The Six Senses

The Six Senses

A Look at the Sensory Systems of Sharks
by Taki Michaelidis

For 13 million years, the shark has, evolutionarily, remained basically unchanged. Even as earth's species have been in a constant state of evolution, earth's present day Chondrichthyes are remarkably similar to the Chondrichthyes of 13 million years ago. In short, Sharks have been so well adapted to their environment, that there is little natural pressure for evolution. Part of this evolutionary perfection can be attributed to the awesome sensory abilities of Sharks. Sharks' six senses provide an incredible array of perceptive tools for hunting, reproducing, eating, and swimming. The abilities of Chondrchthyes make them beautiful creatures, and dangerous hunters.

I.) Shark Sense of Smell

Sharks have paired nostrils on the bottom of their snouts. By drawing water through the nostrils into their olfactory sacs, the shark is able to detect various "smells" in the water. The olfactory bulb in sharks' brains performs the "interpretations" of the received data and is a large component in the brain. Sharks can smell molecules at times when there is only 1 part of the molecule per 25 million parts of water. As a result, sharks can smell things at distances of hundreds of meters. This ability is particularly useful in that sharks can smell blood from far off, and quickly swim after potential prey.

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II.) Shark Vision

In many ways, shark eyes are very similar to human eyes. We both have a cornea, an iris, a pupil, a lens and a retina. However, sharks' vision is slightly better (7x better) than human vision, allowing them to see as far as 50-60 feet underwater in good conditions. In addition, sharks have a structure in their eyes called the tapetum lucidum that increases the sensitivity of the sharks' eyes to light. Furthermore, certain sharks have evolved a special "nictitating membrane" on the lower lid of the eye that protects the eye from thrashing fish during feeding frenzies. While their are a number of visual similarities between different kinds of sharks, the size of the eye, focusing ability of the eye, and strength of the eye vary between individual kinds of sharks.

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III.) Shark Touch and Feel

Sharks' skin is made up of "tooth-like structures" called dermal denticles that provide for a hydrodynamic skin surface. As a result, the sharks are able to swim faster. In addition the skin, called shagreen, serves as a hard, abrasive defensive layer, used to protect the shark and also to wound potential prey. As the sharks grow older, the dermal denticles (placoid scales) are replaced by larger dermal denticles. In other words, the skin doesn't grow directly with the shark.

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IV.) Shark Electrosensory

Electrosensory is the so-called "sixth sense" of sharks and is the "highly specialized bioelectrical system used to detect/find prey and possibly navigate by the ocean's magnetic fields. All sharks have tiny "jelly-filled canals located over the shark's snout," called Ampullae of Lorenzini, that are able to detect and process electrical currents at short ranges. Because all living things produce weak electrical fields, sharks are able to detect these fields in the 0-8 Hz range with electrical field differences of as small as 5 billionths of a volt per centimeter. However, since bioelectrical fields are generally so weak, it is thought that sharks use their Electrosensory systems as a short-range detection device, or as a navigational system to swim by the earth's magnetic field. It is thought that the Hammerhead shark evolved its head in order to increase the surface area of its electrical receptors.

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V.) Shark Hearing

The ears of sharks are located internally inside of the head. By connecting the inner ear, called the membranous labyrinth, to the surrounding water by a number of tunnels and ducts, the shark has a sensitive mechanism for interpreting sounds under water. And since sound travels much further and faster underwater, sharks are able to interpret sounds that are kilometers away.

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VI.) Shark Taste

Sharks lose teeth on a continual basis over the course of their lives. Their teeth are arranged in a set of parallel rows so that as one tooth falls out, another tooth will immediately takes its place. In the course of it's life, a shark may go through anywhere from 14,000 to 20,000 individual teeth. Depending on the feeding habits of individual kinds of sharks, the shape of the teeth for each kind of shark is slightly different. For instance, the Great White Shark has sharp serrated teeth developed for the cutting and slicing of prey. Recently, there have been number of theories proposing that the sharks' sense of taste is highly developed. In short, the theories say that on the first bite, sharks are able to immediately detect the energy content of their prey in order that the sharks may calculate the benefits of killing that particular prey.

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VII.) Sources/Links:

1.) http://www.dive.co.za/graham.htm (Pictures #1, #2)
2.) http://www.mar.dfo-mpo.gc.ca/science/shark/english/ampul.htm (Picture #6)
3.) http://www.enchantedlearning.com/subjects/sharks/anatomy/Senses.shtml (Picture #4)
4.) http://gerber.iwarp.com/descript/physical.html (Pictures #5 and #8)
5.) http://www.learn-animals.com/sharks/03d-scent.html (Picture #3)
6.) http://www.bcsd.k12.ca.us/stiern/senseher.htm (Picture #7)
7.) Class Presentations of MB
8.) Sharks Facts and Stats, A packet from The Center For Shark Research
9.) Anatomical Terminology, Packet
10.)http://www.neaq.org

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