The question “What is biggest fish in the world?” has at lest two answers. There are two families of fish: those with skeleton which consists of bones and those with skeleton made of cartilage.
The first groups winner is Ocean sunfish ( Mola mola). An average adult weighs about 1000 kg. They are found in warm tropical seas. They mostly feed on jellyfish, but since this diet isn’t very nutritious, they have to eat a lot in order to stay healthy.
Another curious fact about this fish is that females can produce more eggs than any other vertebrate. They have a habit of sunbathing on the oceans surface, but it’s a habit that has not been thoroughly studied yet. The largest fish of the other family, the Chondrichthyes family, is the Whale Shark (Rhincodon typus). This fish can be more than 10 meters long. The largest individual shark found was 12.56 meters long.
The heaviest Whale Shark found weighed more than 36 tons. In despite of its enormously big mouth, this shark is a filter feeder and very friendly towards humans.
Filter feeder means that this shark feeds on plankton, microscopic animals and small fish by filtering large amounts of water. The whales feed in the same way (but whales are mammals). Like all members of the shark family, it has numerous rows of teeth. There can be up to 350 rows of teeth.
However, they do not use their teeth for eating. Instead, Whale Shark traps the food in its mouth and then ejects out water through its five specially modified gills. This species was first discovered in 1828. It lives in the same habitat as the Ocean Sunfish, mostly found in tropical and warm oceans in the open sea.