General Morphology Sharks have a flexible cartilaginous skeleton that allows them to be extremely mobile in the water.  Unlike other fish, sharks do not utilize a swim bladder.  Instead, sharks depend on a large, oil-filled liver and their  pectoral and dorsal fins to aid them in buoyancy.  Contrary to popular myth, sharks are not dependent on swimming in order to stay alive although they do have to be in constant motion to avoid sinking.  The oil filled liver opposed to the swim bladder allows the shark to move with ease between the surface and deep within the ocean. Interestingly, sharks go through about 30,000 teeth in a lifetime.  They have two functional rows of these daggers, but they are continually falling out and being replaced by new teeth (Sharks and their Relatives).

 

Respiration Unlike most other bony fish, sharks cannot pump water over their gills to breathe.  They must move through the water so that oxygenated blood passes over the gill coverings.  Reproduction Sharks usually employ oviparous or viviparous methods of reproduction.  While some may lay external eggs, others give birth to live young and nourish them with their nutrient rich placenta in the womb.  They are called a K-selected species because sharks invest heavily in few, large offspring to ensure the survivorship of their young (Sharks and their Relatives).