How do freshwater fish manage their water and ion levels?

Freshwater fish live in an environment where the concentration of solutes (ions) in the water is much lower than in their bodies. As a result, these fish gain water through osmosis because the water moves from an area of lower solute concentration (the surrounding water) to an area of higher solute concentration (inside the fish). To manage this influx of water, freshwater fish must actively take in ions to maintain their internal balance.

By taking in water and excreting excess ions, freshwater fish ensure that they do not become over-hydrated. This actively regulated process allows them to maintain osmotic balance and homeostasis. The kidneys of these fish are adapted to produce large amounts of dilute urine, helping to expel the excess water while conserving the necessary ions their bodies need.

This management strategy is crucial for their survival in a low-salinity environment, thus making the choice of taking in water while excreting ions the correct response.