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Published: 2021-12-17

Just metamorphosed amphibians: imagos or metamorphs?

Institut de Systématique, Évolution, Biodiversité, ISYEB - UMR 7205 - CNRS, MNHN, UPMC, EPHE, Muséum national d’Histoire naturelle, Sorbonne Université, 57 rue Cuvier, CP 30, F-75005, Paris, France
Development growth form metamorphosis larva imago juvenile adult senex metamorph metamorphont heterochrony aneuchrony euchrony neoteny neosystelly argiotrophy ergotrophy


The term metamorph for just metamorphosed amphibians appeared surreptitiously in the batrachological literature. It is shown here that this term is linguistically unjustified and conceptually confusing, as it has never been associated with a clear, formal definition stating in particular when does this developmental stage start and end. The use of the term imago for an individual resulting from the last metamorphosis following a larval stage, which exists for insects since 1767 and for amphibians since 1808, is much preferable. For amphibians, the formal definition of this term was given in 1978 as an animal having completed its metamorphosis, before having substantially grown and until the first major ecological event in its life cycle (such as migration, hibernation or aestivation). In amphibians, this stage is followed by a stage juvenile and a stage subadult until the stage adult is reached, which is defined by sexual maturity and ability to reproduce. Given the diversity of developmental modes in the animal kingdom, it would be vain to try to homogenise the terminology of all detailed developmental stages across all groups. However, the possibility to homogenise the use of the term imago throughout zoology for specimens resulting from the ‘last metamorphosis’ (i.e., drastic change not only of form but also in some anatomical structures), whether followed by growth and minor transformations or not, and whether associated with sexual maturity or not, would certainly be worth considering. This would allow to have a few general descriptive terms to designate the main similar, but not homologous, ‘landmarks’ observed in the development of many animals (egg, larva, imago and adult), just like we have a general term (metamorphosis) for ‘similar’ phenomena which are not homologous. This would not prevent specialists of the various zoological groups to have specific terms for more precisely defined ‘stages’ which are proper to these groups.


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