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1. MegafishesGiant freshwater fish from around the world clock in as 2011's most popular environment gallery on nationalgeographic.com. Increasingly rare because of fishing, pollution, and loss of habitat due to human activity, these so-called megafishes can be indicator species of aquatic ecoystems.National Geographic Explorer Zeb Hogan has wrangled, photographed, and studied many of these freshwater monsters. Some, like the Chinese sturgeon, are "living fossils" that have changed little over millions of years.Pictured is an arapaima fish at an aquarium in Manaus, Brazil. This South American giant can reach lengths of more than 10 feet (3 meters) and weigh upwards of 400 pounds (180 kilograms).Like several megafish species in our gallery, the arapaima is the focus of conservation projects, which may be racing against time to save these animals in the wild.(See the megafishes gallery, scary freshwater creatures, and how you can help protect freshwater ecosystems.)

Photograph courtesy Zeb Hogan