Coral reefs are the jewels of the ocean. Communities of organisms as rich and diverse as any above or below the surface of the planet, they encircle the tropics like an azure necklace.

1998 was designated 'International Year of the Oceans'. It turned out to be the year that coral reefs around the world began to die. Unprecedented mass bleaching swept the world's tropical oceans, in places leaving hundreds of miles of coral coastline - the fringes of entire countries in places - severely damaged. Following a number of similar but lesser events since the 1980s, this latest bleaching event is being touted as unequivocal proof that global warming has begun, and that it will have a greater impact than many think.

SILENT SENTINELS examines these claims and takes a step back to take a broader look at the coral organism and how it has coped with climate change over time. How coral both defines its environment and is created by it. It is a story of a polyp and a plant - one of the most successful biological relationships in the history of the earth.

THE PERILS OF PLECTROPOMUS

In recent years scientists have pieced together the dramatic life cycle of reef fish like plectropomus, the coral trout. They have discovered it's a life lived against the odds.

Today it's not just the natural environment that threatens them; it's the impact of humans. People in the Far East have a tradition of eating fresh fish. In the last 10 years they've developed a taste for live reef fish, attracted by their spectacular color and markings. A live plectropomus can bring in big bucks.

After World War II fishermen used dynamite to catch reef fish. Today they use cyanide to catch them live. In addition to global warming, it's our appetites that are now threatening plectropomus, and everything on the living reef.