The rush is on for silky sharks (Carcharhinus falciformis) and yellowtail snappers (Ocyurus chrysurus) to get their share of food tossed out of a dive boat into Cuba's coastal waters. Aggressive and considered dangerous to humans, the silky shark is nevertheless fished for human consumption. Its meat is eaten fresh or salted. Its skin is processed for leather. The fins are sold in the Asian shark-fin trade, and its liver—high in vitamin A—is extracted for medicinal liver oil. Photograph by David Doubilet

Moving as one, a school of sweetlips explores the waters of Australia's Great Barrier Reef. Sweetlips are often accompanied by the cleaner wrasse, a fish that grooms them and keeps their skin and mouths free of infection-causing parasites. Photograph by David Doubilet

 A Cape fur seal (Arctocephalus pusillus pusillus) undulates in rhythm with fronds of kelp in the waters off Gansbaai, South Africa. Although the animals are clubbed during hunts and die when entangled in fishing nets, they are top predators in the region and among the most abundant fur seals in the world. Photograph by David Doubilet

In a world of clouds and crystalline blue, a pair of stingrays glides just below the surface in the waters of French Polynesia's Tuamotu archipelago. The creatures find safe haven here under the protection of one of UNESCO's Man and Biosphere reserves. Photograph by David Doubilet

Fast Facts

 Type: Fish

Diet: Herbivore

 Average lifespan in the wild: More than 60 years

Size: Up to 10 ft (3 m) Weight: Up to 650 lbs (295 kg)

 Did you know?

 The largest freshwater fish ever recorded was a Mekong giant catfish caught in northern Thailand in 2005. It was nearly nine feet long (2.7 meters) and weighed 646 pounds (293 kilograms).

Mekong Giant Catfish Profile

 The world’s largest scaleless freshwater fish lives a tenuous existence in the murky brown waters of Southeast Asia’s Mekong River. Capable of reaching an almost mythical 10 feet (3 meters) in length and 650 pounds (295 kilograms), Mekong giant catfish live mainly in the lower half of the Mekong River system, in Cambodia, Laos, Thailand, and Vietnam.Once plentiful throughout the Mekong basin, population numbers have dropped by some 95 percent over the past century, and this critically endangered behemoth now teeters on the brink of extinction. Overfishing is the primary culprit in the giant catfish’s decline, but damming of Mekong tributaries, destruction of spawning and breeding grounds, and siltation have taken a huge toll. Some experts think there may only be a few hundred adults left.Mekong giant catfish have very low-set eyes and are silvery to dark gray on top and whitish to yellow on the bottom. They are toothless herbivores who live off the plants and algae in the river. Juveniles wear the characteristic catfish “whiskers,” called barbels, but these features shrink as they age.Highly migratory creatures, giant catfish require large stretches of river for their seasonal journeys and specific environmental conditions in their spawning and breeding areas. They are thought to rear primarily in Cambodia’s Tonle Sap Lake and migrate hundreds of miles north to spawning grounds in Thailand. Dams and human encroachment, however, have severely disrupted their lifecycle.International efforts are underway to save the species. It is now illegal in Thailand, Laos, and Cambodia to harvest giant catfish. And recently in Thailand, a group of fishers pledged to stop catching giant catfish to honor the king’s 60th year on the throne. However, enforcement of fishing restrictions in many isolated villages along the Mekong is nearly impossible, and illicit and bycatch takings continue.