CHARLOTTE AMALIE, U.S. Virgin Islands —  Scientists have issued their strongest warning so far this year that unusually warm Caribbean Sea temperatures threaten coral reefs that suffered widespread damage last year in record-setting heat.

Waters have reached 85 degrees around the U.S. Virgin Islands and Puerto Rico — temperatures at which coral can be damaged if waters do not cool after a few weeks — said Al Strong, a scientist with the U.S. National Oceanic and Atmospheric Administration's Coral Reef Watch, in a telephone interview Monday.

The warning issued Saturday by NOAA urges scuba-dive operators and underwater researchers in the U.S. Caribbean territories to look for coral damage and use caution around the fragile reefs, which are easily damaged by physical contact.

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Coral reefs, which provide a sheltered habitat for fish, lobsters and other animals, die from prolonged bleaching, when the water temperature gets so high that the coral polyps, small stationary animals related to sea anemones, expel the colored symbiotic algae that provide energy through photosynthesis.

Absence of the algae helps the polyps survive in times of extreme stress, but they will die of starvation if new algae are not reabsorbed within several weeks.

The new warning follows two watches issued since July.

Strong said the water was not expected to become as warm as last year, when sea temperatures in the territories hovered near 86 degrees for months at a time and as much as 40 percent of the coral died around the U.S. Virgin Islands.

He said researchers were monitoring how the heat affects coral recovery from last year.

"There is still so much to learn about the physiology of coral" and which species recover fastest, Strong said in a telephone interview from Maryland.

Scientists have not pinpointed what is behind the warm sea temperatures but some speculate global warming might be the cause.

Millions of people visit the Caribbean each year to dive and snorkel over the region's coral reefs, part of a multibillion-dollar tourism industry.