What's that carving on the wood?

Picking up ocean driftwood, you'll sometimes be lucky enough to pickup a piece or two that look like they've been carved with little squiggly lines. Many think that it's worms that have done it (many have thought that I did it!) but no - you're looking at the work of the teredo (Teredo navalis - Wikipedia). This little guy is actually part of the mollusk family - related to muscles and clams. They're much smaller though - and don't have a shell to protect them. They're favorite thing to eat is the layer of soft wood directly under the bark of a piece of wood that's found itself afloat in the ocean. They eat their way through - leaving behind the trails that we see. The bark protects the teredos as they go about their business. Once they've eaten enough of that soft layer, the bark falls from the wood and the teredos move onto another piece of wood starting the debarkation process on the next piece that floats by.