Most flounders sampled in Lake Pulmanki in northern Finland had spent their entire lives in freshwater. Study provide the first evidence of fully freshwater confined life of species.
European flounders ( Platichthys flesus ) are marine fish that often move into river mouths to feed. They are considered to return to marine or brackish water to reproduce. An international research team has shown that the flounders of Lake Pulmanki are an exception: they can complete their entire life cycle in freshwater.
Lake Pulmanki is a former Arctic Ocean fjord that became a freshwater lake after the last ice age and is currently connected to sea via 60 km long river section.
When flounders were first documented in Lake Pulmanki in the 1970s, researchers could not explain their presence. The fish could have been migrating to the lake to feed and returning to the Barents Sea to spawn. Alternatively, flounders could have adapted to a full freshwater life cycle.
The team approached the question in two ways. Using otolith microchemistry, the researchers analysed otoliths, small bony structures in a fish’s inner ear that help it sense balance and movement. As otoliths grow, their chemical composition preserve a record of the waters a fish has occupied.
Using whole-genome resequencing, the team compared genetic variation among flounders from Lake Pulmanki and a few other marine locations. The aim was to find out whether differences in habitat use were associated with particular regions of the genome.
The otoliths records show that most flounders sampled in Lake Pulmanki had spent their entire life in freshwater. This is evidence that the population can recruit without a marine phase. The remaining fish had spent part of their lives in marine or brackish water, which means that flounders with different life histories occupied the same lake.
"This is a striking finding because reproduction poses challenges that go beyond an adult fish’s ability to tolerate freshwater. In fishes, including flounders, salinity affects whether sperm become motile and how eggs develop and remain suspended in the water", says Associate Professor Paolo Momigliano of the University of Tuscia.
"The next question is how they reproduce successfully under conditions that prevent European flounder from doing so in the rest of its range", says Professor Juha Merilä of the University of Hong Kong.
The genomic comparison suggests that the shift to freshwater reproduction is likely to have a complex genetic basis, involving variants on several chromosomes.
The study offers an opportunity to examine how a predominantly marine fish can come to reproduce in freshwater. Flounders with different life histories occur in the same Arctic region and remain genetically similar overall. This allows researchers to pinpoint which traits and genetic changes are associated with the transition.
Marine-freshwater transitions often lead to reproductive isolation, which makes the flounders of Lake Pulmanki a system for exploring one of the processes that may lead to speciation.
PhL Thais Ferreira-Araújo, University of Helsinki, Professor Kimmo Kahilainen University of Helsinki and PhD Petri Kemppainen, University of Helsinki. Professor Juha Merilä, University of Hong Kong, Associate Professor Paolo Momigliano, University of Tuscia. The research team included researchers from Italy, Hong Kong, Australia, Portugal and Finland. The study began as a licentiate thesis by Thais Ferreira-Araújo at the University of Helsinki, supervised by Paolo Momigliano, Petri Kemppainen and Juha Merilä.
Evolution
10.1093/evolut/qpag165/8827781
Evolution of freshwater reproduction in a marine fish: Evidence from genetics and otolith microchemistry
22-Sep-2026