A Gastropod Ate My Ship

Christopher Columbus got stranded in the Caribbean in 1503 because these things ate his boat.  The Great Shipworm is a mollusc, not a worm, but the 'ship' bit is perfectly correct.

Me and my longtime Hebridean travelling companion, were wandering about on the shores of Colonsay and found these chunks of tree trunk riddled with perfectly machined tunnels about 1 cm in diameter and beautifully lined with a hard chalky material.  Much of the linings had eroded away but you can see it in the bottom right hole in the picture below. Also the hole above that seems to have a sort of calcium divider across it - not sure what is going on there.

The molluscs themselves were long since gone but some robust bashing and rattling caused some fragments of shell to drop out.

The tunnels are beautifully formed and those we could see were at least 12 cm long. The timber looked just like someone had been messing about with a drill bit.  Allegedly, they keep out of each others tunnels by inter-twining but never breaking through.  Maybe that's the point of the hard calcium lining? Who knows.

The odd thing is that although the animal can grow up to 20 cm in length, its shells (it's a bivalve) are reduced to tiny bits - bit being the operative word - because the shells are at its head end and it does indeed use them as a drill bit, twisting and turning to drill out the tunnel. 

What I can't work out though is what happens when the animal grows.  How does it keep widening out its tunnel when the drill bit is stuck up at the front end at the coal face.  Can it turn round head over heels and double back? Maybe it just concertinas up and shortens itself? In which case how come the entrance doesn't stay narrower than the inner tunnel? And does it wait until it is fully grown before it lines the tunnel? Ach...so many questions?

Some old references say that shipworms do not eat the wood but just use the tunnel as a home. More recent papers revise this and describe special enzymes that the animal produces to break down the cellulose for digestion - so it really does eat ships after all.  And anything else made of wood.

By another of those delightful coincidences I happened to tune in to BBC 2 last night just at the point where they were describing a wreck that had been covered in sediment until recently.  Its a 17th century armed merchant ship known as the Swash Channel wreck.  From fresh as the day it sank to a swiss cheese in no time and they reckon that within another few years it could all be gone . The culprits - the shipworm of course - along with its partner in crime, the fabulously named Gribble (whose wood crunching enzymes, incidentally, are causing a great excitement in the green energy world.)

If you didn't catch 'Britain's Secret Seas' have a look on the BBC i-player (here) and zip through to the 28th minute for a 5 minute piece including some great samples and photos together with an x-ray of the beast in its burrow.

I also read that in the Baltic Sea, where archaeologists have discovered 100s of  really old and important wooden vessels, shipworms have always been absent and not a concern.  However, in recent times they have begun to appear and are now munching their way through a smörgåsbord of delicious wrecks.  I suppose the real reason is not fully understood but as shipworms don't survive in colder water, a presumption has been put about that that old devil called global warming is behind this, through a rise in sea temperature. Could just be that old devil called evolution of course but the change does seem rapid.

Which is to say - there's more to a hole in a piece of driftwood than first meets the eye.