Listed as critically endangered by the World Conservation Union, the Mekong giant catfish is big but toothless, as shown in this exclusive photo of an animal captured November 13, 2007, in the Tonle Sap River near Phnom Penh in Cambodia.

"This fish was very weak after capture and so we stayed with it in the hopes that it would recover," U.S. biologist Zeb Hogan said.

The fish was released unharmed by Hogan, the staff of the Cambodian Department of Fisheries, and local fishers.

Residents of the Mekong River Basin, which includes the Tonle Sap, depend heavily on fish as sources of protein, but the region's fisheries have come under increasing pressure from rapid industrialization and a growing human population.