A - Mouth - Scroll down the page to see extensive information about a shark's specialized teeth.

B - Eye - Some sharks have a protective covering that slides into place as they attack, called a nictating membrane.

C - Gill Openings (Slits) - Most sharks have five, but a handful of deep-water sharks have six or seven.

D - (First) Dorsal Fin - Some dogfish sharks have a defensive spine on the front of both dorsal fins.

H - Anal Fin - This fin, and the one directly above it, are not present in all sharks.

E - Second Dorsal Fin - Not all shark have a second dorsal fin, or the one directly below it, the anal fin.

I - Pelvic Fins (Left and Right) - Between these fins is the shark's "vent" (common excretory and reproductive canal).  Male sharks have modifications to the fins called claspers, which implant sperm in the female's vent.

F - Subterminal Notch - Most sharks have this unique feature on the upper lobe of their caudal fin (some more prominent than others).

J - Pectoral Fins (Left and Right) - These large fins are used like a submarine's bow planes, assisting in maneuvering and helping to maintain proper depth.

G - Caudal Fin (Upper and Lower Lobes) - Large, fast sharks have somewhat equally sized upper and lower lobes.  Smaller, slower species have little or no lower lobe (see "Types" in gray stripe - above).

K - Spiracle - The above arrow shows the approximate location of this sensory organ, which helps the shark find it's prey.

Most sharks have several important design modifications.

Skeleton of Cartilage, not Bone - Cartilage is rigid enough to act as structural material, but pliable enough to allow maximum flexibility (like our nose and ears).  The ability to rapidly flex the body and change direction makes the shark an agile predator.  The cartilage is important for another reason, which you will learn as you continue to read.

No Air Bladder - Unlike most fish, the shark has no air bladder to assist in buoyancy control.  For that reason, it may need to swim (while using it's pectoral fins as planes) to avoid descending.  It's lighter cartilage skeleton also helps lessen it's tendency to drop to the sea floor.  Also, it has excessive liver oils, which are lighter than water (oil floats).  The big advantage of having no air bladder, is that the shark is able to more quickly change depth.  This allows the shark to use a preferred attack method - rapidly approach it's unsuspecting prey from below.

Multiple Rows of Teeth - If you stick your head inside a shark's mouth (which we don't recommend), you will see a flexible "gum" covering the inner surface of the jaw, just behind the teeth.  Beneath that "gum" are several rows of developing teeth, one row lying against the next farther back, pointing toward the shark's throat.  When an exposed tooth is lost, the space is soon filled by a tooth from the next row back, which rotates forward from is flattened position, eventually pointing upward/outward, as the rest of the primary teeth do.  Throughout the life of a shark, it will lose and replace thousands of teeth.  That is why we find them relatively easily as we fossil hunt.

Teeth of Specialized Shapes - Different sharks can have distinctive tooth shapes, largely dependant on what prey they eat.  Or is the prey they eat dependant on the shape of their teeth?  In any case, sharks with long dagger-like teeth on both top and bottom jaws generally swallow prey whole (their teeth are not designed well for cutting, but are better for catching).  This means that their primary food source will be prey that are smaller than they are.

Other sharks have flat, triangular, serrated teeth in the upper jaw (and maybe also the lower jaw).  These sharks are able to feed on prey of any size, because they can bite and cut the flesh of their prey.  Arguably the most successful group of sharks (Reef or Requiem Sharks) often have cutting teeth in the top jaw and stabbing teeth in the lower jaw.  This allows the lower jaw to grasp the prey and hold on, while the upper teeth cut.  This is accomplished when the shark shakes it's head.  The top and bottom jaws partially dislocate and move back-and-forth across each other.

Another tooth adaptation that is obvious when you see the entire jaw, is that the teeth become more angled as they proceed toward the back of the jaw (pointing toward the throat).  Like the barb of a fishing hook, the teeth hold the prey in the jaw.  The farther back in the jaw the teeth are positioned, the shorter and more angled they become.  Two teeth from the same species, even the same fish, may look very different.  Considering there may be only subtle differences in teeth between two closely related species, you can imagine the difficulty determining the exact species base solely on a single tooth.  So please take it easy on me, if I don't offer an exact species identification, especially among the many Reef shark species.