Corals have been turning up in my social media lately. Not the actual corals, of course, but stories about them. A very large coral reef has just been discovered in the mouth of the Amazon and the Great Barrier Reef is evidently badly bleached at the moment thanks to abnormally high sea temperatures. I figure this is a good time to talk about some coral basics.
Corals are animals in the same phylum as jellyfish—it seems odd that corals are climate losers while jellyfish might well be winners until one remembers that a phylum is a very large group. Sharks, for example, belong to the same phylum we do. So they’re not closely related, they just have very broadly similar structures. Corals are colonial, so what might look like a single coral is actually a whole colony. Individual coral animals, polyps, are rather like tiny upside-down jellyfish, each sitting in its own cup-like exoskeleton.
Many corals depend on symbiotic algae (zooxanthellae) for food, though they also grab and eat plankton. The color of corals depends on the algae in their bodies. Under stress, corals will expel their algae, turning white in the process. That’s coral bleaching. Bleached coral isn’t dead and can get new algae, but until they do they are extremely vulnerable (and, one imagines, hungry). Frequent or severe coral bleaching events can kill corals, as can any additional stresses that might occur while the animals are vulnerable.
Unusually hot water is one cause of bleaching. The warmer the water is, the faster the algae photosynthesize, meaning the more oxygen they release into the coral bodies. While corals do need oxygen to live, too much oxygen is a poison and the corals dump their algae to protect themselves. Corals vary in their heat tolerance, but they live at the upper edge of that tolerance, so even slight increases in temperature can hurt them. Bleaching on a large scale appears to be new–there’s no evidence for it before modern times.
Corals have very narrow habitat requirements, especially those that use zooxanthellae. Their water must usually be clear and sunlit, for photosynthesis, so they cannot grow anywhere more than a few hundred feet deep. The water can’t be too hot or too cold. In theory, a warmer world could support more coral, since a larger portion of the sea would be warm enough for them. In actual fact, though, climate change is moving too quickly—new reefs cannot establish quickly enough to balance out those lost to increasingly warm water in the tropics where they live. Rising carbon dioxide levels are also causing the oceans to become more acidic, and acid water eats away at the calcium-rich exoskeletons corals build. It’s not to the point where corals are shrinking, but they grow more slowly than they used to. Changing ocean currents and storm tracks also can stress corals and are also related to climate change.
This year is especially bad because it’s an El Nino year, which piles its own warmth on top of longer-term climate change.
Corals face risks from other directions, too, such as water pollution and physical damage from boats. So, as usual, the losses we’re seeing come from multiple sources. Between one thing and another, corals around the world are in trouble. Some areas have lost 80-90% of their corals already.
Obviously, corals are intrinsically important themselves, but coral reefs also provide a lot of habitat space for other animals. Some, like parrot fish, actually eat coral. Many others hide in nooks and crannies in the reef or take advantage of different microhabitats in different parts of the reef—a coral reef has a lot more surface area than a barren sea floor, so the reef essentially makes the part of the world it occupies a lot bigger. Something like a quarter of all marine species worldwide depend on corals.
Reefs are the oceanic equivalent of rainforests in terms of their biodiversity. If we lose the reefs, we lose the reef inhabitants–which is another example of how climate change can simplify and shrink the biosphere by taking out many species indirectly.
Lest this seem all like doom and gloom only, remember that we can still do something about climate change if we hurry. So don’t get so distracted by the presidential horse race that you forget to vote climate-sane people into Congress. Despite what you may be seeing on social media, voting matters.