This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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. 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 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.
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