What type of freshwater organism uses gills for respiration?

Fish are the primary freshwater organisms that use gills for respiration. Gills are specialized respiratory organs that extract oxygen from water as it flows over them. This makes them highly efficient for life in an aquatic environment, where oxygen is dissolved in water rather than available in the air as it is for terrestrial organisms.

Fish have evolved various adaptations related to their gill structure and function, enabling them to maximize oxygen uptake in different water conditions. The water enters the fish's mouth, passes over the gills, and then exits through openings on the sides, allowing for continuous oxygenation of the blood.

While some amphibians do use gills during their larval stages, they typically rely on lungs as adults, and other groups like mollusks and crustaceans have their own unique respiratory adaptations. Mollusks may have gills or rely on other respiratory mechanisms depending on the species, while crustaceans often utilize gills or other structures, but fish are the most characteristic and iconic group associated with gill-based respiration in freshwater environments.