Probably the most widely accepted theory explaining the evolution of larval stages is the need for "dispersal". Sessile organisms such as barnacles and tunicates, and sea-floor groups like mussels and crabs, need some way to move their young into new territory, since they cannot move long distances as adults. Many species have relatively long pelagic larval stages (how long a larva is in the water column). During this time, larvae feed and grow, and many species move through several stages of development. For example, most barnacles moult through six "nauplius" larva stages before moulting to a "cipris", when they look to settle. The larvae eat different food from the adults, and disperse.