Shipworms-Shiver the timbers

by Matthew Norton

Life under the sea is not easy for most animals with the search for food being a constant challenge. Often, there is not enough to go around and this can lead to fierce competition over access to what little food there is. Some species get out of this difficult situation by specialising on those foods that most animals wouldn’t want to eat because they are low in nutritional value and hard to digest. So long as the food source can provide the basics to support life, which include (but are not limited to) carbon for making glucose sugars and nitrates (or other nitrogen based compounds) for making proteins and DNA, then it can be a viable food source.

For shipworms, which are worm-like clams, the food of choice is wood and they can burrow into submerged wooden structure, such as trees, boats and piers, and slowly grind it into tiny pieces. They dig into the wood by using a pair of sharp shells at one end (which in other clams would encase the whole animal) to grind away the wood and then pushing their body in. As they move in, shipworms line their burrows with a mixture containing calcium carbonate (which most hard body parts, such as bones and shells are made of) to prevent them from collapsing.

shipworm article image 1
Shipworms are weird, but resourceful animals that completely repurpose their shells, which other bivalve molluscs (e.g. oysters) use for protection, to drill their way through wood.

All this burrowing breaks the wood down into the pieces that shipworms can swallow, but actually digesting the wood material, specifically the cellulose fibres, into glucose sugars that the animal needs is not so easy. In fact, they cannot do it themselves and instead rely on one specific strain of bacteria, called Teredinibacter turnerae T7901, to make the necessary enzymes (molecules designed for making or breaking chemical bonds) for breaking down cellulose.

Bacteria also help shipworms to overcome the other problem with a wood diet, a lack of nitrogen, by extracting dissolved nitrogen from the water and converting into more usable forms. Although, there is some evidence that shipworms also catch food particles floating in the water to supplement the nitrogen in their diet.

Shipworms (with help from their bacteria) are so well adapted to living in wood that once the larvae settle on a suitable block of wood they never leave, with the giant shipworm, Kuphus polythalamia, being the only exception. This species, already noteworthy because of their sheer size, leave their wooden burrows in their youth and instead settle for a life buried deep in muddy seabeds. Without wood to eat they rely on a different strain of bacteria (Thiosocius teredinicola) to make them glucose sugars from carbon dioxide dissolved in the seawater, similar to how plants and seaweeds make their food by photosynthesis. However, a giant shipworm’s bacteria uses chemical energy, instead of light energy, released from reacting sulphides with oxygen or nitrogen-based compounds to power this conversion.

The use of bacterial helpers to boost an animal’s chances of survival is common in nature, even we have thriving communities in our guts that massively outnumber our own cells and provide all kinds of health benefits. However, shipworms take this one step further and use their bacteria to exploit a unique food source and mould their own place in the natural world.

From a human perspective

Because shipworms eat wood, they have been a serious pest for mariners and coastal communities through history, causing serious damage to boats, piers, harbours and any other man-made wooden structure submerged in seawater. This problem is particularly hard to manage because shipworms are so small when they first enter the wood that infestations are only discovered when it is too late. Even inland seas, such as the Baltic Sea, are not safe because of one particular species, Teredo navalis, which is particularly effective at infesting areas that most other shipworms cannot.

The damage and frustration that shipworm infestations can cause is pretty clear from the epidemic that plagued the coasts of the United States during the late 19th and early 20th century. The damage itself cost millions of dollars per year, but some coastal communities were relatively unaffected while others were devastated by shipworm infestations and this led to a lot tension. Teredo, a general term for shipworms at the time, was also used as an insult targeted at certain political groups and ethnicities.

shipworm article image 2
The first Herne Bay peer, depicted in this postcard, was also devastated by shipworms burrowing in the wooden pillars that were not protected by copper plates.

Fortunately, shipworms are also useful because of their unique way of living. For example, in the 1990’s there was a lot of interest in the commercial uses of the enzymes produced by shipworm bacteria to remove stains and contaminating chemicals. These cleaning enzymes do not work at very high temperatures, but they must still be effective if certain entrepreneurs went to the trouble of patenting their use.

shipworm article image 3
The enzymes produced by shipworms’ bacteria can be used for various purposes, such as sterilising contact lenses, cleaning industrial machinery and as an ingredient in detergents.

Shipworm burrowing also inspired the French engineer Marc Brunel (not to be confused with his more famous son, Isambard Brunel) to invent the “tunnelling shield” technique in the early 19th century for burrowing underground. This involves using a movable ‘shield’ as a temporary support structure (sort of like scaffolding) while workers dig and build concrete walls around the tunnel. This was so effective that Brunel used the tunnelling shield to build the Channel Tunnel that connects the south of England and the north of France (under the English Channel).

shipworm article image 4
On the right hand side you can see the tunnelling shield that holds up the tunnel while the rest of the tunnel is built up with concrete wall. This approach was so effective during Brunel’s time that it is still used today.

Shipworms are both pests and a useful source of ideas with a certain historical importance in their impact on our lives. However, I suspect that most people focus on the damage that they cause, which I think unfair as this is really a problem of our own making. Through centuries of building wooden structures in the sea we likely caused rapid increase in shipworm numbers by providing with far more habitat than they would have normally had. Such are the long-lasting consequences of our meddling with the ocean’s ecosystem, consequences that we rarely see coming.

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Thanks for reading

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Image sources

Michael C Rygel. 2006. [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]. https://commons.wikimedia.org/wiki/File:Shipworm_mcr1.JPG

All other images are public domain and do not require attribution

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