The majority of starfish species release eggs and sperm into the water. This allows fertilization to take place in the external environment. The female starfish has the ability to release millions of eggs in a span of two hours. These eggs have a diameter ranging from 0.16 mm to 0.19 mm.
The moment fertilization takes place, the embryo forms. This embryo is a hollow ball and forms the blastula. The cells of the blastula have cilia outside to facilitate swimming. After one day, the blastula develops a deep groove. This is the gastrula stage of the embryo.
Once the gastrula stage of development occurs, differentiation begins, leading to the bipinnaria larval stage. The larva has ciliated bands along the edges, giving it a beautiful appearance. This stage continues for few weeks and then the larva enters into the next stage of development.
The larva has a gut and cilia inside the body. This allows the larva to inhale and carry the food particles to the gut for digestion. The larva feeds on diatoms and plankton in this stage of development.
The brachiolaria is the second and last larval stage of the starfish. After this a rudimentary immature starfish develops, and the organs of the immature starfish begin forming afresh. As the larva undergoes metamorphosis, it latches on to the seabed using its three arms that are star shaped with broad ends. The larva at this stage has a bilateral symmetry.
The entire process of development from egg to the formation of an immature starfish occurs in approximately two months, but is dependent to a certain extent on the temperature of the water. During the larval stage, the larvae are swept by the ocean currents and they end up traveling for many miles. The young starfish will be ready for reproduction in around a year's time.