A European nudibranch new to the Gulf of Maine: Doris pseudoargus Rapp, 1827.

Abstract The Gulf of Maine has seen an increasing number of introduced species, some of which have had significant impacts on benthic community structure. In 2017, a number of specimens of the European dorid nudibranch, Doris pseudoargus, were observed on rocky ledges in waters off Cape Ann, Massachusetts. The presence of numerous specimens and egg masses suggested that the species had been established before 2017. Subsequent observations and collections have discovered specimens ranging from sites off Wedgeport, Nova Scotia to the northern end of the Cape Cod Canal. Genetic analysis has confirmed the species as Doris pseudoargus and similar to specimens from European locations. Historically, the Gulf of Maine had only one species of sponge feeding nudibranch. Unlike this endemic, stenotrophic feeding dorid, the introduction of a large sized, sponge predator known to feed on diverse sponge species has the potential to significantly impact community structure, as evidenced on vertical rocky substrates where its natural history has been observed.