Driftwood with Shipworms
On occasion a piece of driftwood may be found with numerous little holes in it. Some times those holes may still be be lined with a thin layer of white calcium carbonate, a remanant of it's previous occupant, the shipworm.
Genus Teredo and Bankia
Shipworms are not actually worms but wood boring clams. The free swimming larvae will attach themselves to any piece of unprotected wood below the water line where the colony of shipworms will quickly honeycomb the wood until it loses all strength and collapses. Their shells have evolved into a cutting tool that allows them to burrow into the wood where they make their home. The cavity is then lined with a calcerous coating. The shipworm will remain in the cavity for the rest of it's life, where it will only have to extend it's siphon from the hole to feed on plankton. As the shipworm grows it tunnels deeper into the wood weakening the wood still further.
Each year they do a tremendous amount of damage to piers and wooden boats and in the days of the sailing ship they were responsible for many sinkings. Christopher Columbus, himself, lost two of his three ships during his first voyage to the new world due to shipworms. Many treatments have been used over the years to protect wood including coatings of pine tar or creosote as well as copper sheathing and special antifouling paints. Ship worms can only survive in salt water and it once was a common practice to sail up river for the purpose of anchoring a wooden hulled vessel in fresh water for a few weeks to get rid of a shipworm infestation.