[A large reproducing population of Cyathura polita occurs in the Saint John River estuary, New Brunswick, Canada. This is the first record from Canada and represents a 180 km northeastward extension of the known range for this species. A large portion of the Cyathura population occupies deep (20 m), mesohaline (0.1-20‰) water on bottoms of moderate silt-clay content. The population exhibits protogynic hermaphroditism, a phenomenon common to cyathurans. This occurrence, disjunct from the main center of Cyathura polita distribution to the south, may be a result of an earlier widespread distribution during the Hypsithermal Period with a subsequent fracturing of the range during recent cooler climatic conditions., A large reproducing population of Cyathura polita occurs in the Saint John River estuary, New Brunswick, Canada. This is the first record from Canada and represents a 180 km northeastward extension of the known range for this species. A large portion of the Cyathura population occupies deep (20 m), mesohaline (0.1-20‰) water on bottoms of moderate silt-clay content. The population exhibits protogynic hermaphroditism, a phenomenon common to cyathurans. This occurrence, disjunct from the main center of Cyathura polita distribution to the south, may be a result of an earlier widespread distribution during the Hypsithermal Period with a subsequent fracturing of the range during recent cooler climatic conditions.]