Shiploads of Shipworms: Mussel-power and the Industrial Revolution

This boring mollusc is anything but boring

Photo by Zoltan Tasi on Unsplash

On New Year’s Eve, 1731, a storm swept in from the North Sea. Whipped up by gales, waves ripped away timber piles from protective dikes along the coast of Holland and West-Friesland. Without the wooden breakwaters, the earthworks were in danger of collapse. When the wardens inspected the damage, they found the timber riddled with worms. Under the force of the waves, the piles had crumbled.

Some said this plague of worms was a sign from God and the only way to prevent more destruction was by penance and prayer. Others took a more scientific approach: the way to defeat the worms was to understand their biology and use that knowledge to devise a solution.

Over the next few years, natural philosophers, engineers and theologians studied the worms and their impact on the sea defences. The answer to the problem was simple but expensive — replace the timber with stone facing. Paalwormen [pile worms] could not burrow into rock. The polders were safe again.

The animal that caused the crisis was the shipworm (Teredo navalis). Despite its appearance, it is a bivalve mollusc, a close relative of marine and freshwater clams. Although its form is highly modified — long and thin for burrowing — its kinship is revealed in the siphons that draw in and expel water, and the small, paired shells that cover the front end. These shells are too small to enclose and protect the animal. Instead, they are used to scrape away timber. Their surfaces are toothed like a wood rasp; they excavate slowly but effectively.

In 1733, German academic Gottfried Sellius published Historia Naturalis Teredinis seu Xylophagi Marini, a monograph on the shipworm. In it, he described the internal and external anatomy, its reproduction and ecology, and discussed references to it in the classical literature. Linnaeus called it Teredo navalis, a name that emphasised its danger to sea-going vessels. He listed its habitat as ships and wharves and conjectured that it had originated in the warm waters of ‘the Indies’.

Illustration from ‘Historia Naturalis Teredinis…’. Image from Biodiversity Heritage Library.

Shipworms are prolific. Under favourable conditions, females can produce millions of eggs several times a year. To give offspring a head start, the eggs are brooded in the gill chambers before being released into the sea as microscopic veliger larvae. Veligers spend days to weeks in the ocean before they settle on submerged timber. On settling, they tether themselves with a fine thread to avoid being washed away, and begin to tunnel. In six weeks, they are ready to reproduce.

But why had they suddenly appeared in numbers so great they forced the remodelling of Dutch sea defences? The weather. Summer and autumn had been hot and dry, lowering the freshwater outflow from the Rhine. The combination of warm water and increased salinity was optimal for Teredo.

Teredo navalis had always been a problem for wooden ships. From the earliest days of sailing, boats that were in the sea for long periods, whether moored or underway, were colonised by these bivalves. To repair the damage, they had to be careened or put in dry dock and the timbers replaced. It took time. It took money.

Solutions included painting the hull with poisons or sheathing it in expendable timber, which could be stripped off and replaced when required. Lead sheeting was also used, often in conjunction with layers of timber. In the early days of the Vereenigde Oostindische Compagnie (Dutch East India Company), their ships were sheathed in two layers of lead with oak timbers sandwiched between them, and the whole enclosed in pine.

In the 1620s, the Dutch companies experimented with copper sheathing. Piet Pieterszoon Hein, who commanded vessels for the West-Indische Compagnie (West India Company), is reported to have encased the hull of his ship with copper. The rudder sternposts of VOC East Indiamen were often wrapped with copper, although it was an optional modification.

It was not until the 18th Century that the British Navy considered the use of copper sheathing. HMS Alarm was the first vessel to receive the treatment. After two years in the Caribbean, Alarm’s timbers were free of shipworm, and the order was given to extend sheathing to other naval vessels. But in the following years, a problem became apparent: the iron bolts that held the copper in place had corroded. The programme was suspended until the issue could be solved.

The American Revolutionary War and its European sequels soon prompted the British Navy to resume its programme. Copper sheathing made its ships faster and reduced the time they needed to spend in dock. To copper-bottom the entire fleet required huge amounts of metal. Seeing the benefits, merchant shipping adopted the practice. For miners in Cornwall, this was a boom time.

The Cornish copper industry used Newcomen steam engines to pump out water from the deeper shafts, but the machines required large amounts of coal and so were expensive to run. They were replaced by the more efficient Boulton & Watt steam engines. So many were purchased for use in copper mines that Boulton and Watt visited Cornwall for extended periods to supervise installations of their engines and fix problems as they arose. To avoid the repeated relocations, they appointed William Murdoch to oversee in the south-west. Working with Boulton and Watt, Murdoch refined and improved the engines.

Among the inventors who developed the use of steam power in industry was Cornish mining engineer Richard Trevithick. Trevithick constructed a high-pressure steam engine that was lighter and more efficient than the low-pressure B&W machines. In addition to its use in mining, the engine could also be used to drive wheels. In 1801, Trevithick took his steam carriage ‘Puffing Devil’ for a drive through Cambourne, Cornwall.

The Industrial Revolution in Britain was well underway.

“The Crowns at Botallack, Cornwall” by Christopher Dart is licensed under CC BY 2.0

But shipworms contributed one more thing to industrialisation.

Richard Trevithick was invited to London to assist in the construction of a tunnel under the Thames at Rotherhithe. In 1808, after digging more than 305 metres (1000 ft), he and his mining team had to abandon the project due to flooding.

Fifteen years later, Marc Isambard Brunel began work on a new Thames tunnel. He had designed a tunnelling shield that protected miners as they worked. The machine was an iron frame 6.4 m (21 ft) high from which miners hacked away at the face of the London clay. Once the area in front of the frame had been excavated to a pre-determined depth, the structure was moved forward, while bricklayers lined the tunnel walls behind.

Brunel was said to have found his inspiration for the shield from observing shipworms in timber, taking the way their shells worked for the mechanism of excavation and their protective calcareous tubes for the tunnel lining in an early form of industrial biomimicry.

Zoologist, writer, artist, museum fan, enjoying life in the tropical rainforest of Far North Queensland.

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