Darwin's Paradox

VulcanSpirit
Richard & Alison Brunstrom
Tue 29 Mar 2011 12:29
Darwin's first monograph, 'The Structure and Distribution of Coral Reefs' appeared in 1842, some years after his voyage in the Beagle; his paradox was the fact that the reefs teemed with life, despite being surrounded by tropical waters providing very little nutrient - "oases in the desert of the ocean".
Tropical seas are startling clear precisely because they provide so little nutrient. The surface water is warmed by the sun and stays floating at the surface, separated from the denser colder water below by a stable thermocline; little mixing takes place. Anything which dies in the surface layer sinks through the thermocline and the products of its decay and decomposition are not available at the surface. This keeps the surface water clear - but relatively sterile. How, therefore, have coral reefs become amongst the most diverse ecosystems on the planet, home to 25% of all marine species despite covering only 0.1% of the world's ocean surface? 
The answer lies principally with the coral and its quite extraordinary symbiosis (a mutually beneficial relationship) with tiny unicellular algae called zooxanthellae. Reef building corals evolved in the Tertiary, about 250 million years ago. They are small sessile colonial animals closely related to jellyfish and sea anemones. The individuals are called polyps (which look very like a tiny sea anemone), and it is their ability to deposit calcium carbonate (the same stuff that makes snail shells, limestone and chalk) which enables them to build reefs, growing vertically at 3-5mm per year. The coral polyps' bodies contain huge populations of zooxanthellae (about 1million per cubic centimetre). The coral provides the algae with a constant supply of carbon dioxide and nitrogen required for photosynthesis and the zooxanthellae provide the coral with the products of photosynthesis (glucose, glycerol and amino acids) which provide about 90% of the corals energy needs. Neither can survive without the other.These reefs trap nutrients, preventing them being lost to the sea floor and providing a resource base for other animals and plants (some of which also deposit calcium carbonate to contribute to he reef). The complex interdependent ecosystem which evolves provides the resolution of Darwin's Paradox. And the resulting structures can be enormous - witness the Australian Great Barrier reef, visible from space. The Great Orme in North Wales is a nice (small!) example, for those that know it. I find it truly wonderful that such tiny animals, given sufficient time, can build such vast structures.
 
Photographing the reef corals is difficult. The major problem, when snorkelling, is focus. Focus distance becomes critical in close-up photography, but when swimming it is very difficult to maintain position - especially since you cannot hang on to anything in case you damage it (corals are surprisingly fragile, considering that they are made of stone), or it stings you (some corals have a powerful sting), or you get painfully spiked by a large sea urchin. But here are a couple of shots I managed to get off Virgin Gorda in the BVI. I hope and expect that I'll get better with more practice.
 
 
 
This one (above) is a brain coral, Diplora strigosa, so called for obvious reasons. It is about half a metre in diameter and about a metre under the surface. They can grow to 2m in diameter. The coral polyps live in little pits in the valleys between the ridges. There are several other species of brain coral, each with a diagnostic ridge pattern.
 
The picture below is looking down on a metre-high colony of, I think, boulder star coral Monastraea annularis. I'm not entirely sure because there are over 1000 species of coral and I'm a novice at their identification - and this particular species is extremely variable. The individual polyps are out, looking like pinheads, sifting the water for phytoplankton to eat and recycle. Between the living brown coral you can see white limestone exposed - reef which this colony has built.
 
 
 
More lizard photos following shortly Arthur, because pictures of the same people eating yet another meal in yet another beach restaurant on yet another island are just sooo boring.