National Geographic : 2011 Jan
A school of Pacific steephead parrotfish (left) graze on dead coral at Kanton island. "You can hear them going crunch, crunch, crunch," says Greg Stone, a diver and marine scientist. Because Kanton sits near the Equator, the island rarely experiences big shifts in meteorological conditions. But the El Niño of 2002-03 raised water temperatures here by more than 1°C (1.8°F) to as high as 88°F in some places, killing all the coral in the island's lagoon. Once coral dies, seaweed can take hold and grow on top of dead reefs, making resettlement by new coral difficult. That's where parrotfish come in: By grazing on algae, these and other herbivores keep the reef free of seaweed, enabling pink coralline algae to take hold and form a substrate for new coral. The four-saddle grouper (below) passes over the candy-pink algae on which new coral will grow.