Average adult size: Adults can grow to be more than 12 feet long.

Natural history: Natural history: Blue sharks are dark blue above and white underneath. This is called countershading. If the shark is seen from below, it blends in with the lighter surface water above. If the shark is seen from above it disappears into the dark ocean below. Living in a kelp forest, coral reef or the open ocean, it is always a good idea to blend in with your environment. The blue shark is one of the most common open ocean sharks. It rarely comes near the coast. Blue sharks eat sardines, anchovies and squid. They migrate great distances and swim in schools of the same sex. Blues have many more pups than most sharks. Litters of 135 pups have been reported. Pups are born after nine to 12 months and are 15 to 20 inches long at birth. An 8-foot blue shark was nine years old. Courtship and mating in sharks is tough business. The female will have many bite marks on her back. She is protected, during mating, by having skin twice as thick as the male. There is a fishery for blue sharks on the west coast of California. But, since blues grow slowly and don't reproduce until they are more than 6 feet long, they can't support a large-scale fishery. Instead, a more positive approach is visiting the sharks in their world. A business has grown up around the blue sharks of California. Scuba divers can see the sharks up close, from a protective cage, without harming them.

Range: Worldwide in tropical and temperate seas.