The soft-shell clam in the North Sea basin: descendants of ancestors hitchhiking with the Vikings from North America to Europe?

Published: 9 March 2017

Research shows that soft-shell clams (Mya arenaria) were already present in the Middle Ages in the Netherlands, supporting the hypothesis that the animal was re-introduced from North America before Columbus, possibly by Vikings. During the Pleistocene, the coastal marine bivalve became extinct in northwest Europe.

The species survived at the east coast of North America. Datings of Mya shells found in northern Denmark and the southern Baltic Sea suggest that repopulation of northwest European coasts already occurred before Columbus’ discovery of America (1492), possibly facilitated by Viking (Norse) settlers at Greenland and northeast North America , as was suggested by Petersen et al. (1992). Nowadays the soft-shell clam is common along the coasts of the North Sea, the Balthic Sea, northern Adriatic Sea, the Black Sea, Ireland, England, Atlantic France, Spain, and even in the Mediterranean Sea. It has been a very successful invader.

Living Mya arenaria of the Wadden Sea: a once extinct invasive bivalve of up to 15 cm in length; you will be able to find it everywhere along the European coasts (copyright free photo: Essink).

Recent study

A recent study of the Sunken History Foundation, Deltares and the Universities of Utrecht, Groningen and Leiden (Essink et al., 2017) published in Netherlands Journal of GeoSciences dated M. arenaria shells of older coastal deposits. Using a precise isotope analysis method it was shown for four locations in the coastal landscape of the Netherlands that the shells date from the 13th to 15th century AD. It provides further evidence of the re-introduction of this invasive species from its native range in northeast America into European waters in the Medieval Period. As a consequence, it supports the hypothesis of Viking-mediated transfer of this species from northeast America to northwest European waters. To test the hypothesis by Petersen et al. (1992) Essink and Oost are currently investigating in more detail if Viking-mediated transport was indeed possible and if there could be other plausible cross-Atlantic pathways. At the moment the arguments for Mya hitchhiking with the Vikings seem the most convincing.

Albert Oost (geologist Deltares): as Mya arenaria is an invasive species it is exciting to notice that wide spreading in NW Europe already took place in the Middle Ages. It provides an excellent indicator for the age of marine deposits of the North Sea basin.

Shells of medieval Mya arenaria: one of the early descendants of the migrants from North America (copyright free photo: Essink)