Traditionally, shipworms have avoided the Baltic Sea due to its lower salt content. The Baltic therefore holds a number of archaeological finds featuring wrecks of wooden ships and structures which would not likely have survived were it not for the mild salinity levels of Baltic waters.
According to Appelqvist and her colleagues, around 100,000 well-preserved ship wrecks are scattered across the bottom of the Baltic Sea.
But the Teredo navalis species of shipworm, one of 65 varieties found throughout the world, has recently been making its way into the Baltic Sea, having been spotted along both the Danish and German coasts in the southern Baltic.
It was also recently found in wooden supports at the Ribersborg cold bath house in Malmö, further evidence that maritime artifacts are under threat.
Known for boring tunnels up to 30 centimetres deep into all kinds of wood, the shipworm is capable of completely destroying large maritime archaeological finds within ten years.
Three varieties of shipworm have long been common to Sweden's west coast, meaning there are no wooden shipwrecks left.
“They've all been eaten up,” said Appelqvist.
But the spread of Teredo navalis into Baltic waters in southern Sweden, is a relatively new phenomenon which Appelqvist and her colleagues believe may be due to warmer seas caused by climate change.
“The warmer temperatures mean that the shipworm is less stressed and can thus tolerate lower salinity,” she explained.
“The warm water also results in a longer breeding season.”
In order to help protect Sweden's many maritime archaeological finds, the Gothenburg-based team has joined WreckProtect, an EU project designed to determine exactly which archaeological treasures are at risk.
“We just don't know today how far Teredo navalis may spread,” said Appelqvist, although she said that it was unlikely it would spread as far north as Stockholm.
“It will primarily depend on salt content and water temperature, but those can be affected by a number of other variables.”
While waters further north in the Baltic may not be immediately threatened by Teredo navalis, Appelqvist pointed out that there are other species of shipworm which could tolerate the chilly, low-salt waters between Stockholm and the Åland islands.
“They could easily enter that part of the Baltic in the ballast water of a passing ship,” she said.
For now, however, Appelqvist and her colleagues are focusing their efforts on the waters off the coast of Skåne, using climate data and maps of known ship wrecks to determine which archaeological finds are most threatened by Teredo navalis.
In addition to trying to predict where shipworm is likely to spread in the future, the group is also exploring various methods to protect the shipwrecks include covering them with permeable geotextile fabrics and bottom sediment.
A video featuring the exploration of the wreck of the BirgerJarl, a Swedish frigate built in 1808. The wreck now lies in about 28 metres of water between Sweden and Denmark. Courtesy of WreckProtect.