The Secret Life of Corals – Part One

Corals are the oceans’ equivalent of tropical rainforests, with their spiky rainbow of branches weaving their way through our imaginations while our excited dreams dart between the thought of swimming with turtles and swimming with seahorses in the same manner shoals of fish flit between reef outcrops. Yet these almost mythical seeming structures are at risk of crumbling before our grandchildren set eyes on them. If polar bears on melting icebergs are the icon of catastrophe for polar regions, then coral reefs are the symbol for tragedy in our tropical waters.

Most coral reefs lie between the Tropics of Cancer and Capricorn, or more simply, as far North as Florida and as far South as Mozambique as they require salty, clean, sunny waters between 20-30°C to thrive. These creatures could be named the Goldilocks of the sea, not enough sun and they fail to grow to their full height, too warm and they emit a stress response and too many nutrients means that seaweed takes over the reef, stealing sunlight and seafood. Yet some corals flourish outside of this tropical region. One of the largest cold water reefs lies beyond the Norwegian coast, an 8,000 year old structure stretching over 40 kilometres while providing an oasis of food and shelter from the bitter sea for sea urchins, starfish and crustaceans. Back in the shallow azure waters of the tropics, the famous Great Barrier Reef covers almost 350,000 square kilometres, an area the size of Germany! While this may sound astonishingly large, the world’s reefs only cover 0.1% of our planet, yet they support 25% of marine life while an estimated 500 million people directly rely on these ecosystems for food, income and flood protection as their spiny bodies dissipate wave energy before it crashes on the coastline.

Coral’s fascinating appeal doesn’t stop at their looks as these tree-like structures possess some quirky traits. Coral reefs are formed when small larvae called polyps attach themselves to submerged hard structures, be that rock, the skeletons of old corals or manmade structures such as sunken shipwrecks. Once firmly in place, they can grow between 0.3 to 10 centimetres per year, fuelling this growth at night when teeny stinging tentacles unfold from their skeletons to trap unsuspecting tiny critters like zooplankton which make up their daily dinner. Many corals are part of this reef building group which provide a home for an algae called zooxanthellae and while algae sound a relatively dull green imposter to our multicoloured paradise, it is actually the algae itself which gives the coral its vibrant colour as crimson, magenta and indigo shades shine through the corals’ transparent skeleton. The relationship between zooxanthellae and coral is a happy arrangement where both partners get exactly what they want. The algae allowed to grow amongst the shelter of reef which promotes photosynthesis, a process producing sugars which are then used by the coral to form essential proteins, fats and carbohydrates.

Once the corals are settled in their fruitful relationship, reproduction can begin. Different species go about this in different ways; some provide a warm home inside their skeletons for the larvae until their antics become too much when females literally spit their offspring out through their mouth into the real world. Other corals take part in the truly magical event which is coral spawning. Spawning events on many reefs only occur once a year when the lunar cycle, temperature and currents align to allow a mass synchronised dance as corals all release either sperm or eggs into the water simultaneously, leaving them in a free for all to pair up with a partner, a bit like a Saturday night club scene.

Now that you’re even more amazed by the wonder of reefs, keep an eye out for the secret life of corals part two where we talk about the threats facing these majestic environments and what science is doing to help.

By Neve McCracken-Heywood