Swan Mussels are often sold to fishpond keepers as an aid to filtration because they are filter feeders. This is perfectly true as they do filter the particulate matter out of the water at the rate of about one pint of water per hour for a mature mussel. However, as the average fish pond holds in excess of 100 gallons, or 800 pints of water it is easy to see that one would need a very large number in deed to give effective filtration.
When mature Swan mussels measure 6, or 7 inches, or about 15 cm in length, but they start out life as tiny eggs which float around in the water slowly developing. As they develop they go through a stage where they become parasitic on fish. They catch a ride on any passing fish and cling to it boring a tiny hole in the fish. They continue developing and after a few weeks they drop off doing no serious damage and they become free swimming. At this stage they are tiny miniature adults and will cling to the sides of the pool. Here they grow over several years until they reach maturity. Mussels like a good layer of mud on the bottom of the pool into which they can bury themselves. They do this with the use of their pseudopodia, or single foot with which they move.
Swan mussels have an unusual relationship with a fish called a Bitterling. The female fish develops an ovipositor, or egg laying tube in the breeding season down which she lays about 50 eggs directly into the mussel. The eggs develop inside the shell of the mussel until they hatch and then the baby fish swim out. This prevents the eggs from being eaten by other fish. In return the fish act as hosts to the baby mussels.
One problem with this arrangement is that the adult fish find the mussel very tasty to eat as do most other fish especially Tench and Carp. For this reason unless you intend to breed Bitterling, mussels can end up being an expensive type of fish food if added to a pond.