How corals got their colour

by Shreya Yadav

 

A long time ago, in the shallow, sunlit waters of the earth a coral polyp and an alga started a unique relationship that benefitted both. Read on to know what happened to next…

 

About 230 million years ago, when the earliest dinosaurs were beginning to walk the earth and strange lizard-like fish swam the open oceans, a plant and an animal living in shallow, sunlit waters were meeting for the very first time under a very special circumstance.

It was a time when everything was changing. The supercontinent Pangaea was cracking because of strong forces from underground. Water from the Tethys sea rushed to fill these gaps, creating rift valleys and ravines. The climate was hot and dry and forests were bursting with spiders and scorpions and millipedes. If you’d stood on the beach you would have noticed the snouts of crocodiles and turtles all poking out of the crowded sea! And if you followed one of them underwater, you would have seen the world’s very first coral reefs beginning to form.

Sudden boom

Just a little while before this (and by that I mean a few million years earlier), a cataclysm known as the Great Dying had wiped out over 90% of all living things on earth. What caused this is not certain, but it resulted in a drastic change in the earth’s climate and chemistry. Only a few things survived, and in the oceans, some coral-like animals managed to make it through.

These organisms, that till now had been small and solitary, began to thrive well in this new environment, growing in all kinds of shapes and sizes. They formed large boulders and mushroom-like tables. They multiplied in a maze of branches and cauliflower-shaped colonies. Earlier brown and pale, corals now appeared in a full spectrum of colours from deep violets to bright strawberry reds. They took over clear, shallow waters, growing fast and strong and forming extensive reefscapes that stretched for miles underwater. What was driving this sudden expansion?

Helping each other

A single coral colony is actually made up of hundreds of little animals called polyps. Each polyp has a mouth and a ring of tentacles that surround it. They use their tentacles to catch small fish and little creatures floating by, which they then swallow and digest in their stomachs. Until now, this was the only way corals could get energy. But in the changed environment after the Great Dying, they began to do something they had never done before — photosynthesize.

Photosynthesis is how plants, algae and some bacteria make their own food, using light-energy from the sun (photo) together with carbon dioxide and water (synthesis). The coral animal couldn’t photosynthesize, so it began to capture algae that could. The algae got a place to live inside the coral and protection from predators, and the coral used the food the algae produced for its own growth and reproduction. Because the algae (or zooxanthellae; “zoox” for short) were coloured, corals too began to look pink or green or orange depending on the kind they picked up. And that’s how corals first got their colour!

This one symbiotic association between plant and animal gave rise to an ecosystem that today supports thousands of animals, from burrowing crabs to brightly coloured fish and majestic rays. But it’s a delicate bond and can break easily. If the water gets too warm, corals lose their colour, and an entire world begins to fade. And today, it’s the strength of this relationship that’s being tested to its limits.

This article appeared in the Hindu in School on 29 October 2014.

Picture: A shoal of fish in a Lakshadweep reefscape.
Credit: Shreya Yadav/NCF