NAME: SHARKS

SCIENTIFIC NAME: PHYLUM: CHORDATA; SUBPHYLUM: VERTEBRATA;

LOCATION: FOUND IN ALMOST ALL OF THE WORLD’S WATERS, FROM EQUATORIAL ZONES, TO TEMERATE WATERS, AND EVEN BELOW THE POLAR ICE CAPS.  SHARKS THAT LIVE IN TEMERPATE REGIONS (GREAT WHITES, MAKO, BASKING) ARE SOMETIMES MIGRATORY SHARKS, TRAVELING, AS THE SEASONS CHANGE AND THE WATERS BECOME COOLER, TO WARMER SOUTHERN WATERS.  SOME TROPICAL SHARKS (WHALE SHARK) SPEND THEIR LIVES MIGRATING THROUGHOUT THE TROPICAL ZONES OF THE WORLD, WHILE OTHERS, SUCH AS NURSE SHARKS, REMAIN IN THEIR NATIVE HABITAT THEIR WHOLE LIVES.  SHARKS THAT LIVE IN THE POLAR REGIONS (GREENLAND SLEEPER SHARK, PACIFIC SLEEPER SHARK, AND THE BLACK DOGFISH SHARK) HAVE ADAPTED TO SPEND THEIR WHOLE LIVES IN THE COLD WATERS AT THE ENDS OF THE EARTH.  DIFFERENT SPECIES LIVE IN DIFFERENT OCEAN DEPTHS, WITH THE NURSE, HAMMERHEAD, GREAT WHITE, AND TIGER PREFERRING SHALLOW WATERS NO DEEPER THAN 650 FT.  OTHER SPECIES, SUCH AS CATSHARKS, GOBLIN SHARKS, AND MEGAMOUTH SHARKS, PREFER DEEPER WATERS, AND LIVE ALONG THE BOTTOM OF THE OCEAN FLOOR IN THE COLD WATER.

REGOGNITION FEATURES:  STREAMLINED BODY, 5-7 GILL SLITS ON EACH SIDE OF THE HEAD, DEPENDING ON THE SPECIES, THE HEAD IS TYPICALLY STREAMLINED WITH THE BODY, EXCEPT FOR SOME SPECIES, SUCH AS THE HAMMERHEAD, WHICH HAS A T-SHAPED HEAD WITH EYES LOCATED ON THE SIDES.  VARIED TEETH, RANGING FROM DAGGER-LIKE IN CARNIVORES, TO FLAT FOR THOSE THAT EAT SHELLFISH.

DETAILS:

Sharks are probably the most misunderstood and vilified animals that exist, but at the same time, are also some of the most interesting creatures to inhabit the planet.  Certainly, most people know about sharks from the movies, which more often than not, characterize these animals as cold-blooded murderers, which go on ferocious killing sprees (usually on American summer holidays) and don’t stop killing until they have been stopped, or perhaps, the holidays are over.  Contrary to popular opinion, sharks are not the aquatic terrorists that Hollywood would have us believe.  Sharks kill roughly five people per year, while humans on the other hand, are responsible for somewhere around 38 million sharks deaths per year.  It’s enough to make one wonder who are the real terrorists. 

One part of the characterization is true: sharks are killers, only not of people.  Out of the 360 species of sharks in the world, only four are responsible for a significant number of attacks against humans.  The great white, tiger, oceanic whitetip, and bull sharks are big, strong predators, and are responsible for more attacks against humans, but they also are not normally dangerous animals, and most of the time, people can swim with them without incident. 

Sharks are fish that are characterized by having a cartilage skeleton, uncovered gill slits, multiple rows of teeth, and an extensive array of senses. 

The difference between fish with cartilage and those with bony skeletons, is that the bony species have gas-filled swim bladders to give them buoyancy.  Sharks, however, do not have this characteristic, and use a large, oil-filled liver to allow them to float.  The liver can be up to 25% of the body mass. 

The gill slits found on most sharks are different from those found on most fish, although they do function in roughly the same way, allowing the fish to extract oxygen from the water as it passes over the gills.  Since the gill slits on sharks are uncovered, the shark relies on what is called ‘ram ventilation’, which means that water must pass over the gills in order for them to function.  This is the reason that most sharks must remain in a relative state of motion.  Sharks that would be unable to move, would drown. 

All sharks, except for the whale shark, have several rows of teeth, which are mostly temporary.  Some species, such as the great white, tiger, and bull sharks, have razor-sharp teeth, designed to cutting and tearing prey.  Others have flat teeth which are used to crush shellfish.  Over the course of a shark’s life, some species are thought to go through as many as 30,000 teeth in a lifetime.  Whale sharks do not have teeth.  Instead, they use filters to feed on plankton and other small animals, in the same way most whales do. 

Perhaps the most evolved aspects of sharks are their senses.  Sharks possess highly developed senses of smell, sight, hearing, and also electroreception.  So highly advanced are sharks senses of smell that they can detect one part per million of blood in water up to a quarter of a mile away.  While the role of sight in sight in sharks’ hunting is still being debated, what is not in question is that sharks do possess strong olfactory senses that allow them to see in dark water.  Sharks are also able to hear prey from miles away.  They have a small opening on either side of their heads which lead to the inner ear through a small canal.  In addition, the lateral line—a sound and vibration detection system which allows sharks to literally ‘feel’ prey in the water—provides them with yet another sense to rely on.  Finally, electroreceptors are organs that are ‘tuned in’ to certain electric frequencies, which allow sharks to detect the different electric charges that are given off by all fish.  Scientists also believe that sharks use this sense as a sort of Global Positioning Device, as they can monitor the magnetic and oceanic currents of the earth. 

Although most people only think of sharks as dumb beasts, guided solely by their ancient programming to eat and kill, they have also been observed playing and even occasionally working together in groups.  Sharks brain-mass-to-body-mass ratios are similar to those of other higher animals.