Valued for its meat and leather, the addax has declined due to uncontrolled hunting.

The Wildlife Conservation Society has released a list of the "Rarest of the Rare," a dozen animals most in danger of extinction.

The eclectic list includes birds, mammals, fish, reptiles, amphibians, and insects. Some are well known, such as the northern right whale and Sumatran rhino, while others are more obscure, including Abbot's booby, an ocean-going bird that only nests on Christmas Island.

The animals were highlighted today because it is National Endangered Species Day. They are:

Abbott's booby: A large black-and-white seabird that breeds on Christmas Island, a remote Australian island in the Indian Ocean.

Addax: A nocturnal antelope species with long spiral horns, found the sand dunes of the Sahara desert.

Angel shark: Bottom-dwelling, nocturnal predators once common throughout the Northeast Atlantic Ocean and the Mediterranean and Black seas, but now critically endangered.

Bengal florican: A large terrestrial bustard bird native to Cambodia, Nepal, Vietnam, and India.

Black-faced lion tamarin: A small primate that sleeps in tree holes dug out by woodpeckers and feeds on insects, fruit, and plants. Discovered on the island of Superagui, Brazil, in 1990, there are now only 400 in the wild.

Burmese roofed turtle: One of Myanmar’s seven native turtles, once abundant in the major rivers of central and southern Burma, threatened by hunting and egg poaching.

Dragonflies of Sri Lanka: Of the 53 endemic species of dragonfly found in Sri Lanka, at least 20 are threatened.

Golden arrow poison frog: An amphibian native to Panama, threatened by a highly-infectious fungal disease.

North Atlantic right whale: Hunted since the 10th century, only 350 of these slow-moving, 220,000-pound (100,000 kg) cetaceans remain.

Ricord's iguana: A reptile native to two isolated locations in the arid southwestern Dominican Republic

Pygmy hippopotamus: A small hippo from the Upper Guinean Forest of Liberia, Guinea, Ivory Coast, and Sierra Leone.

Sumatran rhino: Also known as the hairy or Asian two-horned rhinoceros, fewer than 300 survive today in the subtropical and tropical dry forests of Indonesia and Malaysia.

Threats to each species vary widely. In the case of Abbot’s booby, the introduction of yellow crazy ants to Easter Island has severely altered their nesting habitat. Meanwhile, the addax has been severely impacted by desertification of its habitat and overhunting. Other species suffer from diseases, as in the case of the golden arrow poison frog, or poaching for the Chinese medicinal trade, which has reduced the population of Sumatran rhinos to fewer than 300 individuals.

The 2007 World Conservation Union (IUCN) Red List revealed a continuing rise in the number of species threatened with extinction. Although only a fraction of all plant and animal species have been evaluated, the number of species listed as threatened stands at 16,306, an increase of 188 species since 2006.

"'Rarest of the Rare' is a snapshot of just a handful of the most critically endangered species that serves to illustrate their plight and inspire the public to join the fight for their continued survival," said Kent Redford, director of the Wildlife Conservation Society Institute.