For about 50 years, flounders in Lake Pulmanki posed a simple question: were they just visiting from the sea, or could they live and reproduce in freshwater? A study of fish from this northern Finnish lake has now found that most sampled flounders had spent their entire lives there.
European flounders are usually described as marine fish. They often enter river mouths to feed, but are thought to return to the sea or brackish water to reproduce. Lake Pulmanki challenges that picture. The research offers the first evidence that the species can complete its whole life cycle in freshwater.
The lake itself has a long connection to the sea. It was once a fjord of the Arctic Ocean, then became a freshwater lake after the last ice age. Today it remains linked to the sea through a 60-kilometer river section. When flounders were first recorded in the lake in the 1970s, researchers could not tell whether the fish migrated there to feed and later returned to the Barents Sea to spawn, or had adapted to living and reproducing in freshwater.
An international team tested both possibilities using two kinds of evidence. First, researchers examined otoliths: small, bony structures in a fish’s inner ear that help with balance and movement. As an otolith grows, its chemistry records the water the fish has lived in. Those records showed that most of the sampled Lake Pulmanki flounders had stayed in freshwater throughout their lives. The remaining fish had spent part of their lives in marine or brackish water, so fish with different life histories were living in the same lake.
The team also compared the fish’s genetic variation with flounders from several marine locations. The results suggest that the ability to reproduce in freshwater has a complex genetic basis, with variants on several chromosomes, rather than a single genetic change.
Reproduction is a demanding test of freshwater living. Salinity can affect whether sperm move and how eggs develop and stay suspended in water. “This is a striking finding because reproduction poses challenges that go beyond an adult fish’s ability to tolerate freshwater,” said Paolo Momigliano, an associate professor at the University of Tuscia. Juha Merilä, a professor at the University of Hong Kong, said the next question is how the fish reproduce successfully in conditions that prevent European flounder from doing so elsewhere.
The discovery gives scientists a rare chance to study how a mainly marine fish can shift to freshwater reproduction. Because fish with different life histories share the Arctic region and remain genetically similar overall, researchers can look for traits and genetic changes linked to the shift. Such changes can sometimes lead populations to become reproductively isolated, an important step in the formation of new species.
The study began as a licentiate thesis by Thais Ferreira-Araújo at the University of Helsinki and involved researchers from Finland, Italy, Hong Kong, Australia and Portugal. Published in the journal Evolution, it turns a question first raised in the 1970s into a new opportunity to understand how life adapts to different waters.