Sharks

Sharks 101

posted: 04/11/12
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Rodney Fox probably wishes he'd seen a sign like this one.
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Sharks are a picture of perfected evolution. Estimates for the length of time they've been swimming the seas range from about 350 to more than 400 million years — that's quite an impressive track record compared with other organisms' longevity rates. Modern sharks are slightly less ancient, but they still hit the scene an impressive 150 to 200 million years ago.

At least 350 to 400 different species of sharks live today, although more are still being discovered. There is great diversity among these creatures, even while they share many common characteristics. Sharks range in size from less than a foot (0.3 meters) to more than 40 feet (12 meters) long. Unconfirmed sightings of whale sharks have even reported individuals up to about 60 feet (18 meters) in length.

Some notable features of sharks include their enormous livers, which are proportionally the largest of any animal, their cartilaginous skeletal structure, which gives them great flexibility, and their ability to sense the world around them in ways we can barely understand.

Take electroreception, for example. Sharks have electrosensory organs that can pick up tiny bioelectrical fields and hone in on them. In the middle of a shark attack, frantic muscle movements and the electricity generated by a terrified nervous system basically paint a bull's-eye on the victim's body.

Sharks use different senses at different ranges, although this varies depending on the species. In general, ears are usually the first organs engaged, roughly followed by the nose, movement-detecting lateral-line organs, heat-sensing pit organs, eyes and ampullae of Lorenzini, the source of that uncanny electroreception. Two other senses — touch and taste — are the last to come into play (and the least enjoyable for any potential prey).

All those adaptations, though, haven't prepared sharks for humans' influence on the Earth's oceans, from commercial fishing to pollution. These factors are delivering a quick one-two punch to worldwide shark populations, so the next million years might turn out to be their last.

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