By Daniel Wallis

MIAMI (Reuters) - The U.S. government pared back the number of reef-building coral species it was considering to label as threatened from 66 to 20 this week, prompting criticism from conservationists.

Environmentalists urged the National Oceanic and Atmospheric Administration on Thursday to extend the protection to all threatened marine species.

"We are concerned with NOAA's unwillingness to acknowledge the widespread threats to the coral species not receiving protections," said Bethany Cotton, wildlife program director for environmental advocacy group WildEarth Guardians.

NOAA was considering 66 coral species when it embarked on its study two years ago. On Wednesday it announced its decision, adding the 20 species to two - staghorn and elkhorn - that were listed as threatened in 2006.

Of the new species, five are found in the Caribbean, including pillar coral and rough cactus coral, and 15 in the Indo-Pacific.

A U.N.-backed study warned earlier this year that most reefs in the Caribbean could vanish in the next two decades, hit by the loss of fish and sea urchins that eat coral-smothering algae.

NOAA said it considered wide-ranging public comments as part of the rulemaking process.

"The final decision is a result of the most extensive rulemaking ever undertaken by NOAA," Eileen Sobeck, assistant administrator for NOAA Fisheries, said in a statement.

"The amount of scientific information sought, obtained and analyzed was unprecedented."

Coral is a stationary animal that slowly grows on sea floors over tens and even hundreds of years. Coral reefs are nurseries for many types of fish, and they also they help protect coasts from storms and tsunamis, as well as attracting tourists.

NOAA was petitioned in 2009 by the Center for Biological Diversity to list 83 of what it said were the most vulnerable coral species found in U.S. waters as threatened or endangered under the Endangered Species Act.

The U.S. agency considered 66 of those species for the protected status.

Miyoko Sakashita, the Center's oceans director, said getting 20 species listed on Wednesday was "great news," but also a "bittersweet victory."

"This is a wake-up call that our amazing coral reefs are dying and need federal protection," Sakashita said.

"But there's hope for saving corals and many other ocean animals if we make rapid cuts in greenhouse gas pollution to stop global warming and ocean acidification."


(Reporting by Daniel Wallis; Editing by David Adams and Doina Chiacu)