What is a coral?

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Photo by Francesco Ungaro on Unsplash (copyright free)

Individual coral animals, called polyps, are typically very small in size and, unlike their jellyfish and anemone family members, form huge colonies. This means when you look at "one" coral; you are actually observing thousands of individual animals... all of which are genetically identical! These colonies can form either hard or soft corals. Hard corals secrete calcium carbonate (limestone) and produce a rigid skeleton that individual polyps inhabit. Hard corals come in various lifeforms, with some resembling underwater boulders, which is why many people initially believe that corals are rocks. In comparison, soft corals do not have a skeleton and instead sway and move with the underwater current.

Photo by Joan Li on Unsplash (copyright free)

How does a coral feed?

Both hard and soft corals are permanently attached to the seafloor and cannot move about the reef, making feeding difficult. However, as we already know, coral polyps have a simple mouth surrounded by stinging tentacles. They use these tentacles to catch prey as it floats past in underwater currents, but this typically only provides corals with 20% of their energy requirements.

The other 80% comes from algae that live inside the tissue of a coral polyp. This algae, called zooxanthellae, has a mutualistic symbiotic relationship with the coral, where both zooxanthellae and the coral polyp benefit.

The zooxanthellae, just like terrestrial plants, undertake photosynthesis, where they use sunlight, water and carbon dioxide to create oxygen and energy in the form of sugar. However, they produce more sugar than they require and give the excess to the coral polyp, providing 80% of a corals energy. In turn, the coral polyp provides the zooxanthellae with a safe space to live within its tissues and waste products to power the photosynthesis reaction.

So back to our original question... is a coral an animal, plant or rock? A coral is an animal that sometimes looks like a rock and has algae living inside its tissues that undertakes photosynthesis. Evolution is amazing!

Maisy Fuller

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