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No cold currents flow through Indonesia, but even temporary rises in sea temperatures can devastate reefs. In 1983 sea temperatures around the Pulau Seribu islands off western Java rose to 33°C (91°F), killing much of the shallow reef coral there. Most has now recovered.

Turbid waters, those carrying a great deal of suspended sediment, deter reef formation. This is a very important in South and Southeast Asia, where rivers dump 70 percent of all sediments delivered to the ocean worldwide. (The Ganges is the champion, carrying almost 1.7 billion tons a year to the Bay of Bengal.) In Indonesia, the larger rivers in Kalimantan and Sumatra produce enough sediment to discourage reef formation a significant distance from their mouths, although even here some rich reefs have formed.

The Biology of Corals

True reef-building or hermatypic corals are animals grouped in the phylum Cnidaria, order Scleractinia. They all have an indispensable symbiotic relationship with dinoflagellate algae called zooxanthellae. (see "Zooxanthellae and Corals," opposite.) These algae are essential for respiration and nutrient uptake, and the vigorous deposition of calcium.