The respiratory organ of a freshwater mussel is the gill, and water flowing over the gills completes gas exchange.

The respiratory organ of a freshwater mussel is the gill, located on both sides of the body within the mantle cavity. Its gills are of the lamellibranch type, with one inner and one outer gill plate on each side, made up of many fine, closely set gill filaments, and the surface of the gill filaments bears cilia. In addition, the mantle can also carry out a small amount of gas exchange and plays a supplementary role.

During respiration, the cilia beat continuously, drawing water in through the incurrent siphon into the mantle cavity. As the water flows over the gill filaments, dissolved oxygen in the water passes through the gill wall into the blood, while carbon dioxide in the blood diffuses into the water; finally the water carries the waste gases out of the body through the excurrent siphon, and one round of gas exchange is complete.

Besides respiration, the gills also take on the duties of feeding and brooding young. Food particles brought in by the water current are trapped by mucus on the gill filaments and then pushed in a directed way by the cilia toward the mouth; in the breeding season, the outer gill plates of the female mussel also swell to form a marsupium, used to hold fertilized eggs and glochidia larvae.

The condition of the gills can also reflect the health level of the freshwater mussel. In farming activities such as freshwater mussel pearl culture, when the water is clean and dissolved oxygen is sufficient, the filtering and respiratory functions of the gills operate normally, and only then are the growth of the mussel and the quality of the pearls guaranteed; if the water lacks oxygen or contains too much filth, the freshwater mussel may show symptoms such as gaping shells, reduced squirting, and weakened activity, and it is necessary to change the water and increase oxygen in time to improve the environment.