WHAT IS A SHARK?

All sharks belong to the class of fish called Chondrichthyes, which unlike bony fish have skeletons made of cartilage. Sharks do not have scales like bony fish but instead have rough skin. Their skin is covered with tiny sharp teeth called dermal denticles. These teeth can scrap the skin off your hand if you rub it the wrong way, which is from tail to head. When rubbed from head to tail sharks feel smooth (with the exception of a few species). The denticles are there to make the shark move silently, and protection. Another thing that separates sharks from bony fish is that their backbone extends into the upper lobe of the caudal fin. The shark also fertilizes internally, and has fewer young then bony fish. Reproduction will be in a different section.







1.Liver
2.Spiral valve
3.Heart
4.Pancreas
5.Stomach
6.Eye
7.Nostril
8.Ampullae of Lorenzini
9.Teeth
10.Pectoral Fin
11.Pelvic Fin
12.Anal Fin
13.Caudal or Tail Fin
14.Second Dorsal Fin
15.First Dorsal Fin
16.Gills

Sharks have highly sensitive senses, a special liver which helps them to float, several rows of teeth, and eyes which aren't so different from humans' teeth.
Like rays, shark skeletons are made of cartilage.
A shark has several pairs of gills on either side of its head, unlike other fish who only have one gill on each side. You probably breathe through your nose and mouth, but a shark only uses its nostrils for smelling. When people don't want to be seen by their enemies they sometimes wear fatigue suits. Sharks are born with their own special colors to protect them from predators above and below them in the water.

SHAPE:

The shape of a shark is specially designed to help it navigate long distances and maneuver around its prey with ease.
"When engineers came up with the shape of a DC-9 airplane, they were thinking about the fastest way to travel using the least amount of energy. If you compare the DC-9 to a shark, you will see that their shapes are amazingly similar. They both have round bodies tapering off at both ends. This shape allows the plane to glide through the air, and the shark to glide through the water, without using up all their fuel before they get where they want to go." (Stephens 32).

SKELETON:

The bones of a fish are made mostly of calcium, but a shark doesn't really have any bones. A shark skeleton is made of cartilage. Bony fish have a gas-filled swim bladder which enables them to float in the water, but sharks have no such bladder. Since cartilage is lighter than bone, it helps to keep a shark from just sinking to the bottom of the ocean. A shark doesn't have as many moveable parts as a bony fish, which in some ways makes a shark a little more clumsy. But cartilage is more flexible than bone, so a shark can turn around in a smaller space than a bony fish. Sharks keep growing cartilage as long as they live, and have extra mineral deposits in their jaws where they need extra strength, (the better to bite you with).

NOSTRILS:

Sharks do have noses, but they only use them for smelling, not for breathing. You'll find their pair of nostrils on the underside of their snouts. Some species, like the Nurse Shark, even have some extra smellers called nasal barbels, which stick out near the nostrils and mouth.

GILLS:

A shark has several pairs of gills on either side of its head, unlike other fish who only have one gill on each side. You probably breathe through your nose and mouth, but a shark only uses its nostrils for smelling.

Sharks have five to seven gill slits on each side of their head, unlike bony fish which have one gill on each side. As water passes over their gills, oxygen is absorbed by the blood in the gills and transported from there to the rest of the body.

Some sharks have small openings called spiracles behind their eyes, at the top of the head. Spiracles are sort of like baby gill slits. The more active, fast swimming sharks seem to have outgrown the need for these spiracles, and only have very tiny ones or don't have them at all.

TEETH:

Shark teeth are in parallel rows. In the front row are the teeth they use to catch and eat their prey. Several rows of replacement teeth keep growing behind the teeth they are using. As their teeth fall out, the new teeth come forward. Sharks keep growing new teeth as long as they live.
Studies by scientists at Mote Marine Labs have shown that, on average, a Nurse Shark will replace each front row tooth every ten days to two weeks during the summer, when actively feeding, and every one to two months during the winter, when they are less active.

THE LATERAL LINE:

The lateral line is a sense that we human do not have. "The lateral line is a fluid filled canal that is lined with tiny hair like receptors that are attuned to vibrations made by prey." (Smith 11). These lateral lines are located just under the skin on the shark's snout and along both sides of the body. These lines are also in bony fish. Fish (sharks included) can also use these lines to sense water currents and pressures and even sounds. Some say that the bumping behavior of sharks allows external taste receptors to see if the organism is edible (this may explain why sharks often bump objects).

BODY TEMPERATURE:

A few of the larger species, such as the shortfin mako, Isurus oxyrinchus, and the great white, are mildly homeothermic - able to maintain their body temperature above the surrounding water temperature. This is possible because of the presence of the suprahepatic rete, a counter current exchange mechanism that reduces the loss of body heat. Muscular contraction also generates a mild amount of body heat. However, this differs significantly from true homeothermy, as found in mammals and birds, in which heat is generated, maintained, and regulated by metabolic activity.

Works Cited

Digest, Reader's. Sharks. New York: n.p., 1998. 

Ellis, Richard. The Book of Sharks . N.p.: n.p., n.d. 

Martin, James. Great White Sharks. Minneapolis: Capstone , 1995. 

Shark Facts. 26 Feb. 2008 .