Is salmon smoltification an example of vertebrate metamorphosis? Lessons learnt from work on flatfish larval development

The terms metamorphosis and smoltification both describe developmental processes. However, the question on what specific criteria define these terms continues to engage scientists. At the same time, various views have been expressed on whether or not smoltification of anadromous salmonids should be regarded as an example of vertebrate metamorphosis. This short overview tries to summarize some of these discussions and starts by determining if smoltification meets any of the criteria used to define metamorphosis. In particular, it broadly compares the process of flatfish metamorphosis with that of salmonid smoltification from a morphological, endocrine, molecular and behavioral perspective. Tools and approaches developed and used in metamorphosis research which could be useful in continued work on smoltification are highlighted.