Female zebra mussels can produce 30,000-1 million eggs per year and males can produce 30 million sperm. Serotonin (a biogenic amine) is present in the gonads. It is implicated in spawning behavior and can reliably trigger spawning. Once the gametes have formed they develop into larvae (called veligers) that begin to form shells. After two-three weeks, the microscopic veligers start to settle and attach to any firm surface using "byssal threads". Temperature, rates of temperature change, food availability, and effects of neighboring mussels seem to be critical variables that determine reproductive responses.