Many marine organisms, both invertebrates and fishes, begin life as minute planktonic larvae. These larval stages are responsible for dispersal and ultimately for habitat selection. They must settle out of the water at a location suitable for growth of the juvenile to the adult. For many marine invertebrates the transformation from planktonic larva to adult is very rapid. Sea urchins can complete this metamorphosis in about one hour.
Crustaceans have complicated life cycles sometimes involving four separate metamorphoses.
Parasitic organisms are also characterized by complex life cycles with striking metamorphic changes. For parasites the different stages may be specialized for dispersal, host-seeking, attachment and penetration, feeding, and reproduction.