[Campbell Biology P.1028] Unlike marine bony fishes, sharks have an internal salt concentrati... | Practice Question

Unlike marine bony fishes, sharks have an internal salt concentration much lower than that of seawater. However, the text explains that water slowly enters a shark's body by osmosis. What is the primary reason for this osmotic water influx?

  • A: Sharks actively transport water into their bodies against a concentration gradient.
  • B: Sharks consume large amounts of food that have high water content.
  • C: The total solute concentration in shark tissues is maintained at a level hyperosmotic to seawater due to high concentrations of urea and TMAO.
  • D: Sharks continuously excrete excess salts, creating an osmotic gradient that draws water in.

Explanation

Sharks maintain a relatively low internal salt concentration compared to seawater, but they accumulate high concentrations of urea and trimethylamine oxide TMAO in their tissues. These organic solutes increase the total solute concentration in their body fluids to a level slightly higher hyperosmotic than that of seawater e.g., above 1,000 mOsm/L . This creates an osmotic gradient that causes water to slowly enter the shark's body by osmosis, rather than being lost to the environment.