Two sympatric species earlier differentiated by means of starch gel electrophoresis within Polydora cf. ciliata from the Sea of Japan are here identified and their adult and gamete morphology are described and illustrated. One species bores into various shells and coralline algae while the other bores only into gastropod shells occupied by hermit crabs. Both species have the prostomium anteriorly incised to rounded, 0–4 eyes, the caruncle extending to the end of chaetiger 2, heavy falcate spines of chaetiger 5 with a lateral tooth, branchiae beginning on chaetiger 7, and the pygidium disc-like to cup-shaped with a dorsal gap to only an incision. Males of the two species differ in aggregations of spermatids. The more opportunistic borer, which occurs in a greater number of hosts, always has 8-cell-aggregations of spermatids. These worms are here referred to as P. calcarea. The other species, in which spermatids are always interconnected in tetrads, is here described as a new one, P. manchenkoi sp. nov. The two species also differ in that prominent brownish-yellow pigment appears on the posterior segments after fixation in formaldehyde in P. manchenkoi sp. nov. but not in P. calcarea. It is suggested that spermatid aggregates and spermatozoan morphology be included in future polychaete descriptions when possible.