Metamorphosis in Crustaceans
Many crustaceans undergo considerable morphological and ecological changes throughout ontogeny. Especially drastic and rapid cases are generally addressed as metamorphosis, which cannot be easily differentiated from nonmetamorphic development; a comparative view is necessary. Evolutionary changes lead to a more metamorphic development either by changing the speed of the developmental process or the morphological difference between earlier and later stages. Five cases of evolutionary changes are differentiated: (1) Skipping stages: An ancestrally gradual developmental pattern becomes more metamorphic as the morphological changes of several molts occur in a single molt; the intermediate stage is skipped. (2) Peramorphosis: A developmental pattern becomes more metamorphic by increasing the morphological difference between early and late stages by “adding” new morphologies to the later part of individual development. (3) Delay and acceleration, single step: A single larval stage becomes delayed in development, more resembling the earlier stage, but differing more strongly from the next stage; hence, this later molt becomes more metamorphic. (4) Delay and acceleration, globally: Several larval stages are delayed in development and hence increase the morphological difference to the later larval stages; this stronger difference is bridged by a single, more metamorphic molt. (5) Caenogenesis: new structures evolve in earlier stages, increasing the difference to later stages; these structures become reduced usually in a single molt, making it more metamorphic. For all cases, examples are presented. Furthermore, terminological issues are discussed, as well as costs and benefits of metamorphic development, followed by a short comparison to insects.