, 143 mm, right form specimen, ocular side; syntype BMNH 1853.11.12.133, 139 mm, left form specimen, ocular side; syntype BMNH 1853.11.12.134, 152 mm, right form specimen

, Hille; KS.3021, 279 mm SL, right form specimen, KS.3020, 242.9 mm SL, right form specimen, 2016.

, Wasström; KS.3049, 228.2 mm SL, right form specimen, western Gulf of Finland, Tvärminne, RävskärBrännskär, 59 ? 51 ? N 23 ? 16 ? E, KS.3022, 266 mm SL, right form specimen, south-western, p.22, 2016.

, Nordahl; KS.3076, 236.9 mm SL, right form specimen, south-western Baltic, Rostock, 54 ? 11 ? N 12 ? 01 ? E, coll. O, 2017.

, Body fully plated (A, phenotype 1): P. flesus, KS.3049, 228.2 mm SL, western Gulf of Finland, Tvärminne, Rävskär-Brännskär, 59 ? 51 ? N 23 ? 16 ? E, FIGURE 2 | UV photographs showing variation in bony scales, representing phenotypes 1-4 sensu Voronina, 1999.

.. P. Momigliano and H. Jokinen, Jokinen; and few plates present along the lateral line and spine (D, phenotype 4): P. solemdali sp. nov., paratype, KS.3041, 230.8 mm SL, western Gulf of Finland, Tvärminne, Furuskär (island), northern side, 59 ? 50 ? N 23 ? 15 ? E, coll. P. Momigliano and H, issue.3, 2017.

?. , coll. U. Wasström, 2017.

, 8 mm SD, right form specimen, western Gulf of Finland, Inkoo, 60 ? 02 ? N 24 ? 05 ? E, 30th, coll. U, vol.3027, 2017.

?. , coll. U. Wasström, 2017.

, Wasström; KS.3030, 186.6 mm SL, right form specimen, western Gulf of Finland, Inkoo, 60 ? 02 ? N 24 ? 05 ? E, 30th, KS.3029, 185.5 mm SL, right form specimen, western Gulf of Finland, Inkoo, 60 ? 02 ? N 24 ? 05 ? E, 2017.

, Wasström; KS.3035, 279.8 mm SL, right form specimen, western Gulf of Finland, Inkoo, 60 ? 02 ? N 24 ? 05 ? E, KS.3034, 204.9 mm SL, right form specimen, western Gulf of Finland, Inkoo, 60 ? 02 ? N 24 ? 05 ? E, p.28, 2017.

, Momigliano and H. Jokinen; KS.3042, no SL available, right form specimen, western Gulf of Finland, Tvärminne, 59 ? 50 ? N 23 ? 16 ? E, SL available, right form specimen, western Gulf of Finland, Tvärminne, 59 ? 50 ? N 23 ? 16 ? E, p.22, 2017.

, P. solemdali is characterized by thymine (T) at position 237 (allele frequency: 92% in P. solemdali sp. nov. vs. 2% in P. flesus), while P. flesus is characterized by cytosine (C; allele frequency: 98% in P. flesus vs. 8% in P. solemdali sp. nov.). Other loci identified by Momigliano et al. (2017a) as under divergent selection in P. flesus and P. solemdali sp, 11% in P. flesus), while P. flesus is characterized by higher frequencies of A (allele frequency: 89% in P. flesus vs. 13% in P. solemdali sp. nov.). At locus, vol.3556, p.3599, 2017.

K. Aarnio, E. Bonsdorff, R. , and N. , Food and feeding habits of juvenile flounder Platichthys flesus (L.), abd turbot Scophthalmus maximus L. in the Åland archipelago, northern Baltic Sea, J. Sea Res, vol.36, pp.311-320, 1996.

D. Akkaynak, L. A. Siemann, A. Barbosa, and L. M. Mäthger, Changeable camouflage: how well can flounder resemble the colour and spatial scale of substrates in their natural habitats?, vol.4, p.160824, 2017.

L. Andersson, The driving force: species concepts and ecology, Taxon, vol.39, pp.375-382, 1990.

E. Aro, A review of fish migration patterns in the Baltic, Rap. Proc. Verb. Ré. Cons. Int. Explor. Mer, vol.190, pp.72-96, 1989.

E. Aro, Fish migration studies in the Baltic Sea-a historical review. ICES Marine Science Symposia, vol.215, pp.361-370, 2002.

E. Aro and V. Sjöblom, The Migration of Flounder in the Northern Baltic Sea. Helsinki: International Council for the Exploration of the Sea, 1983.

P. Artedi, Ichtyologia sive opera omnia de piscibus scilicet: Bibliotheca ichthyologica. Philosophia ichthyologica. Genera piscium. Synonymia specierum. Descriptiones specierum. Omnia in hoc genere perfectora quam antea ulla posthuma vindicavit, recognovit, 1738.

O. Bagge, Demersal fishes, pp.311-333, 1981.

R. Barrett, Adaptive evolution of lateral plates in three-spined stickleback Gasterosteus aculeatus: a case study in functional analysis of natural variation, J. Fish Biol, vol.77, pp.311-328, 2010.

G. A. Begg, K. D. Friedland, P. , and J. B. , Stock identification and its role in stock assessment and fisheries management: an overview, Fish. Res, vol.43, pp.1-8, 1999.

J. Bendtsen, J. Söderkvist, K. Dahl, J. L. Hansen, and J. Reker, Model Simulations of Blue Corridors in the Baltic Sea, BALANCE interim Report, vol.9, p.26, 2007.

S. Björck, A review of the history of the Baltic Sea, ka BP. Q. Int, vol.27, 1995.

E. Bonsdorff and A. Norkko, Flounder (Platichthys flesus) spawning in Finnish archipelago waters, Memoranda Soc. Fauna Flora Fennica, vol.70, pp.30-31, 1994.

J. P. Borg, M. Westerbom, and H. Lehtonen, Sex-specific distribution and diet of Platichthys flesus at the end of spawning in the northern Baltic Sea, J. Fish Biol, vol.84, pp.937-951, 2014.

J. Carstensen, J. H. Andersen, B. G. Gustafsson, and D. J. Conley, Deoxygenation of the Baltic Sea during the last century, Proc. Natl. Acad. Sci. U.S.A, vol.111, pp.5628-5633, 2014.

P. F. Colosimo, K. E. Hosemann, S. Balabhadra, G. Villarreal, M. Dickson et al., Widespread parallel evolution in sticklebacks by repeated fixation of ectodysplasin alleles, Science, vol.307, pp.1928-1933, 2005.

J. Cracraft, Species concepts and speciation analysis, Curr. Ornithol, vol.1, pp.159-187, 1983.

K. De-queiroz, Species concepts and species delimitation, Syst. Biol, vol.56, pp.879-886, 2007.

T. Dobzhansky and T. G. Dobzhansky, Genetics of the Evolutionary Process, 1970.

T. Drevs, V. Kadakas, T. Lang, M. , and S. , Geographical variation in the age/length relationship in Baltic flounder (Platichthys flesus), ICES J. Mar. Sci, vol.56, pp.134-137, 1999.

G. Duncker, Der Elbbutt, eine Varietät der Flunder. (Pleuronectes flesus L. var. leiurus), Schrif. Nat. Vereins Schleswig Holstein, vol.9, pp.275-291, 1892.

J. Erlandsson, Ö. Östman, A. Florin, and Z. Pekcan-hekim, Spatial structure of body size of European flounder (Platichthys flesus L.) in the Baltic Sea, Fish. Res, vol.189, pp.1-9, 2017.

S. Evseenko, An annotated catalogue of pleuronectiform fishes (order Pleuronectiformes) of the seas of Russia and adjacent countries, J. Ichthyol, vol.43, pp.57-74, 2003.

B. Fernholm, W. , and A. , Linnaean fish specimens in the Swedish Museum of Natural History, Zool. J. Linn. Soc, vol.78, pp.199-286, 1983.

A. B. Florin, Flatfishes in the Baltic Sea-a Review of Biology and Fishery With a Focus on Swedish Conditions, 2005.

A. Florin and J. Höglund, Population structure of flounder (Platichthys flesus) in the Baltic Sea: differences among demersal and pelagic spawners, Heredity, vol.101, pp.27-38, 2008.

P. A. Gagnaire, S. A. Pavey, E. Normandeau, and L. Bernatchez, The genetic architecture of reproductive isolation during speciation-with-geneflow in lake whitefish species pairs assessed by rad sequencing, Evolution, vol.67, pp.2483-2497, 2013.

R. Galleguillos, W. , and R. , Genetic and morphological divergence between populations of the flatfish Platichthys flesus (L.)(Pleuronectidae), Biol. J. Linnean Soc, vol.17, pp.395-408, 1982.

R. N. Gibson, R. D. Nash, A. J. Geffen, . Van-der, and H. W. Veer, Flatfishes: Biology and Exploitation, 2015.

L. T. Gronovius, Museum ichthyologicum, sistens piscium indigenorum e quorundam exoticorum, qui in museuo Laurentii Theodori Gronovii, adservantur, descriptiones, 1754.

L. Batavorum, Accedunt Nonnullorum Exoticorum Piscium Icones, Aeri Incisae, vol.1

S. Guindon, J. Dufayard, V. Lefort, M. Anisimova, W. Hordijk et al., New algorithms and methods to estimate maximum-likelihood phylogenies: assessing the performance of PhyML 3.0, Syst. Biol, vol.59, pp.307-321, 2010.
URL : https://hal.archives-ouvertes.fr/lirmm-00511784

B. Guo, J. Defaveri, G. Sotelo, A. Nair, Ä. et al., Population genomic evidence for adaptive differentiation in Baltic Sea three-spined sticklebacks, BMC Biol, vol.13, p.8, 2015.

B. G. Gustafsson and P. Westman, On the causes for salinity variations in the Baltic Sea during the last 8500 years, Paleoceanography, vol.17, pp.12-13, 2002.

J. Hemmer-hansen, E. E. Nielsen, P. Grønkjaer, L. , and V. , Evolutionary mechanisms shaping the genetic population structure of marine fishes; lessons from the European flounder (Platichthys flesus L.), Mol. Ecol, vol.16, pp.3104-3118, 2007.

H. Hinrichsen, C. Petereit, A. Nissling, I. Wallin, D. Ustups et al., Survival and dispersal variability of pelagic eggs and yolk-sac larvae of central and eastern baltic flounder (Platichthys flesus): application of biophysical models, ICES J. Mar. Sci, vol.74, pp.41-55, 2016.

A. G. Hudson, B. Lundsgaard-hansen, K. Lucek, P. Vonlanthen, and O. Seehausen, Managing cryptic biodiversity: fine-scale intralacustrine speciation along a benthic gradient in Alpine whitefish (Coregonus spp.), Evol. Appl, vol.10, pp.251-266, 2017.

, ICES WGBFAS REPORT 2016: Report of the Baltic Fisheries Assessment Working Group (WGBFAS). ICES HQ, 2017.

H. Jokinen, H. Wennhage, A. Lappalainen, K. Ådjers, M. Rask et al., Decline of flounder (Platichthys flesus (L.)) at the margin of the species' distribution range, J. Sea Res, vol.105, pp.1-9, 2015.

H. Jokinen, H. Wennhage, V. Ollus, E. Aro, and A. Norkko, Juvenile flatfish in the northern Baltic Sea-long-term decline and potential links to habitat characteristics, J. Sea Res, vol.107, pp.67-75, 2016.

T. Jombart, adegenet: a R package for the multivariate analysis of genetic markers, Bioinformatics, vol.24, pp.1403-1405, 2008.
URL : https://hal.archives-ouvertes.fr/hal-00428105

T. Jombart, S. Devillard, and F. Balloux, Discriminant analysis of principal components: a new method for the analysis of genetically structured populations, BMC Genet, vol.11, p.94, 2010.
URL : https://hal.archives-ouvertes.fr/hal-00539267

A. L. Jonsson, Morfometrisk och Konditionsmässig Variation hos Platichthys Flesus i Östersjön, 2014.

K. I. Jönsson, Capital and income breeding as alternative tactics of resource use in reproduction, Oikos, vol.78, pp.57-66, 1997.

M. Kottelat, European freshwater fishes. An heuristic checklist of the freshwater fishes of europe (exclusive of former USSR), with an introduction for nonrsystematists and comments on nomenclature and conservation, Biologia Bratislava, vol.52, pp.1-271, 1997.

C. Linnaeus, Fauna Svecica Sistens Animalia Sveciae Regni. Lugduni Batavorum: Wishoff, 1746.

C. Linnaeus, Skånska Resa på Höga Öfwerhetens Befallning Förrättad år 1749. Salviani, Stockholm. Linnaeus, C. (1758). Systema Naturae per Regna Tria Naturae, Secundum Classes, Ordines, Genera, Species, Cum Characteribus, Differentiis, Synonymis, Locis. Tomus I. Edn decima, reformata. Salvius: Holmiae, 1751.

S. Lønning and P. Solemdal, The Relation Between Thickness of Chorion and Specific Gravity of Eggs From Norwegian and Baltic Flatfish Populations, FiskDir. Skr. Ser.HavUnders, vol.16, pp.77-88, 1974.

B. R. Mackenzie, H. Gislason, C. Möllmann, and F. W. Köster, Impact of 21st century climate change on the Baltic Sea fish community and fisheries, Glob. Chang. Biol, vol.13, pp.294-299, 1995.

R. L. Mayden, On biological species, species concepts and individuation in the natural world, Fish Fish, vol.3, pp.171-196, 2002.

E. Mayr, Systematics and the Origin of Species, From the Viewpoint of a Zoologist, 1942.

J. Merilä, Nine-spined stickleback (Pungitius pungitius): an emerging model for evolutionary biology research, Ann. N. Y. Acad. Sci, vol.1289, pp.18-35, 2013.

J. Merilä and A. P. Hendry, Climate change, adaptation, and phenotypic plasticity: the problem and the evidence, Evol. Appl, vol.7, pp.1-14, 2014.

W. W. Mielck, K. , and C. , Fischbrut-und Plankton-Untersuchungen auf dem Reichsforschungsdampfer'Poseidon'in der, 1931.

N. Mikelsaar, Über die Anwendung der Methode der ausgeglischenen Skalen bei der Untersuchung der artinneren Gruppierungen der Flunder, Eesti NSV Teaduste Akadeemia Zoologia ja Botaanika Instituut), pp.287-314, 1958.

N. Mikelsaar, Eesti NSV Kalad: Käsiraamat-määraja, 1984.

P. Momigliano, H. Jokinen, A. Fraimout, A. Florin, A. Norkko et al., Extraordinarily rapid speciation in a marine fish, Proc. Natl Acad. Sci. U.S.A, vol.114, pp.6074-6079, 2017.

P. Momigliano, H. Jokinen, A. Fraimout, A. Florin, A. Norkko et al., Data from: extraordinarily rapid speciation in a Marine fish, Dryad Data Repository, 2017.

J. E. Morrow, The Freshwater Fishes of Alaska, 1980.

T. Neumann, H. Radtke, and T. Seifert, On the importance of Major Baltic Inflows for oxygenation of the central Baltic Sea, J. Geophys. Res. Oceans, vol.122, pp.1090-1101, 2017.

A. Nissling and G. Dahlman, Fecundity of flounder, Pleuronectes flesus, in the Baltic Sea-reproductive strategies in two sympatric populations, J. Sea Res, vol.64, pp.190-198, 2010.

A. Nissling, U. Johansson, J. , and M. , Effects of salinity and temperature conditions on the reproductive success of turbot (Scophthalmus maximus) in the Baltic Sea, Fish. Res, vol.80, pp.230-238, 2006.

A. Nissling, H. Kryvi, and L. Vallin, Variation in egg buoyancy of Baltic cod Gadus morhua and its implications for egg survival in prevailing conditions in the Baltic Sea, Mar. Ecol. Prog. Ser, vol.110, pp.67-74, 1994.

A. Nissling, S. Nyberg, P. , and C. , Egg buoyancy of flounder, Platichthys flesus, in the Baltic Sea-adaptation to salinity and implications for egg survival, Fish. Res, vol.191, pp.179-189, 2017.

A. Nissling, A. Thorsen, and F. F. Silva, Fecundity regulation in relation to habitat utilisation of two sympatric flounder (Platichtys flesus) populations in the brackish water Baltic Sea, J. Sea Res, vol.95, pp.188-195, 2015.

A. Nissling and L. Westin, Salinity requirements for successful spawning of Baltic and Belt Sea cod and the potential for cod stock interactions in the Baltic Sea, Mar. Ecol. Prog. Ser, vol.152, pp.261-271, 1997.

A. Nissling, L. Westin, and O. Hjerne, Reproductive success in relation to salinity for three flatfish species, dab (Limanda limanda), plaice (Pleuronectes platessa), and flounder (Pleuronectes flesus), in the brackish water Baltic Sea, ICES J. Mar. Sci, vol.59, pp.93-108, 2002.

P. Nosil, Ecological Speciation, 2012.

E. Ojaveer and T. Drevs, Flounder, Platichthys flesus trachurus (Duncker), " in Fishes of Estonia, pp.362-370, 2003.

H. Ojaveer, A. Jaanus, B. R. Mackenzie, G. Martin, S. Olenin et al., Status of biodiversity in the Baltic Sea, PLoS ONE, vol.5, 2010.

A. Orio, U. Bergström, M. Casini, M. Erlandsson, R. Eschbaum et al., Characterizing and predicting the distribution of Baltic Sea flounder (Platichthys flesus ) during the spawning season, J. Sea Res, vol.126, pp.46-55, 2017.

C. S. Rafinesque, Analyse de la Nature, 1815.

V. S. Ramachandran, C. W. Tyler, R. L. Gregory, D. Rogers-ramachandran, S. Duensing et al., Rapid adaptive camouflage in tropical flounders, Nature, vol.379, pp.815-818, 1996.

H. Reiss, G. Hoarau, M. Dickey-collas, and W. J. Wolff, Genetic population structure of marine fish: mismatch between biological and fisheries management units, R: A Language and Environment for Statistical Computing, vol.10, pp.361-395, 2009.

J. Sandman, Kurzer bericht über in Finnland ausgeführte untersuchungen über den Flunder, den Steinbutt und den Kabeljau, Rapp Pv Réun Cons int Explor Mer, vol.5, pp.37-44, 1906.

M. R. Servedio, G. S. Van-doorn, M. Kopp, A. M. Frame, and P. Nosil, Magic traits in speciation:'magic'but not rare?, Trends Ecol. Evol, vol.26, pp.389-397, 2011.

P. Solemdal, The effect of salinity on buoyancy, size and development of flounder eggs, Sarsia, vol.29, pp.431-442, 1967.

P. Solemdal, The Reproductive Adaption of Marine Teleosts to Water of Low Salinity, p.30, 1970.

P. Solemdal, Prespawning flounders transferred to different salinities and the effects on their eggs, Vie Milieu Suppl, vol.22, pp.409-423, 1971.

P. Solemdal, Transfer of Baltic flatfish to a marine environment and the long termeffects on reproduction, Oikos, vol.22, pp.268-276, 1973.

M. Spinner, M. Kortmann, C. Traini, and S. N. Gorb, Key role of scale morphology in flatfishes (Pleuronectiformes) in the ability to keep sand, Sci. Rep, vol.6, p.26308, 2016.

L. Stålberg, Morfologisk Variation Hos Skrubbskädda (Platichthys flesus): Finns det en Koppling till Ekologi? Uppsala: Bachelor Degree, 2015.

T. Sterner, Unobserved diversity, depletion and irreversibility The importance of subpopulations for management of cod stocks, Ecol. Econ, vol.61, pp.566-574, 2007.

R. Summers, Life cycle and population ecology of the flounder Platichthys flesus (L.) in the Ythan estuary, J. Nat. Hist, vol.13, pp.703-723, 1979.

F. B. Sumner, The adjustment of flatfishes to various backgrounds: a study of adaptive color change, J. Exp. Zool, vol.10, pp.409-506, 1911.

M. M. Tin, F. E. Rheindt, E. Cros, and A. S. Mikheyev, Degenerate adaptor sequences for detecting PCR duplicates in reduced representation sequencing data improve genotype calling accuracy, Mol. Ecol. Resour, vol.15, pp.329-336, 2015.
DOI : 10.1111/1755-0998.12314

F. Utter, R. , and N. , Genetic markers and mixed stock fisheries, Fisheries, vol.18, pp.11-21, 1993.
DOI : 10.1577/1548-8446(1993)018<0011:gmamsf>2.0.co;2

L. Vallin and A. Nissling, Maternal effects on egg size and egg buoyancy of Baltic cod, Gadus morhua: implications for stock structure effects on recruitment, Fish. Res, vol.49, pp.21-37, 2000.

L. Van-valen, Ecological species, multispecies, and oaks, Taxon, vol.25, pp.233-239, 1976.

E. Voronina, Morphology and systematics of river flounders of the genus Platichthys, J. Ichthyol, vol.39, pp.588-599, 1999.

I. Vuorinen, J. Hänninen, M. Rajasilta, P. Laine, J. Eklund et al., Scenario simulations of future salinity and ecological consequences in the Baltic Sea and adjacent North Sea areasimplications for environmental monitoring, Ecol. Indic, vol.50, pp.196-205, 2015.

I. Wallin, Opportunities for Hybridization Between Two Sympatric Flounder (Platichthys flesus) Ecotypes in the Baltic Sea, 2016.

S. Wang, E. Meyer, J. K. Mckay, and M. V. Matz, 2b-RAD: a simple and flexible method for genome-wide genotyping, Nat. Methods, vol.9, pp.808-810, 2012.

A. Wheeler, The Linnaean fish collection in the Zoological Museum of the University of Uppsala, Zool. J. Linn. Soc, vol.103, pp.145-195, 1991.

A. C. Wheeler, The Gronovius Fish Collection: A Catalogue and Historical Account, British Museum of Natural History, pp.187-249, 1958.

P. J. Whitehead, M. Bauchot, J. Hureau, J. Nielsen, T. et al., United Nations Educational Scientific and Cultural Organization, vol.3, 1986.

E. O. Wiley and R. L. Mayden, The evolutionary species concept, Pecies Concepts And Phylogenetic Theory-A Debate, pp.70-89, 2000.

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