Eurytemora carolleeae is a planktonic calanoid copepod, recently separated from the circumboreal E. affinis species complex. Its native range extends from the St. Lawrence River, Canada to the St. John’s River, Florida. It has been introduced to the Pacific coast of North America in San Francisco Bay, California and Grays Harbor, Washington. It is also known from the Great Lakes and the Gulf of Finland in the Baltic Sea. It is found in salt marshes and estuaries with low-salinity waters. It is abundant in low-salinity waters in San Francisco Bay, where it is a grazer on phytoplankton populations and a major food source for larval fish.