The basking shark is the world’s second-largest fish. In summer, it swims open-mouthed at the surface, filtering out plankton. Every hour, the basking shark passes up to 395,000 gallons (1.5 million liters) of seawater through the huge gills that almost encircle its head. Its liver runs the length of the abdominal cavity and is filled with oil to aid buoyancy. They grow up to lengths of 20–36 ft (6–11 m).