Older specimens of the ribbed saddle-oyster are usually covered by tubeworms and sponges.
Empty serpulid shells can sometimes be confused with the shells of a family of marine gastropod mollusks, the Vermetidae.
Because of all this, the shells resemble the calcareous tubes of certain marine worms, for example worms in the polychaete family Serpulidae.
The empty calcareous tubes of certain marine annelid tube worms, for example the Serpulidae, can sometimes be casually misidentified as empty vermetid shells, and vice versa.