Corals and their algal friends
A coral colony is composed of thousands or millions of small animals called polyps. They range in size from a few millimetres to a centimetre. The polyp lives in roughly the outermost one centimetre of the colony and continuously grows outward by a little more than a centimetre per year by extending the outer limit of the dividing walls. Being animals, the polyps have no method of getting energy from sunlight by photosynthesis – they have no chlorophyll.
After a couple of hundred million years of evolution, however, the polyps have built a partnership with microscopic plants called zooxanthellae (zoox for short) which are like algae. The polyp gets energy from the zoox, and the zoox gets a comfortable home to grow inside the polyp. This symbiotic relationship with the zoox is key to how corals can adapt to different temperatures. The zoox give coral their colour, and also colour to many other marine organisms in which they live, for example, sea anemones.
Infant corals usually have no zoox, but zoox of many different types and species are floating around in the water. The coral somehow grabs them from the water and they grow inside the polyp. But on occasions this cosy relationship breaks down and the coral rapidly ejects the zoox. The coral is now bleached white and is in peril of starving if it does not take on new zoox.
Figure: Coral bleaches when the symbiotic algae are expelled. The algae give the coral its colour. (Picture from NOAA)
Bleaching is not usually a death sentence: it is a survival strategy
Corals eject their zoox under many different types of stress. The best known and most dramatic example is from high temperature. They can bleach from cold water, or if too much freshwater from rivers or rainfall reduces seawater salt concentration. Bleaching is not so much a death sentence as a survival strategy. Corals bleach because the zoox within them have become poisonous, or certainly disadvantageous, to the polyp and must be removed. Coral actively expunge the zoox during bleaching.
Most corals that bleach will survive, albeit a little shaken from the experience. It is akin to many other survival strategies seen in nature. For example, many types of Australian trees shed their leaves during extreme droughts in order to conserve water. They regrow the leaves once the drought is over. For corals, after the stress is over, they take back the zoox, but not necessarily the same type.
The good news for corals is that they are highly adept at “shuffling” zoox which come in many different strains. A particular species of coral has a choice to take on many different types of zoox, and may have a few different types inside them at any one time. Some “high octane” zoox will allow the coral to grow fast but be more susceptible to bleaching from high temperatures. “Low octane” zoox will make the coral grow slowly but be less susceptible to bleaching. Which strategy is better for a particular coral colony at a particular time is like a roll of a dice and will depend on the weather.
For many corals on the GBR, particularly the light and delicate “plate” or “staghorn” corals, their life strategy is to live fast and probably die young. They can certainly grow much faster than the solid blocks of the “massive” corals because they produce a much smaller mass calcium carbonate skeleton to reach the same height. But if a bleaching event does not kill the plate and staghorn coral, their delicate structure will very likely mean they will be obliterated by a cyclone within 20 years. As it happens, the return incidence for bleaching events and cyclones is roughly the same and it is no coincidence that these physically delicate and easily damaged plate and staghorn corals are the most susceptible to bleaching and have a life expectancy of a couple of decades. Taking on high octane zoox and growing fast, while risking being killed by bleaching, is all part of their philosophy.
The risk in taking on low octane zoox is that the coral will be overgrown by the neighbouring coral which had used high octane zoox. It is better to grow fast when it is likely that one is to be killed by the passage of the next cyclone. The opposite are the massive corals that can live for centuries and become a solid block of calcium carbonate, metres across, weighing tons. These grow more slowly, and will generally pass through a cyclone relatively unharmed. They have a long-term strategy, and quick death by bleaching is not part of it.
But corals are very versatile – after they bleach they can take on a different type of zoox which may make them less susceptible to high temperature bleaching. Few other organisms have this type of adaptability to changing temperatures. Whereas most organisms need many generations to change their genetic make-up to adapt to changing temperatures, corals can do this in a few weeks, simply by changing zoox.
Corals thus have a unique ability to deal with changing climates, something that is not surprising considering what upheaval these species have seen over the millennia. But there is another reason why corals must deal with different temperatures even if climate change does not occur – unlike most organisms on land, the progeny of a coral may end up living in a different temperature to its parents.
Reproduction of corals involves release of larvae that float around, without zoox, in the ocean current, usually for a few days, but occasionally up to 100 days, before they finally settle. In this time, they may travel hundreds or, on occasions, perhaps a thousand kilometres into warmer or colder water. The difference in temperature between the north and south of GBR can exceed 2o C and inshore water can be a degree or two hotter in the summer than offshore water, and colder in winter. This is in addition to the 50C to 100C temperature swing between summer and winter. Because the coral progeny may well settle in a region of totally different temperature from its parent, it needs to be able to deal with a range of temperatures considerably greater than the modest rise in temperature that has occurred during the last century (0.5-10 C).
Corals should not be thought of as a particular species. It is a partnership between an animal polyp species and a small plant zoox species. There are many combinations of these pairs. The coral’s ability to “shuffle” the zoox to deal with temperature change is now well known in the scientific literature. It makes them far more able to cope with temperature changes than virtually any other organism. It probably helps to explain why corals have survived hundreds of millions of years, most of which have been far hotter than the present relatively cool period of the Earth’s history.
The real mystery today is why coral’s unique ability in this regard is not well publicised by coral reef biologists.
 Marshall, P. and Schuttenberg, H. (2006). A Reef Manager’s Guide to Coral Bleaching. Townsville, Australia.: Great Barrier Reef Marine Park Authority.
 Baker, A.C. (2003). Flexibility and Specificity in Coral-Algal Symbiosis: Diversity, Ecology, and Biogeography of Symbiodinium. Annual Review of Ecology, Evolution, and Systematics, 34(1), pp.661–689.
 Buddemeier, R.W. and Fautin, D.G. (1993). Coral Bleaching as an Adaptive Mechanism. BioScience, 43(5), pp.320–326.
 Marshall, P.A. and Baird, A.H. (2000). Bleaching of corals on the Great Barrier Reef: differential susceptibilities among taxa. Coral Reefs, 19(2), pp.155–163.
 Guest, J.R., Baird, A.H., Maynard, J.A., Muttaqin, E., Edwards, A.J., Campbell, S.J., Yewdall, K., Affendi, Y.A. and Chou, L.M. (2012). Contrasting Patterns of Coral Bleaching Susceptibility in 2010 Suggest an Adaptive Response to Thermal Stress. PLoS ONE, 7(3), p.e33353.
 See Endnote 5