a bonobo humanity?

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

Are Corals Animals, Plants, or Rocks?

Coral bleaching

https://www.barrierreef.org/the-reef/threats/coral-bleaching