Coral reefs are said to be among the world’s most beautiful locations. They rival tropical rainforests for their biodiversity. But these days, rather than hearing about their wealth of life and colour, we hear a lot about something called coral bleaching: when the corals in an area turn deathly white. If bleaching isn’t reversed, corals usually die. This undermines entire reef ecosystems, and as you may have heard, it is happening more and more often.
But that leaves a lot of questions. Are corals animals or plants? How do they feed and reproduce? And what’s actually behind the bleaching? Corals are animals with an ability that very few can claim to share. It’s a precious talent that their whole reefs rely on, and bleaching is what happens when they lose it.
Corals look like brightly-coloured trees, shrubs or mosses beneath the sea, organic yet somehow stony. These plant-like forms are coral “heads”. Corals thrive in the light, and the shape of their heads helps to catch the sun. Understandably, until the 18th century, scientists thought that corals were plants. It took inspection under the microscope to convince them otherwise. That was the first among several surprises corals have thrown science.
A coral head is actually a colony, more like a hive than a tree. Whether it branches, spreads across the seabed or grows into pillars, a coral head bristles with individual animals called “polyps”. When you look at a polyp, you see a ring of tentacles surrounding a mouth. They’re usually no more than a few millimeters across and a centimeter or so deep, and there’s not much else to them but a stomach. Much larger polyps do exist, but reefs are built by small ones.
Every polyp in a colony is genetically identical – a clone. This is because colonies grow by budding new polyps off old ones. Over months and years, a head extends and expands, its polyps multiplying to increase its surface. A head’s stony skeleton, a chalky substance like sea shells, is secreted by the polyps at their base. It grows at about the same speed as your hair. Big, sturdy heads take many years to form, but more fragile branching ones grow much faster.
Corals “broadcast spawn”, sending out swathes of sperm and eggs out into the sea. Each species of coral synchronises its spawning so that every polyp in every head spawns at the same time, usually around the full moon. When they meet, a baby polyp is formed and finds somewhere to settle. If all goes well, a new head is founded.
By nature, corals are predators and scavengers, eating what comes within reach of their tentacles without ever moving place. Like jellyfish and sea anemones (fellow cnidarians), corals have a type of special stinging cell covering their tentacles. If an unfortunate passer-by touches one, the stings fire into its body, sometimes immobilising or killing it. Having made a catch, the tentacles drag it into the mouth and the polyp curls up to digest it. Corals can kill things like shrimp and fish, but they also grab organic debris that flows past on the currents. So that colourful undersea tree is more like a swarm of hungry clone mouths waiting to gobble whatever happens to get close. (Not that they pose any threat to humans, of course.)
Though corals are animals, they live like plants. They evolved into this vegetative lifestyle living on what they could catch, but that’s not what supports the corals of the great tropical reefs. These corals can get energy from the sun using photosynthesis, like a real plant. Though not unique to corals, this ability is very rare in the animal kingdom, and it allows them to thrive in otherwise very nutrient-poor seas. Charles Darwin called coral reefs the oases of the ocean, because otherwise waters around the equator support very little life.
Corals bought the secret of solar power by forming a pact with a microscopic ally: a type of algae called zooxanthellae, or “zoox”. Millions of years ago, the two organisms set up a “symbiosis”, a relationship that benefits both parties: the corals offer protection and certain scarce nutrients for the algae, who return the favour by converting the sun’s rays into chemical energy and donating the vast majority of it to their hosts. That, at least, is the accepted account. Some marine biologists suspect that the relationship is closer to an enslavement.
Because they can use the sun to grow and produce biomass (living matter), corals with zoox are, like plants, “primary producers”. This means they produce the basic source of food in their community, as well as sometimes the physical foundations. Every other reef species depends either directly or indirectly upon them, as do many people around the world who rely on reefs to provide seafood or protect shorelines from strong waves.
Other corals live without zoox and just get their energy from eating. Some of these do contribute to reef formation, and they can even build them deep in cold oceans, as far north as Scotland and Norway. But the largest, most productive and most biodiverse reefs are built by photosynthetic corals because of the abundant sunlight they have access to. There are also solitary corals, which live as independent polyps without forming heads, some of which keep zoox. These can be big and colourful, so they’re popular with aquarium keepers, but again, they are not key to ecosystems such as the Great Barrier Reef in Australia.
So what’s causing the bleaching? Bleaching is triggered by certain changes in the environment, such as pollution, temperature shifts, chemical imbalances and infections. The stress these cause seems to tear apart the coral-zoox relationship. It’s an alliance that is actually quite tense. Normally, it’s worth it for both parties, but when unexpected problems arise, one of them backs out. Scientists aren’t sure whether the coral ejects the algae or whether the algae leave. Either way, most of the time, this is not a disaster: corals can pick up more zoox when things go back to normal. In fact, some think that the coral boot out the zoox to avoid supporting them while they’ve got other problems on their hands.
The problem is that man-made changes tend not to go back to normal. After going long enough without zoox, many corals will die. Most bleaching occurs due to increased temperature, which we all know is happening gradually all over the world. While pollution can perhaps be avoided or remedied, climate change will probably bring death to most coral reefs.
Corals are also sensitive to increases in ocean acidity. As carbon dioxide accumulates in the atmosphere, more CO2 dissolves into it. This produces acid, and excess acid encourages bleaching. The ocean has become 30% more acidic since the mid-19th century. On top of that, the more CO2 that dissolves, the more difficult it is for sea creatures to make shells and skeletons. So carbon emissions are creating a serious double-whammy for coral reefs.
What can we do about it? I’ve not got a lot to offer. We can change laws and practices to avoid pollution, but global warming is becoming less and less reversible as time goes on. Work is in progress to grow corals from “seed” (usually nubbins taken off wild heads) and replant them and restore reefs. This has promise, but whether corals avoid disaster really depends on how fast they can adapt to changing temperatures. Judging from reef losses so far, this may not be fast enough.