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Friday, November 24, 2006


7 sharp senses tell
sharks if food is near


Last week off the Kona Coast, 25 tiger sharks ate a sick humpback whale. Apparently the whale was too ill to defend itself, and the sharks bit it throughout the morning. By late afternoon the whale was dead.

Officials towed the carcass offshore and secured it to a buoy, and the next day, the whale was gone. The sharks did their job.

Even though I knew this is natural selection at work, it was a particularly gruesome example. To keep myself from thinking about that poor, suffering whale, I concentrated on the sharks. Their behavior was certainly food for thought.

To me, 25 seemed like a large number of tiger sharks to be gathered in one area. I think of these scavengers as solitary creatures roaming tropical waters in search of sick and dying animals. And that's right. But they're so good at searching, it doesn't take much to gather a crowd.

Sharks find their food using a combination of seven senses.

We've all heard that sharks have an exceptional sense of smell, and it's no fish tale. Sharks have two nostrils beneath their snouts. As the fish swims forward, water goes in one nostril and out the other, enabling the shark to smell the water.

Sharks can detect a smell as diluted as one part per billion, a great advantage in enormous oceans.

Sound is another way sharks find food. Sounds waves, which are five times faster in water than air, come to a shark through two pores at the top of its head. Researchers estimate that sharks can hear low-frequency sounds, the wavelengths of some struggling fish, about 100 yards away.

Sharks' eyes are similar to our own and obviously help them zero in on their targets. Sharks also have senses of touch and taste like ours. And sharks do taste their food. When researchers gave sharks squid treated with alcohol, they spit it out.

What we don't have in common with sharks is a lateral line and electro-reception organs.

Most fish have lateral lines, rows of canals beneath the skin that run around the entire fish. These sensors detect changes in water motion, such as splashes of a dying animal.

It's a shark's rare ability to sense electric signals as weak as a billionth of a volt, however, that gives it a leg up (or a fin up). Because all living creatures give off weak electric signals, whether moving or not, sharks' electric receptors help them find even well-hidden animals.

Given sharks' keen senses, it's silly for us human swimmers to think sharks don't know we're there. They do. They're just not interested.

Occasionally, though, a shark mistakes a person as something to eat and takes a bite. When the fish finds it's bitten something alive and kicking, and it doesn't taste right anyway, it lets go.

This explains why so many people survive shark attacks. Sharks have bitten four people in Hawaii in 2006, all on an arm or leg, and all survived.

Sharks might be food for thought, but fortunately, people are not food for sharks. The sharks are smarter than that.

 

 

Marine biologist Susan Scott writes the newspaper column, "Ocean Watch",
for the Honolulu Star-Bulletin, www.starbulletin.com