Buccinum undatum deposits its eggs in clusters of capsules cemented together, often built communally by several females into a mass the size of a fist. Each capsule holds many eggs, but most are not embryos in the usual sense: the majority serve as nurse eggs, consumed by the developing few. Typically only a small number of young emerge from each capsule, crawling out as miniature whelks with no free-swimming larval stage.
That direct development matters for the fishery. Without a planktonic larva to disperse, populations are poorly connected, local depletion is not readily replenished from elsewhere, and stocks recover slowly.
Empty masses detach and drift ashore, where they dry into light, fibrous lumps. Their use aboard ship for washing gave rise to the name sea wash balls. The commonly repeated identification of these lumps as sponges or as some kind of egg foam is wrong; each is a bundle of spent capsules, and holding one to the light shows the exit holes where the young whelks left.