5 things you might not know about sharks

When it comes to humans versus sharks, the numbers are staggering. An average of 6 humans are killed worldwide each year by sharks, while humans kill 100 million to 273 million sharks annually, selling their body parts in markets around the globe.

Sharks were swimming in the ocean depths long before dinosaurs climbed onto land. Researchers say sharks date back 400 million years.

There are 465 known species of sharks, ranging from the tiny spined pygmy shark (about 8 inches long) to the whale shark (about 50 feet in length). Each of these sharks plays a role in keeping other fish populations in check, which is why hunting and killing of sharks can create great imbalances in ocean life.

Richard Dreyfuss' character in "Jaws" claimed that the shark is "a miracle of evolution," and that the only thing it does is "swim and eat and make little sharks, that's all." That's not exactly correct.

Sharks have to eat, like every other living organism, but they don't mate until they are 12 to 15 years old. The females give birth to one or two pups at a time, making it difficult for the sharks' reproduction rates to keep pace with the annual kill rates.

Sharks are extraordinary hunters, who use six senses. In addition to smell, hearing, touch, taste and sight, they also are gifted with electroreception — special pores around their faces detect electrical currents from other organisms and allow them to deftly hone in on their prey.

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