A lot of partially correct info in this thread.
The mussels that cause problems in the Great lakes are Zebra mussels that hitch-hiked to the US in balast water from shipping. There are no natural predators in the US so they have multiplied and choke out water vents etc. with all of the manufacturing plants on the lakes this is a real issue. Yes you should check out the specific species before placing in a pond and check with local ordinances.
With that said my family has used them since 1992 with great success, first at our nursery in the Redlands (Southwest of Miami) and then up north in my 5 ponds. This really maintains a low algal growth with very clear water. I find the fish tend to eat them when they die, but they do reproduce that also indicates a healthy pond, i.e. the fish somewhat behave like bees by carrying around the larval stage (specific name I'll have to look up) before they settle and grow on a hard surface. Commonly on old shells is my observation. They do filter the water of detritous along with microorganisms including algae. For a 2000 gallon pond I started with 1/2 dozen mussels that are common in the creeks in the area. When I clean the pond I find a lot of empty shells, but I also find mussles growing on the shells, so I can't give you a number. In my pond they tend to hide in the anacharis beds.
Most clams and mussels are edible, but you are what you eat
. If you eat oysters think about the polutants around the beds and crabs really are scavengers. Being a carnivore this doesn't bother me so Jerry cook up some linguinni and bring the olive oil and lets have some good eats!
Gary