Reaching an average length of 45 feet and weighing up to 15 tons, the Whale Shark (Rhincodon typus) is generally considered the largest species of fish in the world. One specimen captured in the Gulf of Thailand in 1919 measured an incredible 59 feet! Not related to whales (which are mammals, not fish), the Whale Shark probably earned its name because of its abnormally large size as well as the fact that, like whales, it is a filter feeder, swimming with its mouth open in order to suck up plankton and small sea creatures.