Which of the following is a common characteristic of freshwater fish?

Freshwater fish possess specific adaptations that enable them to thrive in environments where they face the challenge of maintaining their internal salt concentrations. Unlike saltwater fish, which are in a hypertonic environment, freshwater fish are in a hypotonic environment. This means they are constantly at risk of absorbing excess water and losing essential salts. To combat this, freshwater fish have evolved mechanisms that facilitate water conservation: they produce dilute urine to excrete excess water while actively reabsorbing ions through their gills and kidneys. These adaptations are crucial for maintaining homeostasis in a low-salinity environment, making them distinct features of freshwater fish.

The other options, while potentially relevant to some species, do not represent common characteristics across most freshwater fish. Tolerance to high salinity is more relevant to marine species, bright coloration for warning is not a universal trait and may apply to specific species or in particular survival contexts, and the ability to breathe air is more associated with certain species like lungfish or anabantoids, but not all freshwater fish. Thus, the adaptation to conserve water uniquely characterizes freshwater fish.