years ago, corals formed an extraordinary alliance, a friendship as I like to think of it, with a microscopic, plant-like organism called a zooxanthellae. Zooxanthellae are photosynthetic cells that live in coral tissue. They have a symbiotic relationship. Corals provide the zooxanthellae with a safe place to live, and in turn the zooxanthellae photosynthesizes, producing energy that fuels the corals growth and reproduction. Hard corals use this energy to make calcium carbonate, which is the stony outer structure that coral polyps live in and which forms the foundation of coral reefs. This vital step in coral evolution is how minute coral polyps have built reefs the size of countries. However, not all corals are reef builders. Some are soft, lacking a hard calcium carbonate skeleton. Others prefer a solitary life, with no zooxanthellae in their tissues