But in a more temperate setting, with reduced predation by fish and bigger macroalgae, they might shade out corals. The Caribbean is a good example of that - overfishing caused an explosion in the population of a spiny sea urchin which kept the algae down around corals, then a disease killed off 90 per cent of the urchins. Now there were no urchins and no fish and the algae went through the whole reef, smothered the corals and they died. The reefs have gone from coral-dominated 30 years ago to acres of brown algae and of course, when tourism is so vital, it's a disaster. Reef-watchers also blame the aquarium trade for destruction by dynamite and cyanide of many reef habitats, as poachers hunt beautiful and valuable reef fish and shells to feed the black market.
Collectors also take specimen shells, and some can fetch thousands of dollars in the right markets.
But the news isn't all bad - in the Philippines fishers who used to blast and poison and spear everything are now setting aside areas because local people still need food and hasve become very active in stopping the destruction. Catastrophic oil spills tend to be overrated - oil is a natural product and something like seven million tons goes into the oceans each year, of which about half comes up through cracks in the ocean floor. The main problem is that spills are concentrated, but usually marine communities can tolerate episodic, catastrophic impacts. Surveys of reefs flooded by oil during the Gulf war have shown them recovering with relatively low levels of damage. We've go to look hardest at insidious degradation.