A shipworm
isn't a worm at all - it is a marine mollusc called Teredo. It eats and destroys wood
(they actually eat plankton
as well but that is not part of our story). People have known about shipworms for thousands of years because of their habit
of wrecking wooden ships and piers.
There are many different types of shipworm, the largest of which is up to 2 metres long. The worm has a head with two shells (they do the damage) and a wormlike body that follows behind. The worms can only live in water - take them out and they die.
Christopher Columbus was said to have been marooned on Jamaica in 1503 after his
ships were eaten by shipworm. It is also said that Brunel got his idea for lining the
tunnel under the Thames from shipworms that line their burrows.
The shipworm
invades the wood while it is still in the tiny larval stage. The entrance hole may be too small even to see.
The worm then burrows into the wood and grows quickly.
The shipworm uses the shells on its head to burrow. Their ridged and rough surfaces rub the wood away as the worm first turns its head one way and then the other. This cuts away a perfectly circular tube that is just a bit larger than the shell itself. The worm then eats the wood it has cut away, turning the cellulose in the wood into glucose that it uses for energy.
The wormlike body follows behind the shell, producing a substance like chalk to line the burrow. You can see this whitish lining in the picture.
The worm gets its oxygen from water. It draws the water in then passes it out again through two tubes on its tail called siphons. These stick out from the opening of the burrow but can be pulled in and the burrow closed by special small plates called pallets. These seal the tube so tightly that shipworms can survive when the timber is temporarily out of water. This is very important if the wood is exposed during low tide.
A piece of wood may be infested with shipworm, but they will deliberately avoid each other's tunnels. Instead they twist and turn their tunnels until the wood becomes a mass of tubes and holes, and eventually collapses.
This may be the first hint of a problem and is far too late for the wood to be saved.
We can protect timber from shipworm. Several hundred years ago it was realised that shipworms don't like copper, so nails were made from copper and hammered into the ships' hulls. A lot of the copper was mined from Parys Mountain on Anglesey in North Wales (the so-called copper mountain). Wood can still be coated in copper or another poison (usually creosote). The poison can also be injected into the wood, making it even more unattractive to shipworms. Modern boats are often made from fibreglass rather than wood so are protected.
Despite these steps the shipworm remains a major pest, particularly for very old ships that are of historical value. An example of one of these is the Jhelum, a 19th century sailing ship beached in the Falkland Islands. You can read about this ship below.
It should be remembered that shipworms also have their uses. If they did not eat so much wood the oceans and beaches would be full of bits of old wood.
The Jhelum is a ship that has problems with shipworms. When she was active she was a 428-ton sailing ship, built in Liverpool
in 1849. She sailed regularly to the west coast of South America for copper ore and guano. This painting, of the Helen of
Liverpool Arriving in the Mersey, (painted by Samuel Walters) shows how the Jhelum may have looked in its heyday. The ship is
shown at the mouth of the river Mersey and is painted from three different angles.
In 1871 she was damaged and abandoned in Port Stanley in the Falkland Islands, and there she remains. She is special because she is the only accessible example (she is beached) of a mid-19th century Liverpool-built wooden sailing ship in the world.
The shipworms were found in the timbers of the Jhelum in February 2002 and are making rapid progress. It is thought that their appearance may be connected to the increase in sewage released into the harbour.
This piece of wood was taken from the Jhelum. You can see a large shipworm tube about a centimeter thick (it is a different species to the worms that did the damage in the piece above - they're a lot bigger). The smaller holes have been made by gribbles. They look like wood lice, are about half a centimeter in length with 14 legs. Like shipworms they eat wood.