The nudibranch Doris pseudoargus is commonly called 'Sea Lemon' for its oval shape, rough texture, sometimes yellowish color, and size, up to 120 mm. It is one of the largest common nudibranchs on its native European coasts, where it ranges from Scotland and Norway to Spain and Greece. Beginning in 2017, this nudibranch was spotted by divers, from the Cape Cod Canal to Nova Scotia, across the Gulf of Maine, and identified by genetic and morphological means. Doris pseudoargus is a specialized predator on sponges, and could have impacts on these important constituents of rocky intertidal and shallow subtidal communities. This nudibranch could have been introduced through ballast water transport of planktonic larvae, or on sponges in hull fouling.