Scientists from the U.S. and Greece are working from opposite sides of the ocean to save the Hawaiian (pictured here) and Mediterranean monk seals. The IUCN Red List of Threatened Species has declared both species to be critically endangered. Their relative, the Caribbean monk seal, has already gone extinct.

A study conducted in 2010 led researchers to conclude that one-fifth of the world's vertebrates are threatened wtih extinction. Meet some of the marine vertebrate species that are among the threatened in the Backbone of Biodiversity of Risk image gallery.