There are 78 species of whales, dolphins and porpoises, and shark species number more than 400. Some whales and sharks can reach gargantuan sizes, while others are smaller than your hand. A great deal of variation exists between the species in each order of animals, but the similarities between sharks and whales are a classic example of convergent evolution, in which two distinctly different animals exhibit similar traits in the same environment.
Sharks and whales both live in the world's largest oceans at a vast range of latitudes, depths and seafloor environments. Some whales and sharks prefer open ocean, like the largest members of each order, the whale shark and the great blue whale. Others, like the sperm whale and the frilled shark inhabit some of the world's deepest regions, thousands of feet below the surface. With such a great distribution in the oceans, it's no surprise that many whales and sharks exhibit similar physical features.
Whales are mammals, while sharks are a type of fish and get their oxygen from gils. Both animals are completely marine, however, spending their entire lives in the ocean. To get around, their appendages are all fin-shaped for pushing through the water, and their bodies are streamlined and smooth. Both sharks and whales use a single tail fin for propulsion, and many sharks and whales also share a dorsal fin. These similar methods of locomotion are an example of convergent evolution, in which the most effective swimmers in a species were selected for breeding over millions of years.
Because of the enormous size of many species of whales and sharks, these animals consume huge volumes of food. Almost all species of sharks and whales are predators or filter-feeders, which filter minuscule animals out of the ocean. Whale sharks feed on planktonic and nektonic creatures like small crustaceans, phytoplankton, algae, schooling fishes and some larger animals. Similarly, blue whales feed on krill by filtering ocean water and inevitably scoop up other small creatures like fish and copepods. Many species of whales and sharks are also active predators with toothed jaws, feeding primarily on other large fish and mammals.
Unfortunately, one thing that nearly every shark and whale species shares is a threatened habitat or population. Human abuses of the ocean, including overfishing, pollution and overharvesting, have placed many large species of whales and sharks on the Endangered Species list. Whale oil and shark fins both cause extreme harvesting, reducing certain species' numbers dramatically. Eight out of 13 of the great whale species of the ocean are endangered or vulnerable, and over one-quarter of shark species are being commercially exploited.