How Do Starfish Mate?

Starfish reproduce sexually by spawning. Spawning means that the sex cells are released into the water. Starfish gather in groups to reproduce, which increases the likelihood the sperm and eggs will find each other. Starfish have sexual organs, or gonads, in each arm. During a breeding season the males' gonads fill with sperm and the females' gonads fill with eggs. When starfish spawn, the males release sperm and the females release eggs in great numbers. Female starfish may release millions of tiny eggs into the water during a spawning session.

When the eggs become fertilized they develop into zygotes. Some species of starfish brood their eggs after they're fertilized. They may sit over their eggs, or form a brooding basket in which the eggs are sheltered. The eggs may be held on the starfish's central disk or in sacs between the arms. Once the eggs hatch they are released into the open ocean to drift with the current and search for food.

In the larval stage the starfish is a miniscule, jelly-like blob that floats around feeding on microscopic flora and fauna. These larva are too small to be seen with the naked eye. The starfish larva metamorphosizes through several stages until it develops into a juvenile. The larva have bilateral symmetry, but in their adult form they'll have radial symmetry. Bilateral symmetry means the left and right halves resemble each other, where as in radially symmetrical animals the limbs radiate from a single point in the center of the body.

In the last stage of larval development the starfish abandons its free-floating planktonic lifestyle for a benthic (surface-dwelling) existence. The larva anchors itself to a surface and transforms into the juvenile stage. A juvenile starfish looks like a small version of an adult. Juvenile starfish spend most of their time in seclusion, hiding in crevices and under rocks to protect themselves from predators. After doing some growing they explore the ocean floors and reefs eating algae and other particulate matter. At about two years old the starfish is grown enough to be considered an adult starfish and can begin the reproductive stage of its life.

If a starfish's arm is removed along with part of the central disk (the center of the body from which the arms radiate) a whole new sea star can form from that detached piece. The amputated starfish can also regenerate a new arm to replace the lost limb. Because two starfish have been formed from one, this is considered asexual reproduction. If you see a starfish that has some arms significantly longer than the rest, this is a sign the animal has regenerated some of its limbs.