Sea slug invasions: the non-native Archidoris pseudoargus (Nudibranchia: Dorididae) moves south and the native Doriopsilla pharpa (Nudibranchia: Dendrodorididae) moves north on the U.S. Atlantic coast

The European sea lemon nudibranch Archidoris pseudoargus was introduced by shipping to the Northwest Atlantic Ocean sometime prior to 2017, the year it was discovered in the Gulf of Maine, USA and in Nova Scotia, Canada.In 2023, it was discovered in Long Island Sound, a large estuary of national significance south of the Gulf of Maine.The native lemon drop nudibranch Doriopsilla pharpa, originating in waters south of New England from Maryland to Florida, was first detected in the Sound subtidally in 2019.The two species, both well-known sponge predators, are not known to co-occur as of mid-2025.Sponge population declines in the Gulf of Maine have already been linked to A. pseudoargus predation and other bioinvasions, especially non-native colonial ascidians which compete with sponges for space.Here, we report the first discoveries of these nudibranch species in Long Island Sound and consider their potential for deleterious effects on the benthic communities of southern New England.