Salmon eggs hatch in freshwater. The fish then migrate to the ocean (a hypertonic solution) and, after several years of feeding and growing, return to freshwater to breed. How can these organisms make the transition from freshwater to ocean water and back to freshwater?

A) The rectal gland functions in the ocean water, and chloride cells function in freshwater.

B) Different gill cells are involved in osmoregulation in freshwater than in salt water.

C) Salmon in freshwater excrete dilute urine, and salmon in salt water secrete concentrated urine.

D) Their metabolism changes in salt water to degrade electrolytes.

Different gill cells are involved in osmoregulation in freshwater than in salt water.

Explanation:

Salmon is a euryhaline fish that migrate between seawater and freshwater. Salmon has specific mechanisms for osmoregulation. When in the ocean, salmon carry out osmoregulation by drinking seawater and excreting excess salt from their gills. The specialized chloride cells of their gills actively transport chloride ions and sodium ions (Na+) follow passively. Kidneys serve to excrete an excess of calcium, magnesium, and sulfate ions with the loss of only small amounts of water.

When they migrate to freshwater, salmon produce large amounts of dilute urine. In freshwater, a different type of gill cells of salmon starts taking up salt from the dilute environment. This mechanism is similar to the one exhibited by fishes that spend their entire lives in freshwater.