There has been some taxonomic confusion with the closely related C. submersum L. or C. muricatum Chamisso, which occur in Eurasia and can be defined in a narrow sense to exclude American plants (as reviewed in W). C. echinatum is widely scattered in eastern North America and the Pacific northwest, but it is much less frequent than demersum, especially in a zone from the mid-west to the mid-Atlantic coast (K, NS). In Ky., Tenn. (Ch) and Mo. (Y) echinatum is known from a few natural ponds or sloughs that are somewhat removed from major river valleys, occasionally even in sinkhole ponds or springs. The alien Myriophyllum spicatum could be a serious competitor in these habitats, especially with increase in eutrophic conditions.