The polar snap enveloping much of the United States in record cold has been killing off coral reefs in the normally balmy warm waters off the Florida Keys, experts said Monday.

The unusually chilly weather so far this year has seen sea temperatures plummet in southern Florida -- a fatal development for the coral, which dies when exposed for an extended time to temperatures below 15 degrees Celsius (59 degrees Fahrenheit).

Especially in the lower Keys, "temperatures have been lower... there is higher mortality," Diego Lirman, a University of Miami expert on coral, told AFP.

Florida's usually mild and sunny winter weather has given way to record low temperatures during the historic cold snap in recent weeks.

In Miami, the thermometer in January and February regularly dropped below 35 degrees Fahrenheit (1.6 Celsius), the coldest temperatures since 1970.

The cold snap also has led to "bleaching," in which the coral loses pigmentation and ultimately dies.

Destruction of coral having a negative effect on delicate tropical eco-systems in the region, Lirman added, with micro-algae living within the coral forced to leave their habitat for lack of a food source.

Some of the worst affected species are the large brain and star coral, which can take several hundreds of years to grow into the vibrant underwater colonies.

"The Keys have not seen a cold-water bleaching event like this since the winter of 1977-78, when acres of staghorn coral perished," said Billy Causey, southeast regional director of NOAA's Office of National Marine Sanctuaries.

Florida's coral reefs are considered a unique natural heritage area in the United States for their proximity to the coast and their expansiveness, running from north of Miami in the Atlantic Ocean to the Gulf of Mexico.

The state's myriad of tropical animals also have been impacted by the cold snap so far this year, with iguanas dropping from trees and manatees huddling around waters warmed by power plants.

The cold-blooded iguanas' comfort level begins at 73 degrees Fahrenheit (23 Celsius) and they positively thrive at 95 degree Fahrenheit (35 degrees Celsius).

But when temperatures drop below about 60 degrees Fahrenheit (15 degrees Celsius), they become immobile, and below about 40 degrees Fahrenheit (five degrees Celsius), they become completely immobile due to a lack of blood flow.

Unable to hold on, the helpless mohawked lizards that shelter in tree branches have been seen falling to the ground, and wildlife officials have offered guidelines to revive them.