European flounder surprises with freshwater life cycle

An unexpected discovery has revealed that European flounders in Finland’s Lake Pulmanki are completing their entire life cycle in freshwater, contrary to long-held assumptions about their marine origins.

The study, published in *Evolution*, marks the first evidence that this species can thrive and reproduce without ever returning to saltwater. Researchers sampled fish from the lake, which was once an Arctic fjord but now connects to the sea via a 60-kilometer river system.

For decades, scientists puzzled over how flounders—typically marine fish that migrate to river mouths for feeding—had persisted in Lake Pulmanki. Possible explanations included seasonal migrations or adaptive freshwater survival. But the new research shows a more radical shift: some flounders here reproduce entirely in freshwater.

The findings rely on two key methods. First, researchers analyzed otoliths—tiny ear bones that record a fish’s movement and chemical environment—as they grow. Most sampled flounders showed no marine exposure, suggesting they spent their entire lives in the lake. A minority had spent part of their lives in brackish or marine waters, indicating mixed life histories within the same population.

Genetic analysis further supported the finding. By comparing flounder genomes from Lake Pulmanki with those from marine sites, researchers identified genetic variations linked to freshwater adaptation. The shift appears complex, involving multiple chromosomes, and challenges traditional views on how marine species reproduce in freshwater.

The discovery offers a rare natural experiment for studying speciation. Since flounders in the lake remain genetically similar to their marine relatives, researchers can track which traits enable freshwater reproduction. This could help explain how marine species adapt to new environments, a process that may drive evolutionary divergence.