Medieval shipwrecks threatened by the spread of shipworm into the Baltic Sea

The shipworm Teredo navalis is spreading to the Baltic Sea, threatening to destroy archaeological artefacts. Researchers* at Gothenburg University suspect that climate change is what’s making it possible for this species to spread and are now joining the EU project WreckProtect, a cooperative effort to assess which archaeological treasures are at risk. The project includes researchers from Sweden, Denmark and the Netherlands, as well as experts from France and Germany.

Not really a worm
Shipworms are not actually worms but saltwater clams with much reduced shells. They are notorious for borrowing into and gradually destroying wooden structures in saltwater; earning the nickname “termites of the sea”.

There are 65 different know species of shipworm but Teredo navalis is the only one currently known to spread into the Baltic Sea via the Great Belt. Teredo navalis forms up to 30 cm deep tunnels in submerged wood and is difficult to detect since it remains hidden inside the tunnel. It has a life expectancy of 3-4 years.

Teredo navalis can survive in a salinity of 4-6 practical salinity unit (PSU) for short periods of time but can not reproduce unless the salinity is at least 8 PSU. The salinity of the Baltic Sea decrease the further north you get with the Stockholm Archipelago sporting an average salinity of roughly 5 PSU.

The shipworm is capable of completely destroying large maritime archaeological finds in only 10 years, and while it has avoided the Baltic Sea in the past, since it does not do well in low salinity water, it can now be spotted along both the Danish and German Baltic Sea coasts.

14th century shipwrecks under attack

“Wrecks that have been resting unharmed since the 14th century have now been attacked off the coast of Rügen in Germany, and we are also noticing attacks along the Swedish coast, including destruction of the Ribersborg cold bath house in Malmö,” says Christin Appelqvist, doctoral student at the Department of Marine Ecology, University of Gothenburg.

Appelqvist and her colleagues suspect that increased water temperatures may be helping the shipworm to tolerate a lower salinity.

One of the objectives of project WreckProtect is to develop methods for the preservation and protection of shipwrecks. It might for instance be possible to cover the wrecks with geotextile and bottom sediment.

100,000 wrecks may be at risk
Thanks to the absence of Teredo navalis there is currently around 100,000 well preserved shipwrecks resting in the Baltic Sea, a true treasure for historians and archaeologists. If the shipworm continues to spread these ships may vanish before anyone has a chance to explore them.

“Around 100 wrecks are already infested in the Southern Baltic, but yet it hasn’t even spread past Falsterbo. We know it can survive the salinity of the Stockholm archipelago, although it needs water with higher salinity than that to be able to reproduce,” says Appelqvist.

* Christin Appelqvist, Department of Marine Ecology, University of Gothenburg
http://www.marecol.gu.se/Personal/Christin_Appelqvist/

* Jon Havenhand, Department of Marine Ecology, University of Gothenburg
http://www.tmbl.gu.se/staff/JonHavenhand.html

Picture credit: http://www.science.gu.se