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‘Rise above yourself and grasp the world’ Archimedes – attribution

fountains of good stuff 4: coral

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This is, at last, a new topic for my fountains of good stuff podcast series.

In 2009, as part of the celebrations of Darwin’s birth in 1809, I read The voyage of the Beagle, and very enjoyable and fascinating  it was. Within it was a treatise, of sorts, on coral reefs, much of which I’ve forgotten, but it set me to wondering, what exactly is coral? I’ve gathered it’s something alive, yet it doesn’t seem so, it seems like bone, or chalk, or some kind of highly porous rock. Anyway Darwin’s writings, and those of others on coral reefs, haven’t helped me to any sort of understanding of these creatures, so it’s time for some research.

So what’s the first thing that happens as I begin this post? David Attenborough’s The Blue Planet comes on TV, and it’s all about coral reefs, and even quite a bit about coral…

There are more than 400 species of coral on the Great Barrier Reef alone, and they’re often said to combine to make that reef the largest living organism on earth, but that’s a bit of a cheat, because it’s better described as an ecosystem, made up of 3000 separate reefs, just as coral is actually made up of individual but genetically identical multicellular organisms, called polyps. But before going into that, I should mention, and photographically illustrate, the exquisite beauty of coral in all its variety – a beauty I’ve only recently discovered as a non-scuba-diving armchair adventurer. Coral is non-technically divided into two types, bony and soft. The soft kind include sea pens, which inhabit tropical and temperate waters around the world.

a spectacular sea pen from the waters around East Timor

a spectacular sea pen from the waters around East Timor

An amazing example of bony coral is the aptly named brain coral, found in shallow warm-water reefs around all the world’s oceans.


where’s the corpus callosum?

These two photos will do for now, but there are many other spectacular varieties of coral, bony and soft, including blue coral, black coral, pillar coral, sea whips, sea feathers, stag-horn coral and many more.

Now let’s get back to those polyps. These spineless organisms are only a few centimetres long, and radially symmetrical (that’s to say, there’s no left or right, only top and bottom), with tentacles at one end, surrounding the mouth, and at the other end a basal area that exudes a calcareous exoskeleton called a calicle, which is structured so that the polyp can retreat into it when threatened by predators.

So it’s by means of these exoskeletons, built up over generations by these tiny individual polyps that make up the coral head, that we recognise particular species.

Rather confusingly, corals reproduce both sexually and asexually. The coral head grows by means of asexual reproduction of polyps, but mostly corals sexually reproduce by spawning, with polyps of the same species releasing gametes, or sex cells, all at the same time, around the period of the full moon. Most corals gain most of their nutrients and energy from algae living within their own tissue. These one-celled flagellate protozoa, called zooxanthellae, have a neat symbiotic relationship with their hosts, trading the products of photosynthesis for inorganic nutrients. Some corals also catch small fish and plankton, using stinging cells in their tentacles.

Most corals live in shallow, warm-water communities, contributing greatly to reef structures. When they become stressed, due more often than not to temperature or climate change, the zooxanthellae, which give corals their particular colour, are sometimes expelled, leading to the effect known as ‘coral bleaching’. To give more detail, some of the environmental triggers that lead to bleaching include increased water temperatures; acidification; a decline in zooplankton leading to starvation; solar irradiation; increased sedimentation; bacterial infections; changes in salinity; herbicides; exposure to wind, and elevated sea levels. In fine (to resurrect Henry James), they’re very sensitive souls. Whatever the trigger, it affects the coral’s ability to supply ammonium and carbon dioxide to the zooxanthellae, nutrients essential for photosynthesis. The coral is unable to prevent the zooxanthellae from dividing. The algae retains more of the photosynthesis-derived carbon, leading to an energy imbalance, whereby the coral is unable to maintain parasitic control over the symbiont. Or something like that.

Not all corals are shallow-dwelling, warm water inhabitants. Some live in cold waters as deep as 3000 metres, and don’t have intimate relations with algae. And zooxanthellae (I’m becoming a wiz at spelling this) don’t just have intimate relations with coral, as many a sea anemone will tell you.

Anyhow, this is just a fraction of the information available about coral that I’ve looked at. You’d do well to try your own research, or better still, get snorkling and have a look yourself, wherever you can. Sadly, some 80% of the coral reefs in the south-east Asian region are endangered, though they’re doing somewhat better in other regions.

Written by stewart henderson

June 10, 2013 at 10:50 pm

Posted in coral, science

Tagged with ,

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