Final answer:
Marine fishes produce concentrated urine because they live in a hypertonic environment and need to conserve water, while freshwater fishes excrete dilute urine due to their hypotonic environment causing them to take in excess water.
Step-by-step explanation:
When contrasting urine concentration in marine versus freshwater fishes, we can observe that marine fishes typically have concentrated urine while freshwater fishes have dilute urine. Freshwater fishes live in a hypotonic environment relative to their internal cells, leading to water influx; thus, they must excrete the excess water and maintain their electrolyte balance by producing dilute urine. On the other hand, marine fishes reside in a hypertonic environment, meaning they lose water to their surroundings and must drink seawater to stay hydrated, excreting the excess salts through their gills and conserving water by producing concentrated urine.