What is a common adaptation of fish living in a hypotonic environment?

Fish living in a hypotonic environment, such as freshwater, face the challenge of osmoregulation due to the lower concentration of salts in the surrounding water compared to their bodily fluids. Consequently, they tend to gain water by osmosis, which necessitates a physiological response to maintain their internal salt and water balance.

Excreting large amounts of dilute urine is a primary adaptation that allows these fish to expel the excess water they absorb while simultaneously retaining essential salts. By producing dilute urine, these fish can effectively manage their internal environment, ensuring they do not become overhydrated and can concentrate the necessary solutes within their bodies.

This adaptation contrasts with those of fish living in hypertonic environments, where conserving water and excreting salt would be necessary. In addition, the other adaptations listed do not directly address the osmotic challenges faced by fish in hypotonic environments, hence reinforcing why excreting large amounts of dilute urine is a key survival mechanism for these aquatic animals.