My sister asked me the other day what made sharks sharks. I was all prepared to explain in detail but something came up. It did give me an idea for a series though. Many websites on Shark information have a page on this and I hope my take on it is somewhat accurate.
Unlike most other fish, sharks do not have bones. They have cartilage which has a lower density and is both flexible and durable. This helps the shark to stay afloat and swim better.
Sharks have tooth-like scales (placoid scales) all over their skin which reduce drag and turbulence in the water, hence helping them swim faster and more efficiently.
Fins have fins that can't fold down like those on other fish. Their fins have various special functions. Those aren't really the names of those fins, in case you were wondering. The big "scary" fin on top is the first dorsal fin and is used to balance the shark. The pectoral fins on the side help them steer and the caudal fin or tail fin help them to propel forwards. In addition to these, sharks have pelvic and anal fins.
Sharks have gill slits rather than the flappy gills most fish have. Sharks usually have five gill slits but there are some sharks with six or seven. (This Shark only has three because it's a tiny cartoon shark). Most sharks swim with their mouths open to keep water flowing through the gill slits.
Depending on the species, sharks either lay eggs (fewer than fish) or give birth to fully developed young. Shark egg cases vary in size and shape.
In addition to the five senses they share with us (although their sense of smell and sound is probably better), Sharks also possess two additional senses that help them be completely aware of their surroundings. You know, unagi. Their Ampullae of Lorenzini allow them to detect even minute electrical fields generated by other creatures. They can also detect changes in pressure around them through a Lateral line. Cool huh?
Shark teeth come in all sorts of shapes depending on the shark and its diet. The cool thing about shark teeth is that they get replaced by new teeth periodically - this keeps them sharp always!
Unlike fishes (that have swim bladders), Sharks have large oil-filled livers than help them with buoyancy.
ps- That bath mat needs to be mass produced.
Sharks have quite a few traits that make them unique. On top of these special features, sharks have additional features (some sharks have particular features while others have other features) such as third eye-lids, temperature control, disjoint upper jaw.
Sources: http://ww2.kqed.org/quest/2008/10/02/what-makes-a-shark-a-shark/
http://www.elasmo-research.org/education/classification/what_makes.htm
https://www.sharktrust.org/shared/downloads/educational_resources/marine_educators_toolkit/what_makes_a_shark_factsheet.pdf
