Helen - We've all seen pictures of colourful coral reefs in magazines and TV, but what are coral reefs made of? Are they living, are they dead? What are the animals which actually live on the reef?Annelise - Well yes, as you say Helen, most people will have seen coral reefs, either first hand by snorkelling or scuba diving, or on television, in newspapers; Coral reefs are a hot topic at the moment. Coral Reefs are actually comprised of tiny animals called Coral Polyps, which lay down a calcium carbonate limestone skeleton. It's this skeleton that, over hundreds of thousands of years forms the very large reef structure. Within those coral animals, they have a symbiotic relationship with microscopic algae called Zooxanthellae, and these enable the coral to lay down the calcium carbonate skeleton, so they're a very important part of the coral reef ecosystem.Helen - And as we see in these beautiful pictures, and those of us who are lucky to see them in person when diving, there are obviously many, many other animals which live around the reef. Aren't they one of the most diverse ecosystems in the world, on a par with rainforests?Annelise - Yes, they're often described as the rainforests of the sea, they host numerous coral species, fish species, invertebrates... Not only are they very important for their biodiversity, but hundreds of people around the world depend on coral reefs for their financial gain, through tourism, fisheries and the structures that reefs form are an important structural protection, a barrier against wave impact. Due to the location of coral reefs in the tropics, often these surround low-lying areas, and so are very important barriers against wave erosion.Helen - As part of your job working with the Living Oceans Foundation, you went out to Aceh to look at the effect after the tsunami...Annelise - That's correct, we actually went out 10 months after the tsunami hit. We went up the west coast of Sumatra and surveyed on the off shore islands. We didn't find a huge amount of damage, we were actually quite surprised; over half the reefs we surveyed there was no tsunami damage at all, that's the same as studies in Thailand. We were surprised that the waves passed over the coral reef itself and although it did have a big impact on land, the reefs are very resilient to such powerful wave energy.Helen - I guess, in a way, what we found after the tsunami wasn't so much that it had damaged the reefs, but more that where we have damaged the coral reef, such as for buildings, or by using destructive fishing techniques like dynamite fishing...Annelise - Absolutely. Another point is that where they've cleared mangroves for coastal development, mangroves are very important for coastal protection. So where the reef has been mined for the limestone for building material, and also the mangrove has been cleared as they've developed the coastline, that's where the worst tsunami impact was.Helen - Do we have a figure for how many people live within 100km of a coral reef?Annelise - About 8% of the world's population, so that's a huge number of people living very close to a coral reef. Most of them, their livelihoods are driven from the coral reef itself, so the pressure put on coral reefs at the moment in incredibly high.Kat - You've been talking about coral reefs in the tropics, do you find corals anywhere else around the world?Annelise - Yes, you can find corals around the UK for example, but they do not form large reef-like structures. We're talking about specifically tropical coral reefs, and these are actually the ones which have the symbiotic Zooxanthellae, so they're called Hermatipic corals, they're the ones which lay down the skeleton and build this huge, huge structure.Kat - So they're less important around islands like ours?Annelise - I wouldn't say they're less important, but certainly we don't depend on them as strongly as people in the tropics who don't necessarily have alternatives to depend on.Helen - There are always the deep-sea corals, which don't have the Zooxanthellae in their tissues, they live very deep down in the dark parts of the ocean. We've already touched a little on the effects that were having on the reefs around the world, what do you think the main problems we pose for reefs might be?Annelise - The biggest problems we've seen in recent years actually is a natural phenomenon, the El Nino ocean warming that happened in 1997-1998 had a very, very severe impact on the corals. This happens because the water temperature increased, the corals became stressed and when they become stressed they spit out their Zooxanthellae, microscopic algae or the algae become degraded in situ within their tissue. Without these Zooxanthellae the coral can't actually live. This is called coral bleaching.Helen - It's called that because they turn completely white?Annelise - Well it's not that the corals turn white, but the Zooxanthellae actually give them their colours, so if the Zooxanthellae is not there the coral is actually transparent and the white that you're seeing is the coral skeleton itself. An increase in water temperature causes this bleaching phenomenon, and this was very prevalent in 1998 especially in the western Indian Ocean, where over 90% of some corals were bleached, in the Maldives and Seychelles especially.Helen - So the corals can't feed if they don't have the Zooxanthellae, do they definitely die, or can they recover?Annelise - They can recover, if the stress is not prolonged then the corals can actually regain their Zooxanthellae and then they will regain their colour, and go on living, it may just be that their health is degraded for a short while. If the stress is prolonged, if the temperature stays high for a few months, then the corals don't actually regain their Zooxanthellae and they actually die. Helen - You've recently been using a technique to study coral reefs from the air. How does that work?Annelise - This is a very clever technique really, we employ a sensor called a Compact Airborne Spectrographic Imager (CASI) fixed onto a seaplane and flown over an area of interest. It records the amount of light reflected from different things on the seabed, so it can tell you what's coral, algae or sea grass. We still need to have divers in the water though, collecting GPS data to calibrate the sensor. It allows you to collect a much greater amount of data than using divers alone.