among the numerous types of sharks, there are some common general characteristics. sharks have rough skin that is covered with tiny teeth called dermal denticles. the dermal denticles point backwards and help the shark swim with less resistance and more fluidity. for centuries, this abrasive skin was used as sandpaper.

the teeth of a shark are disposable. the teeth are arranged in rows, and as the front row wears out, the back rows move forward to replace them. These replacements shift forward every few weeks.

sharks are generally extremely adept at buoyancy control. they have very large livers that contain a large amount of oil, and this, along with their lightweight cartilage skeletons, helps them stay buoyant. most sharks also stay in constant motion in order to keep water flowing through their gill slits, and this also helps them to stay afloat.

most sharks are cold-blooded, but some species, such as the mako and porbeagle, are warm-blooded, which means they can control their own body temperatures. some species are known to warm their body temperatures up to 10°C warmer than the temperature of the surrounding waters.

sharks have a special sense called electrolocation, the ability to sense electrical signals in the water. every living animal in the water emits electric signals around its body, which are called bioelectric potential. this bioelectric potential is even stronger around a cut in the flesh, and this puts an animal at higher visibility. while hunting, sharks hone in on electrical energy so that they can make a precise attack.

electrolocation also helps sharks navigate over long distances, through multiple oceans and a diverse array of habitats. sharks are known to travel up to 10,000 miles in straight lines while following magnetic fields, sometimes in the deep seas that provide no other navigational indicators (such as polarized light, wave cues, etc). their sensitivity to electromagnetic fields is astounding. a shark can sense the electricity emitted by single battery in the sea from over a mile away!

sharks have inhabited our seas for 450,000,000 years, since the time of the dinosaurs, but humans know comparatively little about them. although many species have not been identified, there are currently over 450 known species of sharks.