Zebra mussels are bivalve molluscs related to oysters, clams, and native freshwater mussels. They are also related to the exotic Asiatic clam (Corbicula fluminea), which is a biofouler in the central and southern United States. Zebra mussels can grow to 5 cm, although most specimens collected in this country have been less than 3.8 cm long. They have an elongate, somewhat pointed, thin shell usually with a zebra-like pattern of stripes. An individual mussel can attach to an object with up to 100 proteinaceous byssal threads that are secreted from a gland at the base of its muscular foot. These threads are extremely tenacious; an attempt to remove the animal by hand usually results in breaking the shell or damaging soft tissue. Native freshwater mussels and Asiatic clams have a single, thin byssal thread that is present only in the juvenile. The zebra mussel is the only bivalve in this country that retains these threads as an adult. The main reason zebra mussels cause problems in industrial and domestic water supplies is that large numbers of mussels attach themselves to hard surfaces within raw water systems by their byssal threads.