When a whale shark is feeding, its mouth can gape 1.5m (4.9ft) wide. This creates tremendous suction that makes it impossible for small animals to swim out of the way. Whale sharks have over 300 rows of teeth but each tooth is quite small and they don’t appear to serve any purpose.
Sharks have cartilage rather than bone, and no ribcage. Whale sharks beached in shallow water are quickly crushed to death by their own body weight. In the open sea, though, a cartilage skeleton saves weight because it has half the density of bone. This makes the whale shark a much more efficient swimmer.
The real business of feeding occurs at the gills. As well as extracting oxygen from the water, the gills have ten filter pads that sieve out anything over 2-3mm (0.08-0.12in). Although efficient, sometimes the pads can become blocked and the shark must ‘cough’ to clear them.
10cm (3.9in)-thick skin provides a rigid covering that maintains the shark’s streamlined shape; when we swim, the water causes our skin to ripple, which leads to drag. Shark skin is also covered with tinyplacoid scales. Each one is structured like a tooth, with enamel, dentine and a pulp cavity; the scales reduce microturbulence.
Whale sharks have quite small brains compared to other sharks. Brain tissue requires a lot of energy, so big brains must earn their keep. These sharks don’t actively hunt, and their huge size keeps them safe from most predators, so they don’t need the same cunning as other shark species.
Instead of a swim bladder, sharks use an oil called squalene to maintain buoyancy. This is stored in the liver and, since oil isn’t as light as gas, the whale shark needs lots of it – indeed, the liver can weigh up to two tons! Fishermen in Kenya hunt whale sharks for their liver and the oil from a single shark can last them for years.