Brain Coral

This brain-like mask is actually a whole colony, or "head," of brain corals. An individual coral is a tiny, tentacled creature, called a "polyp", which lives inside a hard shell, or "exoskeleton." This shell is formed from calcium carbonate: the shells of snails, shellfish, and eggs are made of the same thing. Over the years, as new generations of corals appear, the old skeletons pile up underneath to form huge coral reefs. Reefs can be absolutely gigantic, and take ages to grow— most brain coral colonies grow at a rate of only a few milimetres a year, or less than a metre a century! The biggest brain coral head we know of, off Tobago in the Caribbean, is almost 5 metres wide.

Reefs are only found in warm tropical ocean waters, but they cover around 300,000 square kilometers of the ocean floor, bigger than the UK and Ireland put together. Compared to empty, sandy parts of the ocean floor, reefs are teeming with life. They offer a source of shelter, protection, and food for all kinds of creatures— a healthy reef is usually swarming with fish, crabs, worms, and every other kind of marine organism you can think of. Their role in these undersea ecosystems is a lot like that of plants on land. They can't make their own food from sunlight, but their bodies contain algae called Zooxanthella which do it for them by photosynthesis.

Coral reefs can be suprisingly delicate! Careless tourism or fishing can break off chunks of the reef that took centuries to form. They're also sensitive to small changes in the acidity, salinity, and temperature of the water that surrounds them, so global warming is bad news for the corals. When a coral's not feeling well, it expels the energy-giving algae from it body and can eventually die. Coral's vibrant colours also come from the algae, so they turn white as well, in a process known as coral bleaching. Don't give up hope yet, though! People around the world have put a lot of thinking into how to protect and restore them. One cunning strategy is to give the coral something to hold on to: cement balls, old train cars, or other objects are dropped onto the ocean floor to form the starting point for new reefs.