European flounders are generally marine fish that may enter river mouths to feed before returning to marine or brackish waters to reproduce. A new study shows that a population in northern Finland can instead complete its entire life cycle in freshwater.
Researchers examined flounders from Lake Pulmanki, a former Arctic Ocean fjord that became a freshwater lake after the last ice age. Although the lake remains connected to the sea through a 60-kilometre river section, most of the sampled fish had spent their entire lives in freshwater.
The finding resolves a question that had remained open since flounders were first recorded in the lake during the 1970s. They might have been travelling there to feed before returning to the Barents Sea to spawn, or they might have adapted to permanent freshwater living.
To investigate, the international team analyzed otolith microchemistry. These small structures in a fish’s inner ear retain chemical information about the waters the animal has inhabited as they grow. The researchers also used whole-genome resequencing to compare Lake Pulmanki flounders with fish from several marine locations.
The otolith results indicated that most of the Lake Pulmanki flounders had never entered marine or brackish water. This shows that the population can recruit without a marine stage. Other fish in the lake had experienced marine or brackish conditions, meaning that individuals with different life histories coexist there.
Reproduction in freshwater presents challenges beyond an adult fish’s ability to tolerate low salinity. Salinity can influence sperm motility, as well as how eggs develop and remain suspended. The researchers said the genetic comparison points to a complex basis for the shift to freshwater reproduction, with variants on several chromosomes likely involved.
The population offers researchers a way to study how a predominantly marine fish adapts to reproduction in freshwater. Because flounders with different life histories occur in the same Arctic region while remaining broadly genetically similar, the system may help identify traits and genetic changes linked to the transition. Such shifts can lead to reproductive isolation, a process associated with speciation.
The study involved researchers from Italy, Hong Kong, Australia, Portugal and Finland. It began as a licentiate thesis by Thais Ferreira-Araújo at the University of Helsinki under the supervision of Paolo Momigliano, Petri Kemppainen and Juha Merilä. The findings were published in Evolution under the title “Evolution of freshwater reproduction in a marine fish: Evidence from genetics and otolith microchemistry.” DOI: 10.1093/evolut/qpag165.
Sources: Phys.org.