European flounder can complete whole life cycle in freshwater

European flounder can complete whole life cycle in freshwater Gaby Clark Scientific Editor Andrew Zinin Chief Editor Most flounders sampled in Lake Pulmanki in northern Finland had spent their entire lives in freshwater. The study provides the first evidence of the species living entirely in freshwater. The work is published in Evolution. 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 the sea via a river section 60 km (37 miles) long. Cracking a 50-year-old mystery 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. Two lines of evidence The team approached the question in two ways. Using otolith microchemistry, the researchers analyzed otoliths, small bony structures in a fish's inner ear that help it sense balance and movement. As otoliths grow, their chemical composition preserves 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 otolith records show that most flounders sampled in Lake Pulmanki had spent their entire lives 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. Why the finding matters 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. Researchers 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 Momigliano, Petri Kemppainen and Merilä. Publication details Thais Ferreira-Araújo et al., Evolution of freshwater reproduction in a marine fish: Evidence from genetics and otolith microchemistry, Evolution (2026). DOI: 10.1093/evolut/qpag165 Journal information: Evolution Provided by University of Helsinki