Metamorphosis is broadly defined as a more or less radical morphological change between 2 multicellular life stages within an organism's life phase, often marking the transition from pre-reproductive to reproductive stages. It involves structural reorganization and major physiological changes, generally under the control of endogenous and exogenous factors and often resulting in changes in habitat use. This concept has been applied to the crustose-to-erect thallus transition of some red algae and the present study evaluates the validity of such hypotheses. Available literature suggests that the crustose-to-erect thallus transition involves significant morphological and habitat changes, separating pre-reproductive and reproductive stages. The onset of the morphological changes (for example axis differentiation) appears regulated by endogenous signals (growth factors) and growth is modified by environmental factors. The algae do not exhibit structural reorganization, however, probably due to their simple morphological structure and the lack of several cell and tissue mechanisms involving cell motility. The presence of cell walls in the algae impairs cell motility and maintains the cell in a fixed position within the plant. These are important differences restricting the extension of the definition of metamorphosis to the crust-to-erect thallus transition. The above restrictions also seem to apply to other macroalgae, fungi, and terrestrial plants.