By David Gregory 

20.01.23

Download the article here! 

“Scientific inquiry starts with observation. The more one can see, the more one can investigate”.

Nobel Laureate Martin Chalfie’s word ring true in the case of the three shipwrecks from the 1700s that were found and surveyed by the Sea War Museuem Jutland in the  deep waters east of Gotland. The ROV documentation of the wrecks by Ocean Discovery and supplementary Multi Beam Echo Sounder by JD Contractors, allowed us not only to see what lay on the seabed but following the surveys and post-processing of results, to revisit them in a digital universe. But why the wrecks still there, and conversely why not all wrecks are so well preserved when we find them, has been a question the Sea War Museum, JD Contractors and the National Museum have been investigating in recent years in order to better preserve under water cultural heritage.

Understanding how and why a shipwreck survives involves a complex study of how a ship and its materials interact with an environment they were never designed to be in

 As Gert Normann Andersen stated in a previous posting, “In the North Sea, all wrecks are destroyed in record time. All the woodwork is eaten by shipworms and wave movements and heavy fishing gear take care of the rest”.

The location of the wrecks, near the Gotland Deep in the Baltic sea, is key to their exceptional survival. For the first, their depth has protected the wrecks from fishing and other human activities. Equally important, the wood eating shipworm (Figure 1), the scourge of wooden shipwrecks under water, do not survive in this part of the Baltic Sea.

 

Figure 1. The shipworm Teredo navalis. The head of the animal, with two white shells that bore into wood can be seen to the right. It respires and gains nutrients from the seawater using two tube like appendages (siphons), that protrude from the wood into the seawater, which can be seen to the left. Photo: David Gregory, Nationalmuseet.

 

 There are around 65 difference species of shipworm around the world, four of which we have in and around Danish waters. Interestingly they are not a worm at all but a very specialized saltwater mussel / clam, taxonomically they are a marine bivalve mollusk of the family Teridinidae, first classified by the Swedish botanist and zoologist Carl von Linne in 1758. Teredo Navalis, or the common shipworm, is one of the major species in north west European and Scandinavian waters. Its head is covered with two white shells that it uses to bore into wood. At its tail end are two siphons (Figure 2) – one used to take in sea water for respiration and feeding and one for expelling waste and to assist in reproduction.

Teredo Navalis, eller den almindelige pæleorm, er en af de vigtigste arter i nordvesteuropæiske og skandinaviske farvande. Dens hoved er dækket af to hvide skaller, hvormed den borer sig ind i træ. I bagenden har den to rør (Figur 2), hvoraf det ene bruges til optagelse af ilt og næring, mens affaldsstoffer sendes ud gennem det andet. Begge rør har desuden en funktion i forplantningen.

Figure 2. Left: A side view of the one of the shells of T.,  its sharp ridges help the animal to bore into the wood (x100). Right: Siphones (x20), which are the only part of the animal that can be seen outside of the wood. Photos: David Gregory, Nationalmuseet

Although it has evolved to live in wood it starts its life in the open water (Figure 3). When water temperatures reach about 15°C reproduction starts, with the male releasing sperm into the water and the female picking this up through her siphon. The eggs spend up to five weeks in the mothers gill chamber until they are released into the water as free swimming larvae called veligers. Females will spawn 3-4 times in a season, releasing 1-5 million larvae each time. The veligers will drift in the water for 2-3 weeks during which time the siphons, gills and foot develop, and the young shipworm will hopefully find a piece of submerged wood to settle on.

 

Figure 3. The lifecycle of Teredo navalis (efter Nair og Saraswarthy, 1971).

At this stage the shipworm is very small and only makes millimeter-sized holes in the wood – why it can be so difficult to see when a shipwreck has been attacked by shipworm (Figure 4).

 

Figure 4:  A section of wood attacked by shipworm. Upper: the outside of the wood where only the small entrance holes of the young shipworm can be seen. Lower: The same piece of wood seen from the inside showing extensive attack by shipworm. Photo David Gregory, Nationalmuseet.

Once in the wood the shipworm never leaves its burrow and after orientating itself with the grain of the wood uses the two shells on its head in a rasping action to burrow into the wood, creating a perfectly circular tunnel in the wood. The shipworm can sense the presence of other nearby shipworm in the wood and will deviate away from adjacent tunnels.  The saw dust that is created during boring, is taken into the mouth of the shipworm and ingested. Ironically the shipworm does not utilize the wood itself but it can extract carbohydrates from it with the help of a unique endosymbiotic bacteria (Teredinibacter turnerae) that produce cellulolytic enzymes. The inside of the tunnels it makes are lined with a thin layer of calcium carbonate, which provides protection of the soft body of the shipworm. The shells and the linings of the tunnels help us when carrying out research on shipworm as they show up clearly on X Ray (Figure 5).

 

Figure 5: A piece of pine wood that has been exposed to shipworm infested waters in Denmark for 6 months. The shells and the calcaerous lining of the animals tunnels are clearly visible with X-Ray. Photo: Nationalmuseet.

 

Should the piece of wood the shipworm is living in be taken out of the water, or be exposed to air, the worm can withdraw its siphons and seal the hole using two small “pallets”. In this manner the shipworm can actually survive for several weeks without oxygen as it can survive on sugars stored in its body.

 

Figure 6. Left: Siphons seen along side the two pallets. Right: The siphones have been withdrawn into the tunnel and the hole closed with the two pallets. Photos: David Gregory, Nationalmuseet.

The lifespan of a shipworm is normally two to three years but within this time they can grow up to 30cm long, but specimens of a full 60cm in length and 1 to 2cm in diameter were observed on the remains of the large frames and keel of the Medieval Cog found in Kolding Fjord (Figure 7).

Figure 7.  Attack of timbers from the Koling Cog by shipworm. Photo: David Gregory, Nationalmuseet.

 

Often, when we are carrying out research using small modern blocks of pine or oak wood, we experience that they can devour them within 6 months to a year (Figure 8).

 

Figure 8: Three modern pine wood blocks that have been exposed for one year in Lynæs Harbour, Denmark. Photo: David Gregory, Nationalmuseet.

 

Thankfully shipworm do not survive in the area where the wrecks were found – this is because of the environment and in particular the temperature, salinity and dissolved oxygen in the water. Using what is called a CTD logger, (Conductivity, Temperature, Depth) (Figure 9), profiles of salinity, temperature and depth from around the wrecks were measured. By attaching it to a winch and dropping it from the side of Sema, the logger automatically measured and stored (logged) the various parameters every 50cm as it descended through the water to the seabed.

Figure 9: CTD datalogger.  Photo JD-Contractor / Nationalmuseet.

 

On retrieval, the stored data was downloaded to a computer and the results confirmed that the seabed around the wrecks was 150 metres deep. At these depths there is no sunlight and the darkness of the water has prevented the growth of seaweeds on the wreck surfaces – as seen in the ROV imagery and the 3D models. The temperatures around the wrecks were around 12° C and the Salinity (calculated from the conductivity) at the seabed was around 12 PSU (Practical Salinity Units) (Figure 10).

Figure 10: A CTD profile around one of the wrecks. Data JD-Contractor / Nationalmuseet

 

Interestingly these temperatures and salinities are conducive to the growth and spread of shipworm but there is another very important factor – the amount of dissolved oxygen in the water. As mentioned shipworm do no survive on wood alone and gain nutrition by sucking in seawater and filtering out nutrients and dissolved oxygen. Without oxygen they cannot survive. Although it was not possible to measure dissolved oxygen on this occasion, the Gotland Deep is well studied and monitored (Feisterl et al., 2004) and at the location and depths of the wrecks, there is no dissolved oxygen – and thankfully  no shipworm.

However, no oxygen does not mean no decay. Decay of wood and other materials like the rigging and iron artefacts including cannon, fastenings and nails will still occur but slowly – over centuries or even millenia and is the subject of our ongoing research and will be described in a future posting.

Further Reading

Björdal, Charlotte Gjelstrup, and David Gregory, 2012, WreckProtect: Decay and Protection of Archaeological Wooden Shipwrecks. ArchaeoPress, Oxford. www.wreckprotect.org

Eriksen, A. M. and D. Gregory (2016). "Degradation of Archaeological Remains by Shipworm." Conservation and Management of Archaeological Sites 18(1-3): 30-39.

Feistel, Rainer, Günther Nausch, Wolfgang Matthäus, Elzbieta Lysiak-Pastuszak, Torsten Seifert, Ian Hansen, Volker Mohrholz, Siegfried Krüger, Erik Buch, and Eberhard Hagen 2004. Background data to the exceptionally warm inflow into the Baltic Sea in late summer of 2002. Meereswissenschaftliche Berichte IOW 58:1.

---