a little bit of the Great Barrier Reef
So I want to get my head around corals, so to speak, just a bit more than it is now.
Corals are invertebrate animals, somewhat related to jellyfish, which they resemble, in an upside-down way. They’re anthozoans, the largest class of organisms within the Cnidaria phylum. Jellyfish also belong to this phylum, in the class known as scyphozoa. I won’t remember this for long. Nor will I remember zooxanthellae, of which more soon. And being animals, they have to breathe, something to note.
Corals are colonial animals, not like I’m a colonial Australian, but more because they live in colonies, methinks. They’re made-up of these little plant-pot things called polyps, in their many many thousands. Each of which, I’m told, is an individual animal. So they’re animals colonised by animals. Or a colony of animals creating a superorganism. Or a super-organisation of animals called a colony. As they grow they somehow create skeletons which hold the colony together. Somehow? Well, each polyp has a corallite skeleton, made of aragonite, a crystalline form of calcium carbonate (limestone), within which it sits, apparently. And the polyp makes the skeleton, somehow, I think. It seems that individual polyps wouldn’t be able to survive in isolation, so the super-organisation is essential, as is the skeleton. They build their skeletons by lifting themselves out of the corallite and depositing limestone in the vacant space.
So the corals absorb seawater from which they obtain calcium in the form of bicarbonate (HCO3), aka sodium bicarbonate – but where’s the calcium in that? Ah, it must somehow be related to the limestone mentioned earlier. But I’m still confused. Anyway, these elements form calcium carbonate within the polypian tissues, and that’s how the skeleton is made. Calcium is fairly abundant in sea water, at around 400 ppm.
So every individual coral, aka polyp, sits in this cup-shaped skeleton or corallite. It has a central mouth surrounded by tentacles. When the coral polyp dies, its skeleton, the corallite, adds to the structure of the reef. So, a polyp consists of a stomach, within the corallite skeleton, and a mouth on top, more or less concealed by all those tentacles. The tentacles have stinging cells, which can stun their prey, after which they pull the prey into their mouth. That prey includes tiny fish and plankton.
Within their tentacled tissue can be found nematocysts and zooxanthellae. Nematocysts are intracellular organelles that are found in Cnidarian critters. They contain toxins which are very handy for the aforementioned stunning and capturing. The nematocysts’ structure and action are pretty amazing. They exist within the tissue, in a capsule just below the surface, as a barb and a thread, coiled under some pressure. This is separated from the outer sea world by a tiny flap called an operculum, and when the unwitting prey mooches by, it triggers the operculum to open, shooting out the harpoon-like barb on its thread. Stunning stuff. The weakened or killed prey are pulled into the polyp mouth by those tentacles, and the nematocyst returns to its capsule.
Apparently this nematocyst system only provides about 10% of a coral’s food, so let’s look at zooxanthellae. They live in the tentacles of the polyps and they look like algae. In fact, that’s what they are. They have a symbiotic relationship with the coral and are able to photosynthesise, that’s to say, gain energy, or ‘food’, from sunlight. The way they, or any other plant, can do this, is fiendishly complex, and is explained in Oliver Morton’s book Eating the Sun, one of the most intellectually challenging books I’ve ever read. Needless to say, I’m no scientist. Anyway, these zooxanthellae provide the rest of the coral’s energy – yes, 90%. It’s a symbiotic relationship. Corals, being animals, breathe in oxygen and breathe out CO2, which the zooxanthellae utilise as well as sunlight. The zooxanthellae create lipids and sugars from photosynthesis, which the coral also profits from.
We usually think of coral reefs close to the surface that people can snorkel around in, but they can be found in much deeper waters. These corals and reefs are generally very different from the familiar ones. Deep reefs are relatively new to us, and continually being discovered, but they’re not outside of our influence, apparently. Scientists are telling us that ‘all corals are struggling as a result of human activities’, no matter where they’re situated – Antarctica, for example.
Whether deep or shallow, coral reefs are created by this foundational species – coral – around which huge ecosystems are built. But the deeper corals don’t have those photosynthesising single-cell algae known as zooxanthellae, the loss of which causes coral bleaching in shallower corals. So the vibrant colours we see in corals are due to their zooxanthellae, which raises the question – are the variations in colour due to different types of zoothanthellae, or are they due to different types of reaction, with sunlight, sea-water and such? Here’s what google’s ‘AI overview’ has to say, in my potted version:
Corals come in many colours because of the zooxanthellae inside them and the chlorophyll pigments they produce. This occurs as a part of photosynthesis, which gives the coral its colour. The number of zooxanthellae and the amount of chlorophyll determine the coral’s colour.
So that sort of answers the question. And as we go deeper into the seawater, between 50 and 100 metres, there is still zooxanthellae and photosynthesis, but less and less of it, and less sugar production. Corals do have fluorescent proteins in their tissues that can convert the available light into more algae-friendly wavelengths, but there’s basically no available light past 200 metres. Yet plenty of corals live beyond such depths, without the symbiotic algae. And that doesn’t mean they turn white, like more surface corals do. In fact, deep sea corals come in a great variety of colours. Varieties known as ‘black corals’, for example, are named only for the colour of their skeletons, which are built differently from shallow water corals. Instead of calcium carbonate, these deeper corals build their skeletons from protein and chitin. The chitin turns black when dried out. These skeletons are more flexible but still strong enough to support the coral.
So these deep sea corals are quite different in appearance from your barrier reef corals – they don’t leave behind those massive bone-like skeletons. They’re more like individual plants or trees. Still, they’re found in forest-like collectivities which the cognoscenti label as reefs, some of which are thousands of miles long.
All coral reefs are vulnerable, and one of the major threats is ocean acidification. As more CO2 is added to the atmosphere it reacts with the ocean’s water to form carbonic acid (H2CO3). This affects coral skeletons, making them more porous. Because the process is similar to osteoporosis in humans, this effect has been labelled coralporosis. The acidification of the oceans also reduces the coral’s ability to build their skeletons in the first place, due to a reduction of essential aragonite. And there’s also a problem with deep sea oil drilling affecting coral habitats…
Anyway, that’s enough on coral for now…
References
Coral anatomy virtual lesson (Keys Education video)
The hidden world of coral reefs (SciShow video)
Oliver Morton, Eating the Sun, 2007