A Post from the Past 
Despite the name, shipworm is not a worm but a bivalved mollusc called Teredo navalis Linnaeus. The photograph above shows a piece of driftwood on the sand at Rhossili looking rather like the head of a crocodile. The wood is riddled with tunnels created by the shipworm.
Below, you can see how the entire piece of wood has been attacked until it resembles a honeycomb. You don’t need a lot of imagination to understand how big a threat this pest was to timber-built ships in the past.
Each tunnel is lined with a chalky deposit which you can see in the close-up photograph below.
For more information on the Great Shipworm see the pages of the Marine Life Information Network.