Complex sentence: One of the largest species of freshwater bivalves is the swan mussel, in the family Unionidae; it can grow to a length of 20 cm, and usually lives in lakes or slow rivers.
Split into simpler sentences: One of the largest species of freshwater bivalves is the swan mussel, in the family Unionidae. The swan mussel can grow to a length of 20 cm. It usually lives in lakes or slow rivers.