A recent study reveals that the Saimaa ringed seal is evolutionally much more differentiated than previously known. Indeed, researchers suggest that instead of a subspecies, it should be considered its own independent species.
Ringed seals are geographically the most widely spread Arctic seals and, in addition to the Arctic Ocean, they are encountered in the Sea of Okhotsk north of Japan, the Baltic Sea as well as lakes Saimaa and Ladoga. Lake seals have traditionally been considered close relatives evolved from seals trapped in inland water bodies as land rose after the Ice Age.
The study, conducted in collaboration by the University of Helsinki, the University of Eastern Finland and the University of Copenhagen together with the Tokyo National Museum of Nature and Science and the Natural Resources Institute Finland, examined for the first time the genomes and structural characteristics of almost all ringed seal populations in the world. The results show that Saimaa ringed seals diverged from other ringed seals already more than 60,000 years ago - that is, long before Lake Saimaa came to existence 10,000 years ago.
"For a long time, genetic studies have implied that Saimaa ringed seals are more different than you would think on the basis of the birth history of Lake Saimaa. But it was only through the analysis of all Arctic ringed seals that we could date the differentiation of the Saimaa ringed seal lineage from the others," says Ari Löytynoja from the University of Helsinki.
While ringed seals from the Baltic Sea and Lake Ladoga originate in the Atlantic, Saimaa ringed seals have probably spread to Finnish territory from the east - an area that was home to extensive ice lakes during the Ice Age. On their way to Lake Saimaa they have cross-bred with Western ringed seals.
"Based on comprehensive genomic analyses, the hybridisation starts to be more a rule than an exception in the history of living organisms. Human evolution is also based on the intertwining of lineages," says Petri Auvinen from the University of Helsinki.
Teeth separate Saimaa ringed seals from other ringed seals
The long evolutionary history of Saimaa ringed seals led researchers to examine their ecological differentiation. The teeth of marine ringed seals are adapted to eating both fish and crustaceans, such as amphipods. Like baleen whales they suck the prey into the mouth and sieve out the water with the teeth and tongue. According to the new study, the teeth of Saimaa ringed seals are the most specialised in fish nutrition of all ringed seals.
"Measurements from hundreds of specimens show how exceptional the teeth of Saimaa ringed seals are. These differences support the results of genome analyses of an extended independent development history," says Jukka Jernvall from the University of Helsinki.
Adaptations to a different diet were found also in the tongue and the digestive tract of Saimaa ringed seals. Taken together, genetic and morphological differences support the species status.
"Saimaa ringed seal was identified as distinctly different from other ringed seals already in the 19th century. Since then our understanding of the evolution of species has increased and raising the Saimaa ringed seal to its own species, Pusa saimensis, reflects its independent development path," says Jaakko Pohjoismäki from the University of Eastern Finland.
A unique and endangered remnant from the time of ice lakes
Even though, thanks to active conservation measures, the Saimaa ringed seal population has gradually grown to approximately 500 specimens, it continues to face many threats. Its status as Finland's only endemic species is expected to increase interest in conservation efforts.
"We have a truly unique relic, the last representative of the ringed seals of the world's ancient ice lakes, living in Finland. It is sobering to think how close it has been to becoming permanently lost," says Mervi Kunnasranta from the University of Eastern Finland.
Species description of the Saimaa ringed seal was the result of a long-term multidisciplinary collaboration. From the University of Helsinki alone, researchers from three faculties and two independent institutes took part in the project.
The study was funded by the Jane and Aatos Erkko Foundation, the Research Council of Finland and the LIFE Programme of the European Commission (LIFE19NAT/FI/000832).
Original article
Löytynoja, A., Pohjoismäki, J., Valtonen, M., Laakkonen, J, Morita, W., Kunnasranta, M. Väinölä, R., Olsen, M. T., Auvinen, P. & Jernvall, J. Deep origins, distinct adaptations and species level status indicated for a glacial relict seal. Proceedings of the National Academy of Sciences, USA 122: in press (2025). https://www.pnas.org/doi/10.1073/pnas.2503368122
