Is salmon smoltification an example of vertebrate metamorphosis? Lessons learnt from work on flatfish larval development Björn Thrandur Björnsson a, ⁎, Ingibjörg Eir Einarsdottir a , Deborah Power b a Fish Endocrinology Laboratory, Department of Zoology/Zoophysiology, University of Gothenburg, Box 463, SE40530 Gothenburg, Sweden b Centro de Ciências do Mar and Faculdade de Engenharia de Recursos Naturais, Universidade do Algarve, Campus de Gambelas, P8005-139 Faro, Portugal abstract article info Article history: Received 23 October 2009 Received in revised form 24 February 2011 Accepted 3 March 2011 Available online 9 March 2011 Keywords: Metamorphosis Smoltification Parr–smolt transformation Development Flatfish Salmonids 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. © 2011 Elsevier B.V. All rights reserved. 1. Introduction The title of a recent overview paper asks the question “What is metamorphosis?” (Bishop et al., 2006). This echoes a similar fundamental question frequently raised in discussions at the International Workshops on Salmon Smoltification and similar forums—“What is salmon smoltification?” While the questions are seemingly straightforward—we all “know” what we mean when we use these terms, their simple definitions slip away when we start to consider borderline examples. Do roundfish species undergo meta- morphosis? Do Salvelinus species smoltify? The purpose of this short review is to examine the related and intriguing question if salmon smoltification is an example of vertebrate metamorphosis? A more descriptive synonym for smoltification is parr–smolt transformation, and linguistically, the words metamorphosis and transformation are close synonyms. The question is therefore, in the case of smoltifica- tion, if they are biologically synonymous. Such exercise is not just about terminology and semantics. Instead it highlights the issue of whether (and what) molecular, physiological, morphological, behavioral and ecological research and understanding of salmon smoltification as a developmental process can gain from insights and comparisons with metamorphic processes in other fish species such as lamprey, eel and flatfishes. Defining the terms metamorphosis and smoltification are ongoing discussion topics. However, from a physiological perspective, the most important question is if researchers working in the different fields can gain information and/or research approaches from each other. 2. Discussion 2.1. What is metamorphosis? Metamorphosis is a widely used term in various fields of biology, but there is no clear consensus on its definition (see Bishop et al., 2006). Its literal translation from the Greek metamorphoum is “changing form” or “transform”, with meta meaning change and morphe meaning form. Thus, in its broadest sense, the term metamorphosis describes biological “transformation” and is synony- mous with morphogenesis. It can thus be argued that it can be used broadly to describe a life stage transition between a vegetative and sexually reproductive stage of any multicellular fungi, plant or animal (Bishop et al., 2006). The term metamorphosis is, however, most commonly used to refer to major developmental changes in holometabolous insects, marine crustaceans, amphibians, and certain fish species, most notably flatfish, eels, and lampreys (Youson, 1988). In line with the literal translation, a distinguishing feature of the developmental changes in these groups is the degree of morphological change, with larval and juvenile forms so distinct that they've often been thought to be different species, such as the case with the Leptocephalus larvae of the eel. In the context of salmon smoltification, anecdotally it can be noted that a similar confusion existed in early biology regarding different Aquaculture 362–363 (2012) 264–272 ⁎ Corresponding author. Tel.: + 46 31 7863691; fax: + 46 31 7863807. E-mail addresses: thrandur.bjornsson@zool.gu.se (B.T. Björnsson), ingibjorg.einarsdottir@zool.gu.se (I.E. Einarsdottir), dpower@ualg.pt (D. Power). 0044-8486/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.aquaculture.2011.03.002 Contents lists available at ScienceDirect Aquaculture journal homepage: www.elsevier.com/locate/aqua-online