Partnerships between different species always intrigue. What is the relationship between the partners? Is the symbiosis friendly and cooperative, opportunistic, mutually beneficial, or to the detriment of one partner? In European seas, there exists a relationship between a marine bristleworm (polychaete) and a hermit crab. It is well-known to marine biologists and fishermen, but may not be immediately obvious to casual observers. The worm is Neanthes fucata, a nereidid related to the King Ragworm that anglers often use as bait, and its host is the Common Hermit Crab Pagurus bernhardus. This hermit crab is the largest to be found in Welsh coastal waters, from the lower shore to over 100m, and it most frequently inhabits an empty shell of the Common Whelk Buccinum undatum