Clams filter everything, but I have never heard of clams helping to remove turbidity. Usually, they don't do well in those conditions. There are a number of species of clams and or mussels native to North America that can live in ponds. When the pond was flooded last year, they probably made entry then and have liked it ever since. The larger sized clams probably don't have much value as food except when they die, catfish may feed on them. However, big clams make little clams and I am sure RES, PS and others love to feed on them. I introduced Asiatic clams to my pond this year, we'll see if they take hold.