Final answer:
A shark's liver secretes squalene oil to assist with buoyancy in water. The oil's lighter specific gravity allows the shark to maintain buoyancy without a swim bladder. Additionally, urea and TMAO in sharks' blood aid in osmoregulation, enabling isotonicity with seawater.
Step-by-step explanation:
A shark's liver secretes a substance known as squalene oil, which helps with buoyancy in the water. This oil has a specific gravity that is lighter than water, aiding the shark in staying afloat. Since sharks lack a swim bladder like bony fishes, which can adjust buoyancy, they have evolved to utilize the large amount of squalene oil in their liver for this purpose. This adaptation is an example of the complex osmoregulatory mechanisms evolved by sharks and other marine creatures to survive in their environments.
The high concentration of urea found in sharks' blood along with trimethylamine oxide (TMAO) also contributes to their osmoregulation, allowing them to be isotonic with their marine environment. In contrast to bony fishes, which use mainly active transport of salts through their gills to maintain electrolyte balance, sharks store large concentrations of urea to maintain the right saltwater balance and prevent protein disruption through the stabilizing effects of TMAO.