A. amphitrite is a hermaphrodite and individuals have both male and female reproductive organs. Sperm is introduced into the mantle cavities of adjacent barnacles through an elongated penis and the fertilized eggs are brooded within the mantle cavity for several months. Free-swimming larvae called nauplii are then released into the water column where they become part of the zooplankton. In temperate areas, spawning is mainly in the spring and summer but in warmer waters it may continue throughout the year. Individuals can release up to ten thousand eggs per brood and there may be many broods per year. After several moults the last larval stage, the cyprid larva, settles and crawls around looking for a suitable place to cement itself.