Swan mussels are sold as “an aid to filtration” because they filter feed on planktonic algae (green water) but they produce larvae that are parasitic on fish. Depending of the size and maturity of the adult, the larvae may be released in hundreds or hundreds of thousands at a time.
The larvae, called glochidia, are released into the water through the adult females’ exhalent siphon and sink to the bottom or are scattered by water currents. They can open or close their shells but cannot move independently so, once shed by the female, the glochidia must find a suitable host, usually within 24 hours, or they will die. Some mussels require a specific host fish in order to complete their life cycle; others can use a variety of fish species.
The glochidia of some species of mussels have hooks or teeth on the edges of their shells that enable them to attach themselves to the host. They can attach themselves to soft exterior parts such as the fins of fish resting on the bottom or they may be breathed in during the respiration cycle and attach themselves to the gill filaments. Once attached, they form a cyst so that they can complete their development and transform into the juvenile form after which they break free and drop off to begin an independent life and grow into the adult form. The period of attachment varies from about a week to six months depending on where they are attached on their host and the water temperature.
There are many different species of mussels that may be sold as “an aid to pond filtration” and there is some debate as to whether the parasitic larval stage causes harm to their host. Some sources say that they just use the host as a taxi, some say that they feed on body fluids. Personally I wouldn’t advise taking the chance. Once the glochidia use their hooks to attach themselves, there is the risk that they will inject bacteria as they do so or that any pathogen in the pond will gain entry into the koi’s bloodstream through the fresh wound.
If that isn’t bad enough, when the glochidia are sufficiently developed, they break free of the cyst which will leave an open wound. This damages the delicate gill epithelium and is another breech in the fish’s defence against disease that is even more serious than the original damage they caused. It is also possible that, if enough of these parasites break free at any one time, the resulting breaks in the epithelium will cause osmoregulatory problems. This will cause the kidneys to have to work overtime to expel excess water, which may cause them irreparable damage. Or, if the kidneys cannot cope with the extra inflow of water and cannot expel it quickly enough, the wounds may lead to death by dilution of internal body fluids.
Swan mussels also like to bury themselves in mud so anyone who would like to put them into their pond will need to make sure that it is suitably dirty.
As far as I can see, they only have one saving grace. They are sensitive to changes in environment, so if your pond is not as dirty as they like, they will soon die