Corals are sedentery animals, so how do they reproduce? One way is sexually through spawning, when the corals release eggs and sperm into the water (often at the same time due to some sort of trigger). External sexual reproduction occurs when colonies of coral release huge numbers of eggs and sperm that are often glued into bundles (one bundle per polyp) that float towards the surface. In this picture a purple hard coral (Acropora cerealis) releases such a bundle of pink eggs glued together with sperm.