Jean-Michel Cousteau : Ocean Adventures . Sharks at Risk (original) (raw)

In-depth: Shark Senses

The Superlative, Sensitive Shark
by Robin Marks

Legend has it that as the waves carried the Hawai'ian shark god Kauhuhu to his cave at the base of a cliff on Molokai, his senses told him there was a stranger hiding nearby. "I smell a man!" he thundered as he approached his home within the rocks. "Who is in my cave?" Kamalo, a priest who had come to ask the shark god for help in avenging the death of his sons, crouched silently under a rubbish pile of taro peelings. He knew his scent would give him away. When Kauhuhu's adept nose led him to the priest, Kamalo was prepared. The shark god heard Kamalo's story and agreed to help him.

Native Hawai'ians aren't the only ones who have been in awe of the shark's amazing senses. Nature appreciators the world over have long admired the bevy of detection mechanisms that make the shark the best-equipped predator on the planet.

Nothing to Turn Your Nose Up At

The notion that the mighty great white can smell blood from a great distance has been central to modern shark mythology. Indeed, sharks do possess some of the most amazing olfactory capabilities in the natural world. Smell is so vital to a shark that two-thirds of its brain is devoted to it. They can detect some scents at concentrations as low as 1 part per 25 million, which translates to about a third of a mile away in the open ocean.

Sharks smell through a pair of nostril-like holes, called nares, on the undersurface of their snouts. The olfactory organs that lie within these holes are extremely sensitive layers of tissue with many folds that increase the surface area. A shark uses its sense of smell to navigate toward its prey. When its olfactory sensors detect the odor of a potential catch, the shark will turn into the current that is carrying the chemical. In addition, a shark's olfactory talents are so refined that it can often tell which of its nares is getting the stronger scent signal, guiding it even more precisely toward its prey.

Shocking Snout Sensitivity

The most intriguing and alien sense that sharks possess also resides in their snouts: the ability to detect minute electric fields. In fact, sharks are almost as good at sensing electricity as the best equipment in a physics laboratory is.

This ability, called electroreception, originates in clusters of pores scattered around the shark's head. The pores form different patterns - called long trails, V shapes or ovals - in different shark species and in different areas of the head. Inside these pores are small cells, called ampullae of Lorenzini, which are filled with a gel-like substance that can conduct electricity. Each cell also has a tiny hair within it. When a charge goes through the gel, it also passes through the hair, which triggers a sensory signal. Researchers have also found that the gel can translate minute temperature variations - which might be caused by potential prey - into electrical signals, making each ampulla not only a conductor of electricity but also a sort of thermostat.

All animals generate electricity around them as their muscles contract in movement and their heart beats. Researchers have found that sharks can detect these unimaginably small electrical fields. For example, the bonnethead shark appears to sense a field of 1 nanovolt per square centimeter - that's about equal to the voltage you'd find if you connected two flashlight batteries with a wire 10,000 miles long. It amounts to a sensitivity to electricity that's 5 million times greater than what we humans can feel.

A hammerhead shark, with these detectors scattered across its oddly shaped head, can swing its "hammer" back and forth above the ocean floor, looking for prey buried in the sand, as if it were a metal detector looking for coins.

The ampullae may also provide a sort of electrical "sight" for the shark after it has attacked its prey. Sharks roll their eyes into their head to protect them during attack. At these times, they rely on their electrical sensors to know where their prey is. A bonus for the shark is that wounds leak electrolytes, enhancing the electric field around the prey. This phenomenon helps the shark sense what is happening as its chosen victim struggles to escape.

Catlike Eyes

A shark's eyes are adept at seeing movement in the light-deficient deep-sea environment. They are similar to those of a cat, in that they contain a row of small plates that reflect incoming light through the retina for a second time. This allows sharks to see their prey even in dim ocean waters.

Motion Detector

Like most other fish, sharks can detect movements in the water around them via a set of small fluid-filled canals that run along their sides. These canals make up an organ called the lateral line. The lateral line can be thought of as an extension of the shark's ear because both organs are triggered by low-frequency vibrations commonly associated with underwater sounds. Like our ears, a shark's ears and lateral line canals are lined with small hairs. When sound vibrations, currents or turbulence in the water passes over the hairs, they send signals to the shark's brain. Through the lateral line, a shark can also feel movement in the water around it at a distance of up to 330 feet.

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