Tridacna gigas is the largest of the giant clam species, reaching more than 59 inches across in length and weighing more than 550 pounds. It is found on coral reefs throughout the Indo-Pacific, but may have become extinct in large parts of its former range. The adductor muscle of these clams is considered a delicacy in some areas, which has led to its overexploitation. Giant clams filter in microscopic plants and animals known as phytoplankton and zooplankton. Single celled phytoplankton, known as zooxanthellae, are photosynthetic and grow within the tissues of the clam providing it with nutrients and oxygen. For this reason, Giant clams are dependent upon light for survival.
Called Arapaima in the Guianas, Paiche in Peru, and Pirarucu in Brazil, this relative of the arowana is one of the largest purely freshwater fishes in the world, reaching nine feet and exceeding 400 pounds. They have been overexploited as food fishes, so are vulnerable to extinction. These fast-growing predators surface frequently to take air at the surface. While aquarium visitors may mistake them for Alligator gars, they are not related. Their closest North American relatives are the herring-like Mooneyes and Goldeneyes.