What are the features of the structure of the respiratory organs of fish associated with the habitat?

The respiratory system in fish is represented by gills. The gills are a unique adaptation for breathing underwater. Through the gill slits, water enters the gills, where gas exchange takes place in the gill capillaries. Then water is thrown out from under the operculum, which also protect the gill from mechanical stress. In fish, the respiratory system is closely related to the circulatory and nutritional systems.