You often find keel worms on fish crates and other drifting objects which wash ashore. There are all kinds of species. The large tubes in the picture above are from the Pomatoceros triqueter, which you also find on stones. The small spiral tubes are Spirorbis spirorbis, which are often found on large brown seaweed species such as bladder wrack. They eat plankton, which they filter through a crown of feathery tentacles.