Mollusks begin as eggs, transform into larvae, then into veligers before emerging as adults.The mollusk development process and life cycle is complex, particularly in the early stages of development. Upon reaching adulthood, mollusks make their homes in oceans and on land surfaces around the world, but all breed and reproduce in water.
The process of mollusk reproduction is among the oldest and most primitive among prosobranches. The egg fertilization process begins with the release of egg and gametes into the surrounding water. Fertilization takes place in the aquatic surroundings rather than the female’s body. After fertilization, eggs hatch into larvae called trochophores, which swim freely about the area. Mollusks grow quickly in this stage: days after hatching, they develop ciliary girdles and heavily ciliated lobes. This development signals the start of the veliger phase; in this life stage, mollusks are still classified as larvae, but have much more developed systems. As veligers, mollusks undergo physical transformations called torsion, during which time their bodies rotate 180 degrees. Torsion twists their bodies to bring the posterior parts into an anterior position behind the head, which gives mollusks the body shape they will then have for the rest of their adult lives. The end of torsion brings the emergence into adulthood, and life spans of grown mollusks vary depending on the species.