Snippets from current journals
In deep water
Reef-building corals have an intimate relationship with microscopic algae. (Relationships don't get more intimate than this—the algae are embedded in the coral's tissues.) Algae provide their host with food, the host supplies carbon dioxide. It's all very cosy.
The algae (zooxanthellae) assist the coral to build its skeleton by increasing the rate at which calcium carbonate is laid down. They allow their slowcoach associates to compete with faster-growing organisms that might otherwise smother them. If you wander out to a reef, you'll see how successful the relationship is. As far as coral is concerned, zooxanthellae are life-savers.
But the success of the relationship depends more on the zooxanthellae than on the corals. Photosynthesis is the key. This is the biochemical process by which algae and plants harness light energy to power their production of carbohydrates. The problem is that light decreases with depth. It's fine at the surface but at some point the light becomes so weak that it can't provide the energy required. Photosynthesis stops. Reef building grinds to a halt.
Around the continental shelf, the water is turbid and murky with nutrients. Light doesn't get too far. But in clear oceanic waters, light penetrates much. Around the mid-ocean islands of Hawaii, reef-building corals live as deep as 120 m. At Johnston Atoll, in the middle of the Pacific, the coral Leptoseris hawaiiensis grows even further down at 165 m. That's a long way.
Kahng, S.E. & Maragos, J.E. (2006). The deepest, zooxanthellate scleractinian corals in the world? Coral Reefs 25: 254–254
Some day my alga will come
How do zooxanthellae get together with their hosts?
If a new polyp is formed by budding from another one, the zooxanthellae are passed down from the 'parent'. They like to keep it in the family—a functional heirloom. But if a new colony results from sexual reproduction (a fertilised egg becomes a mobile planula that settles to produce polyps), the zooxanthellae has to make a big effort.
They follow chemical cues in the water. The zooxanthellae pick up the waterborne scents of corals and swim towards the source. Not very quickly, admittedly but they're okay in currents up to 0.5 mm/s. (If you want to see that in other units, do the maths yourself.)
It's easy if a virgin polyp is nearby. What happens if it's too far away? If the zooxanthellae are wandering in a chemical free zone? They keep swimming but they turn a lot. Sure, they don't travel a huge distance from where they started (as the clownfish swims) but they do cover a great deal of territory while not getting anywhere.
Pasternak, Z., Blasius, B. Abelson, A. & Achituv, Y. (2006). Host-finding behaviour and navigation capabilities of symbiotic zooxanthellae. Coral Reefs 25: 201–207.
When reef-building corals are stressed they evict their algal chums. Empty of photosynthetic zooxanthellae, the polyp tissues turn pale. This is the phenomenon known as coral bleaching. (Few corals are coloured. In most cases, the hues of living animals are due to their symbiotic algae.)
Among the hardest hit is the fire coral, Millepora. Exposed to higher than normal temperatures (thank you, global warming), Millepora ejects its zooxanthellae without a by your leave. Colonies often succumb to the stress. Millepora seems to take it harder than other corals. But it also springs back quicker than the rest. Easy come, easy go.
Lewis, J.B. (2006). Biology and ecology of the hydrocoral Millepora on coral reefs. Advances in Marine Biology 50: 1–55.