Archive for the ‘coral’ Category
much ado about corals… just because
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
Charles Darwin, coral reefs, bleaching and all that
a stony coral polyp
I’ve read a lot of stuff about, and by, Charles Darwin over the years – not only in various passing depictions and interpretations by the likes of Richard Dawkins and Steven Jay Gould, but whole books, such as James Moore and Adrian Desmond’s big biography, Darwin (1992), David Quammen’s The Kiwi’s egg (2007) and Rebecca Stott’s Darwin and the barnacle (2003) – a real favourite. And I finished his Voyage of the Beagle only a few days ago, trying to get my head around the last sections, in fact the penultimate chapter of the book, in which he deals with ‘coral formations’. I seem to remember from one or more of those biographical books that he expanded his brief but dense – I mean complex – account in his Voyage, into what we might nowadays call a separate scientific paper [‘On the structure and distribution of coral reefs‘], and that his understanding of these formations was mostly correct, and ground-breaking. So for my sins I’m going to try to fathom these mostly undersea marvels, with the help of Darwin and others.
But before that, just one more thing about Darwin biographies. I’ve recently returned from a very pleasant holiday on Kangaroo Island, where we stayed at an ‘air b & b’ on the coast just outside of Kingscote, very comfy-cosy, and with a very varied lounge-room library. One book caught my eye – another Darwin biography, Charles Darwin: voyaging (1996) by Janet Browne. I read the first few pages and was – well, smitten might be the word. The comparison between Darwin’s social world and that of Jane Austen, one of my favourite authors, was brilliant and completely engrossing. Of course I didn’t have time to read much more, what with my own reading and all our excursions round the island, but I’m looking out to get myself a copy asap.
So Darwin starts out with the kind of basic but fresh wonderment that even I got in observing the rounded, rust-coloured boulders heaped up on the shore at Cape Willoughby, the eastern tip of Kangaroo Island. What were the processes….?
But Darwin, of course, went much further. Of reefs, he starts… ‘such formations surely rank high amongst the wonderful objects of this world’, and goes on:
We feel surprise when travellers tell us of the vast dimensions of the Pyramids and other great ruins, but how utterly insignificant are the greatest of these, when compared to these mountains of stone accumulated by the agency of various minute and tender animals! This is a wonder which does not at first strike the eye of the body, but, after reflection, the eye of reason.
So Darwin reflected on the ‘three great classes’ of coral reefs – atolls, barrier and fringing reefs.
Atolls, as he teaches me, are ‘ring’ islands, or sets of islands, encircling a central lagoon, and I have to quote, as Darwin does, a French adventurer’s exclamation from 1605:
C’est une merveille de voir chacun de ces atollons, environné d’un grand banc de pierre tout autour, n’y ayant point d’artifice humain.
I suppose they could also be called ‘reef islands’, and the ‘land’ or reef rings can extend to a diameter of many kilometres. I won’t be using Darwin’s descriptions for the following, as his antiquated language is headache-inducing, but atolls are apparently the ‘third and final stage of Darwin’s subsidence theory’, so I should put them in order.
With the first stage, the fringing reef, volcanic activity forms an island, rising up from the ocean, and corals, which I’ll attempt to describe later, begin to form, and they build up as the land formed by the volcano begins to subside. This is because the coral needs sunlight as a source of energy. The corals form a more or less circular fringe around the subsiding land.
In the second stage, with more subsidence, a kind of barrier – think of it perhaps as a kind of natural ‘moat’ – forms between the reef and the now almost submerged land in the centre.
In the case of an atoll, the land is wholly submerged. And yet, the coral seems to form islands around this central lagoon? Anyway, here’s how one presumably reliable source puts it:
The Deep Sea Drilling Project sought evidence of volcanic cores beneath coral reefs and found it. First, in 1952 at the Einwetok Atoll in the Marshall islands, and again, in 1960 at the Midway Atoll, teams found volcanic rock strongly supporting Darwin’s theory that coral reefs form around submerging islands. Today, Darwin’s theory is universally accepted as a means of explaining these reef formations.
However, as this source, linked below, puts it, not all reefs fit this pattern (and I’m thinking that Australia’s Great Barrier Reef surely doesn’t). Other reefs known as patch reefs and bank reefs are found in the Caribbean region.
But I want to get down to the real basics. Coral reefs are built by coral, or corals, or what? Micro-organisms? What is coral? I’ll start, and probably finish, with Wikipedia, the most comprehensive and reliable encyclopedia ever devised, but there are many other reliable sites, linked below.
Corals are tiny invertebrate animals, in the phylum Cnidaria (of which there are more than 11,000 species, including jellyfish and sea anemones). Generally they form colonies of individual polyps, long thin little creatures with tentacles. They can reproduce asexually to form colonies, and sexually by spawning – releasing a mix of eggs and sperm into the water, as most marine creatures do. For most of their lives they’re sessile (immobile), and these colonies of genetically identical individuals can number in the millions. Stony coral polyps produce a skeleton of calcium carbonate, essentially composed of calcium, carbon and oxygen (CaCO3). The stony coral we’re familiar with, Scleractinia to the cognoscenti, have been around for about 250 million years, from the Middle Triassic, but we can trace coral ancestry back much further, to the Cambrian, 535 million years ago. They were quite rare, though, until the Ordovician, 100 years later, and they were of a very different type from ‘modern’ corals. It seems that different coral types came and went, with a particularly massive disappearance due to the Permian-Triassic extinction event 250 million years ago, which killed off 75% of all marine species.
So, a little more about their anatomy, before I go on to to coral bleaching, and current threats. I’ve mentioned the calcium carbonate skeleton, deposited by the polyps and also by the coenosarc, a layer of tissue that connects these polyps by secreting coenosteum, a stony material made of calcium carbonate in the form of aragonite (a more spongy and porous form). There’s also an extracellular matrix called mesogloea – it’s complicated!
Aragonite is also the material from which corallites are made. These are cup-shaped depressions into which the polyp can retract. The individual polyps and their housings can grow to form enormous colonies of very variable shapes and sizes:
Colonies of stony coral are markedly variable in appearance; a single species may adopt an encrusting, plate-like, bushy, columnar or massive solid structure, the various forms often being linked to different types of habitat, with variations in light level and water movement being significant.
It would be frankly ridiculous of me to go into much more detail, there’s way too much ground, or stone, or ocean, to cover. Better to focus on coral’s apparently self-imposed bleaching behaviour. When corals are stressed, usually due to the over-heating of reef waters, they expel a particular form of algae, known as zooxanthellae, from their tissues. Why they do this seems unclear, as the zooxanthellae provide food and photosynthetic energy essential for their growth and reproduction. It has to do with oxidative stress, apparently, and I’m sure they know what they’re doing. And perhaps ‘bleaching’ should be dumped as a term, because it surely gives the wrong impression. The pale skeletons that remain are not in any sense bleached, but….
Anyway, Queensland’s Great Barrier Reef has suffered several mass bleaching events in the last few years, the most recent being earlier this year (2024), following the hottest year, globally, on record. Corals do recover from such events, gradually, but the strain on them is accumulating.
References
http://coraldigest.org/index.php/DarwinsTheory
https://en.wikipedia.org/wiki/Coral
https://www.barrierreef.org/the-reef/threats/coral-bleaching
fountains of good stuff 4: coral
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
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.