Polydora commensalis (Polychaeta, Spionidae) and closely related species construct characteristic tubes in gastropod shells occupied by hermit crabs (Crustacea, Paguridae). Similar tubes are common as trace fossils, herein named Helicotaphrichnus commensalis, in Pliocene and Pleistocene strata of California and Baja California, and they provide fossil evidence for the persistence of the spionid-pagurid commensal relationship at least from Pliocene time to the present.