The Coral Triangle is a roughly triangular area of sea around Indonesia, Papua and the Philippines characterised by its coral reefs. Today is Coral Triangle Day, on which the biodiversity and beauty of the coral reef there is celebrated, and of course the threats and damage to those environments are highlighted.
I’ve already talked about the linguistic diversity of Papua in passing. A similar diversity characterises the life in the seas around the island and its neighbours. Over three-quarters of coral species live in the Coral Triangle, more than a third of reef fish species, half of the razor clams and it’s also the site of the world’s largest mangrove forest. It seems an odd coincidence that this area is diverse in these two very different ways, so I wonder if something about the topography of the sea bed is responsible for causing this. On the land, isolated valleys have led to hundreds of isolated linguistic communities. Is it possible that a similar isolating situation continues under the water which leads to this huge biodiversity.
Here’s a map of the area:
Author
Obsidian Soul, map derived from File:WorldMap-B with Frame.png (created from DEMIS Mapserver)
With reference to yesterday’s post, I’ve decided to put south at the top of this map but haven’t the time to turn the captions round. It’s interestingly different to the Wallace Line, which divides terrestrial fauna between Sunda and Sahul, the terrestrial floral and faunal regions of Southeast Eurasia and Australia as a continent (which includes New Guinea), and includes Wallacea itself, a heart-shaped region in the middle of the triangle which has similar mega-biodiversity on land. There is complicated ecological stuff going happening on these islands.
Reefs in general need not be coral. The above picture shows stromatolites, which are the earliest reefs and still form in some places, from blue-green “algæ”. They peaked something like 1.25 æons ago, and by the Cambrian were in decline. By then, new reef-building animals, the archæocyathids who are rather like sponges, had evolved in Siberia and were spreading across the planet’s seas. They were extinct by the middle Cambrian, around 516 million years ago and rugose and tabulate corals began to evolve. Bryozoa (“moss animals”) and other animals such as crinoids, brachiopods and bivalves, built later reefs, along with microörganisms. I feel like I’m getting ahead of myself here.
Stromatolites are made by cyanobacteria, which are the organisms which caused everything to become a bit unstuck early in the history of life. Before them, photosynthesis seems to have been carried out by purple organisms which absorbed green light to run their metabolism. Not having the option of using green light, another group of microörganisms evolved to take advantage of the red portion of the sunlight and were accordingly glaucous in colour. These underwent a mutation which was catastrophic for the rest of the biosphere when they began to release the highly reactive oxygen into the water and atmosphere, thereby poisoning most of the world. Anærobic organisms still survive but are confined to habitats where oxygen can’t reach unless they are facultative and can tolerate oxygen. These cyanobacteria are what we tend to call “blue-green algæ”, except they aren’t really algæ at all but bacteria able to combine water and carbon dioxide to produce sugar with the by-product of mainly toxic oxygen. These bacteria were also later incorporated into the larger cells of the ancestors of the true algæ and more familiar plants, enabling them to photosynthesise. Stromatolites form when cyanobacteria anchor themselves in shallow water using calcium compounds, which then build up in layers forming reefs. They were an early example of macroscopic life on this planet, and lasted for æons.
Archæocyathids are, as I said, kind of sponge-like:
They consist of two-layered structures anchored to the sea bed with cells in between the layers and seem to have fed like today’s sponges, either sucking water through their pores or allowing it to flow while they fed off the nutrients and plankton within it. Some of them lived alone and others in colonies, and their skeletons are made of calcium carbonate, so they too built reefs. They were in fact the first reef-building animals, and didn’t last long, but they did support the usual plethora of other species found in reefs.
Corals themselves are cnidaria. The cnidaria are a major phylum characterised by their possession of stinging cells, and include jellyfish, Portuguese men o’ war and sea anemones. Along with the ctenophores, cnidaria are two-layered animals with a central cavity for digestion, and are together referred to as cœlenterates. There are two rather similar classes whose species tend to resemble flowers in the main part of their life cycle, plus two other similar classes resembling jellyfish in the main part of theirs. These are in fact upside down versions of each other and may have a stage like the other form in their lives. That is, a jellyfish is essentially an inverted sea anemone and reproduces by producing polyps which sit on solid surfaces, and hydroids tend to be sessile but produce jellyfish-like young. This even happens in fresh water, so there are in fact freshwater jellyfish, even some who are dominantly jellyfish such as Craspedacusta sowerbii, who is in fact an invasive species found in rivers and canals in Southeast England. Corals are related to sea anemones and classified as anthozoa – “flower animals”. All cnidaria are radially symmetrical.
Like other reef-builders, corals produce structures made of calcium carbonate, alias limestone. Individual polyps secrete this at their base, bud, die and are replaced by other polyps who do the same thing, building up various structures, often tree-like. Although they capture plankton with their tentacles, most of their nutrition is from algæ within their tissues known as Symbiodinium. I’m actually playing it a bit fast and loose with the word “alga” in this post, so I’ll be more specific. Symbiodinium are in fact dinoflagellates, which are alveolata like the Colpoda I mentioned the other day, and don’t really fit neatly into the category of plants or animals. As a group, algæ are not closely related to each other and their common ancestor would not have been an alga, and was probably also ancestral to animals. Dinoflagellates are related to red seaweed and are notorious for including outrageously toxic forms which cause the deadly red tides, but these are not the organisms in corals. Because they rely on symbiotic dinoflagellates, the kind of corals found in tropical reefs need to live in clear water near the surface, where their symbionts can photosynthesise. The polyps, being animals, don’t photosynthesise on their own but do respire ærobically, exhaling carbon dioxide which the dinoflagellates use for photosynthesis. Since their exoskeletons are made of calcium carbonate, these corals are also quite dependent on the water not becoming too acidic, which is what carbon dioxide does to it, and they are therefore vulnerable to increased carbon dioxide for a reason completely different to anthropogenic climate change.
The coral reef paradox was first noticed by Charles Darwin. Tropical coral reefs are usually surrounded by relatively barren, desert-like areas of sea bed, and yet are fantastically rich in organisms themselves. They also need the water above them to be clear, meaning that plankton cannot be very dense in those layers. Nonetheless, they are very rich in life, and the reason for this seems to be that they are constantly supplied by upwelling currents from below supplying nutrients which are then kept in the reef by cycling through the food chain there. This makes such reefs vulnerable to changes in deep ocean currents caused by climate change.
Biodiversity is extreme in coral reefs of this kind. A quarter of all known marine species live in them. The diversity should make them more resilient than other ecosystems, although right now they’re looking pretty fragile. It’s probably obvious that this diversity also means there are likely to be various useful medicinal compounds in the organisms, although obviously there’s the question of vegan ethics there. An example of a rare compound found in reef organisms would be the only true blue pigment found in vertebrates, which is present in two species of mandarinfish found in the Phillipines. All other blue vertebrates, reef fish included, are blue via structural processes. This includes the Blue Tang, also known as “Dory” from ‘Finding Nemo’:
For what it’s worth, tropical coral reef organisms produce anti-viral and anti-bacterial compounds as well as those useful in treating arthritis and Alzheimers disease, but as a vegan herbalist it’s only fair for me to point out that animals don’t need to be exploited to address these problems since there is also a hugely diverse array of such substances among terrestrial plants.
Tropical reef corals derive their colour from the dinoflagellates living with them. If water temperature in a reef of this kind rises by as little as one degree, or there is too much sunlight, the coral either expels or consumes the zoöxanthellæ (as the dinoflagellates are known in this setting), leading to bleaching. This doesn’t immediately kill the coral, but since most of the nutrition is from these organisms, it may eventually do so. They may also turn bright orange, pink or purple to attract the zoöxanthellæ back.
Tropical coral reefs are whown on this map, and occur in three main regions: the aforementioned Triangle, which can be seen on either side of the map bordering the Indian and Pacific Oceans and stretching into the latter. There’s another cluster near the purported location of Lemuria, which makes me wonder if the sea is shallower there due to the sunken land in that region, and a further concentration off Madagascar and the Red Sea. This means that there will probably be new coral reefs in the current Rift Valley in East Afrika when the new continent splits off there. Something like 90% of the tropical coral reefs are accounted for by those, but there are further reefs in the Caribbean and in sporadic parts of the Atlantic. This map also shows the cold water reefs, notably the big one off Northwest Scotland.
Deep water and cold water corals are less studied than the tropical reefs but don’t have the same reliance on light and dinoflagellates as their counterparts. They may exist near the surface but can go down as deep as two kilometres below sea level, in the permanently dark abyss where water temperature stays at 4°C because that’s the densest water gets – the bottom of any deep body of water will always be at that temperature unless there’s a heat source. The British reef is referred to as the Darwin Mounds, and is located east of the Rockall Trough and slightly southwest of the Faroe-Shetland channel, covering about 100 km2. This is made largely of Lophelia, a very common deep water Atlantic coral:
Deep water reefs are not just sparse, cold ecosystems. The coral is, unexpectedly for the scientists, growing on sand rather than rock and the reef is also home to Syringammina fragilissima, a twenty centimetre long unicellular organism:
There are also various echinoderms and fish, including the Orange Roughy:
British corals are also found in shallower waters. A particularly creepy one is Alcyonium digitatum, the Dead Man’s (sic) Fingers, a soft coral which looks like what it’s called when washed up on beaches but is a lot prettier when it’s alive:
Eunicella, the Sea Fan, is another, found in the Channel:
There’s also the Devonshire Cup Coral:
This is a true madreporarian coral like the ones who form tropical shallow water reefs, but since this species has no zooxanthellæ they can survive in deep water.
Scleractina are the reef-forming corals, and have been around since the Triassic, but there were other kinds who were common before them.
Rugose corals, one of whom I used to have, were common for most of the Palæozoic. These were of fourfold symmetry and are also known as “horn corals”, and also formed reefs. Living at the same time were the tabulate corals, which unlike the rugosa were always colonial:
These reefs were also pretty biodiverse, with plenty of trilobites, crinoids, orthocerid cephalopods and brachiopods.
Finally, there is a coral reef in muddy water at the mouth of the Amazon, although this is still in sea water, though perhaps less saline than most of the ocean. It has an area of almost 10 000 square kilometres and was discovered in the 2010s during exploration for oil drilling. It isn’t as diverse as many shallow sea tropical reefs and is already under threat.
There are a few subjects I haven’t touched upon here, such as the how the famous white sandy beaches of coral islands are made of parrot fish excrement from ground coral, the different zones of coral reefs and the different types, and my personal quandary about the origin of the white sandy beaches of Na h-Eileanan an Iar, but I think that’ll do for now.
A Box Of Chocolates 
