Pycnopodia helianthoides is strictly dioecious (two sexes) and the sexes cannot be detected externally. The gametes are shed externally through the gonopores, known as broadcast fertilization, in the spring, with May-June being the peak. Some subtidal  spawning has been observed in December and January. Spawning is thought to be induced by the presence of the opposite sex cells in the water. After fertilization the microscopic eggs (120 micrometers in diameter), are destined for larval development in the plankton. By seven weeks the swimming and feeding bipinnaria larvae transform into brachiolaria larvae. Larvae feed on single celled plants, diatoms and protozoans. At nine to ten weeks after the larval period,  larvae metamorphosis into the miniature juvenile Sunflower Star and settle to the substrate, feeding initially on the thin layer of single-celled plants on the bottom. Initially, the juveniles have 5 rays.    P. helianthoides can grow up to a meter in diameter and are thought to live for decades.