The shipworm bores its way through wooden piles, dolens and harbour structures – causing millions in damage, closures and disruption for recreational boaters. And it continues to spread.
It is barely bigger than a finger, is misleadingly known as the ‘shipworm’ and causes damage on a scale that is increasingly causing difficulties for port operators on the North Sea and Baltic coasts. Bremerhaven is currently suffering particularly badly from this pest: In early summer, the access ramp to the Weser ferry in Bremerhaven developed cracks and subsided. It has been closed ever since, and ferry services are operating via an alternative jetty. An inspection carried out three years earlier had not identified any defects. The culprit: the shipworm Teredo navalis.
A mussel brings Bremerhaven to a standstill
When Bremerhaven built its quay facilities 100 years ago, the drill mussel was still considered a rare visitor, according to the Thünen Institute – the salinity of the Weser’s brackish water was too low. It is now increasingly infesting the wooden piers of the historic harbour facilities, presumably aided by improved water quality and warmer water temperatures as a result of climate change.
According to …, the consequences are Antenne Niedersachsen Serious: The North Pier has collapsed, the connecting canal between the Old and New Harbours is in danger of collapsing, and the quay at the New Harbour is in a state of disrepair. The bascule bridges leading to the Havenwelten are closed, and a historic crane needs to be shortened. The 240-metre-long North-East Quay and the two historic bridges on the connecting canal between the Old and New Harbours are also considered according to a report by ZDF no longer considered safe. The result: road closures and diversions – for years to come.
Infestations of zebra mussels and zebra shrimp have been detected along approximately eight kilometres of shoreline reinforced with wooden pile foundations – in both the overseas and fishing harbours – since the 2010s, reports Antenne Niedersachsen, citing Bremen’s Environment Department. “The spread is not uniform, but occurs in waves, much like an epidemic.”
“We knew the zebra mussel was there. But we couldn’t have predicted that its population would have grown so rapidly in recent years,” says Heiner Behrens of the Alter/Neuer Hafen Development Company, who is leading the regeneration project. According to ZDF, he estimates that the renovation costs for the connecting canal between the Old and New Harbours alone will amount to around 32 million euros. It is still difficult to estimate how high the total costs will be – the structures and their uses vary too greatly, reports Antenne Niedersachsen.
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From the North Sea to Mecklenburg
The problem is not confined to Bremerhaven. Along the coast of Lower Saxony, the mussel has been causing increasing damage since the 1990s. How it spreads depends on factors such as salinity and current conditions, whilst the damage it causes depends on the design of the structures.
This has already led to severe damage in the past at the island harbours of Wangerooge, Spiekeroog and Langeoog. In the Cuxhaven area, the drill mussel completely destroyed untreated softwood dolphins within two years. In Emden, at the turn of the millennium, divers found that 90 per cent of the round-timber walls in a lock had been destroyed right down to the heartwood. Monitoring is currently being carried out in Norddeich – the first results are expected in the autumn.
Damage has also been observed along the groyne systems on the Baltic Sea coast between Boltenhagen and Zingst, up to and including Hiddensee, according to the Ministry for the Environment of Mecklenburg-Western Pomerania. In the district of Rostock, wooden piles have already been replaced in Heiligendamm and Graal-Müritz – the cost in Heiligendamm alone amounts to around 200,000 euros. Further work is underway at the Hohe Düne seaside resort in Rostock, which has been suspended since April due to nature conservation and tourism considerations.
In Wismar, the shipworm has the West Harbour firmly in its grip: the first areas have been closed off since 2021. From June 2026, the eastern side of the harbour basin will also be off-limits to vessels. Along the quay edge, 80 posts prevent cars from passing through – the wooden posts beneath the paving are made of Teredo navalis corroded. Estimated cost of refurbishing the Ostkai: 6.4 million euros; work is expected to begin in 2028. The cordon will remain in place until then. The Brunkowkai in the overseas port basin had already had to be refurbished at a cost of 8.5 million euros.
A worm that isn’t really a worm
The name is misleading: the so-called shipworm is not a worm, but a bivalve – as Dr Michael Zettler, a zoologist and marine biologist at the Leibniz Institute for Baltic Sea Research in Warnemünde. Teredo navalis It belongs to the wood-eating Teredinidae family; on the North Sea coast it usually grows to between 10 and 20 centimetres in length, but can reach up to 45 centimetres. There are around 80 other species of shipworm worldwide.
As a small larva, it attaches itself to a wooden surface and eats its way inside. The tunnels are often sealed up again on the outside – meaning the infestation remains hidden for a long time. “They live in the wood, grow and form tunnels until only a very thin outer shell remains,” explains timber researcher Christian Brischke from the Thünen Institute. Only when the load-bearing cross-section of the timber has been significantly reduced does the extent of the damage become visible. By then, it is often too late.
“This isn’t a new issue,” says Zettler. “In the past, oak trunks were used in many harbours. Many have long since been replaced by iron piles because the mussels had eaten away at them.” The species has always had its ups and downs – in some years it has been more common, in others less so. “All wooden piles placed in the water provide food for the mussel.”
What recreational boaters should know
Despite the enormous damage, there is no need for sailors to panic just yet. Most marinas are relatively new and are already built using steel and concrete – materials that the shipworm cannot infest. In Bremerhaven, too, it is primarily the historic harbour facilities dating from the Gründerzeit era that are affected – not the marinas.
For recreational boaters, the impact of the refurbishment works will, if at all, be felt only indirectly: closed quays, diversions or restricted facilities in historic harbour areas may lengthen the journey to a mooring or mean that certain mooring options are temporarily unavailable. Anyone moored in harbours with old wooden infrastructure – such as in traditional town harbours – should take a look at the condition of the dolphins and jetties.
Annoying, but nothing new
Every year, new species are introduced into native waters either in the ballast water of ocean-going vessels or as fouling on ship hulls. Some spread rapidly along coastlines or in inland waters. “Once a species has become established, there is absolutely no chance of intervening,” says Zettler. This also applies to the shipworm: as early as 1503, on his fourth voyage to the Americas, Christopher Columbus complained that the so-called shipworms were boring holes in his ship. It is believed that the defeat of the Spanish Armada in 1588 was also linked to damage caused by the barnacle. In the Netherlands in 1731, sea defences collapsed during a storm surge – the barnacle had eaten through the wooden dyke gates. Its origin remains unclear to this day – it probably arrived in Europe from the Indo-Pacific on early wooden ships; it reached the North Sea by the 17th century at the latest, and has been permanently established in the Baltic Sea since 1993.
Ursula Meer
Redakteurin Panorama und Reise