What Is The World Largest Fish. Hidden beneath the murky waters of our freshwater rivers and lakes are monsters swimming. In addition to being the world's largest fish, megalodon may have been the largest marine predator that has ever lived. (Basilosaurids and pliosaurs may have been just as large.) Megalodon was an apex predator, or top carnivore, in the marine environments it inhabited (see also keystone species).

Though they reach the size of a school bus, whale sharks eat tiny plankton and fish eggs, which they filter feed as they swim slowly along with their giant mouths wide open.
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A fish called a whale shark is a true fish and is the largest in the world. The sharks are carnivorous, but they aren't likely to take a nibble out of an unwary surfer - they filter plankton through their enormous mouths while swimming at the surface. Most farmers across the world have started embracing aquaculture so as to increase fish production to satisfy the rising demand for seafood.