Zebra mussels are dioecious (a population that consists of males and females). When water temperatures reach 11 or 12 ºC, females release eggs. Females can reproduce when 12 to 18 months old (age at maturity decreases with growth rate; sexual maturity is at a shell length of 10 mm) and are capable of producing 40,000 eggs per season. Males release sperms directly into the water and eggs are fertilized externally. The fertilized eggs hatch into a free swimming veliger larva that range in size from 0.04 to 0.07 mm. Because zebra mussels do not release sperm and eggs at the same time, spawning, once initiated, can occur over extended periods. In waters of the United States, veligers can be found from May to October. Native freshwater mussels reproduce at a specific time, usually in the spring and typically are not reproductive when older than 5 or 10 years.