The reef builders that generate their own water currents
At first glance, coral polyps seem to be totally at the mercy of their environment. They can’t move from where they settled down, and their food is in the water around them. So how do they manage to stay alive?
Dr Karl Kruszelnicki explains.
There’s two things most of us know about the Great Barrier Reef.
First, it’s big, and second, once every year, all the coral on the entire reef do their «spawning» thing in a giant sexual paroxysm, synchronised to the very second all the way up and down the entire reef: yes, according to the gutter press, «the largest orgasm on the planet».
Well, the Great Barrier Reef is huge. But coral spawning doesn’t happen everywhere at the same time.
The Great Barrier Reef has a surface area about one-and-a-half times bigger than the state of Victoria. It’s home to over 1,600 species of fish, 133 species of sharks and sting rays, and over 30 species of whales and dolphins. The Great Barrier Reef is about 2,300 km long, and is located between 16 and 160 kilometres offshore.
And it was made entirely by trillions of stationary animals called coral polyps. These animals are shaped like cylinders — a few millimetres wide, and a few centimetres long. One end sits on rock or other coral. At the other end is their mouth, surrounded by tentacles.
Corals go back a long way
Around 550 million years ago, evolution produced the first animals to have hard shells. They were Cloudinia: tiny, filter-feeding animals. Within 2 million years, they had built the oldest known coral reef in what is now called Namibia.
Back then, this reef on a shallow equatorial seabed was about 300 metres thick, and about seven kilometres long. Cloudinia attached themselves to fixed surfaces, and to each other, by extruding a «cement» based on calcium carbonate.
Making a «reef» had many advantages. They got protection from predators. The reef also «created» currents that were rich in nutrients.
Back around 1020 AD, the Islamic polymath, Al-Biruni, was one of the first to correctly classify corals as animals. He argued, cleverly, that coral were animals even though they never changed their location, because when he touched them, they responded.
Moving forward to today, there are some 400 different species of coral polyps, both hard and soft, on the Great Barrier Reef.
At first glance, coral polyps seem to be totally at the mercy of their environment. They can’t move from where they settled down. And their food is in the water around them.
They do get a reasonable flow of water over their bodies when the tidal flow is high. But a few times each day, when the tide is on the turn, there’s not much flow.
And what about those coral polyps who have been unlucky enough to land in a quiet lagoon, or inside a densely branched section of a reef, with tiny currents?
How do they survive?
It turns out that coral polyps make their own water currents. They wave their cilia (little hairs) and generate swirling currents that keep a thin layer of water moving parallel to their surface, like a mini conveyor belt. This moving current, up to 2 mm thick, simultaneously brings in foody goodness and takes away waste products.
On a good day, coral polyps can lay down 10 grams of calcium carbonate on each square metre. They have to do this continually, to keep ahead of the erosion caused by waves and other forces.
However, because the reefs are platforms made from calcium carbonate, they are very sensitive to changes in the rising carbon dioxide levels of the oceans.
One major study looked at 328 coral colonies from 69 reefs covering most of the 2,000+ kilometre length of the Great Barrier Reef. These corals ranged in age from 10 years to over four centuries.
The study showed that the rates of calcification between 1990 and 2005 dropped by 14.2 per cent. This decline was seen in both onshore and offshore reefs.
Similarly, brain corals in Bermuda have suffered a 25 per cent decline in growth and calcification since 1959. So this is a global ocean problem, not just one of declining quality in local coastal waters.
Maybe the coral will need to evolve to suit the changing ocean chemistry, caused by burning fossil fuels and dumping carbon dioxide into the oceans.
And evolution is linked to reproduction — and that brings us back to coral spawning, so I’ll talk more about that, next time.