Newly hatched zebra mussel veligers have a velum that supports a ring of cilia that are used for swimming and feeding. Larvae tend to swim up at night and move down during the day but are unable to swim horizontally toward specific objects. They colonize new areas by being carried passively on water currents. Veliger densities have been reported to range between 70 and 400,000 individuals/m2.
The veliger feeds and grows in the plankton for about 10 to 14 days. Gradually the velum begins to decrease in size and the veliger settles to the substratum and metamorphoses into a shelled juvenile. The newly settled mussel resembles an adult and is no more than 0.2 or 0.3 mm long; hence it is easily overlooked. Settlement of immature mussels takes place in areas with velocities less than 1.5 to 2.0 m/s. However, once attached, zebra mussels can tolerate velocities greater than 2.0 m/s. Zebra mussels usually attach to surfaces that are covered with a film of algae or bacteria. This film can develop on a clean surface within a few days.
The ability of immature zebra mussels to remain suspended in the water column for up to 2 weeks allows them to be dispersed great distances in rivers. The immature stage of most native mussels are not free-living but must spend a developmental period on the gills or fins of a specific species of fish.