Whale shark. The whale shark (Rhincodon typus) is found in tropical seas, and is the world's largest fish, reaching an average length of 8 metres. It is harmless to humans, moving slowly through the water as it uses specially adapted gill arches to filter the water, removing anything edible. Its 1.5 metre diameter mouth can circulate up to 6000 litres of water an hour. It is thought to live to an age of 60-150 years. Photographed off Holbox island, Mexico.