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

The Gulf of Maine (GOM) has seen an increasing number of introduced species, some of which have significantly impacted 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 the species had been established before 2017. Additional field observations and literature searches revealed specimens occurring from Nova Scotia to the northern end of the Cape Cod Canal, which poses the question, when and where did the initial introduction take place and how? Genetic analysis confirmed the species as Doris pseudoargus with genomic similarities to specimens from Northern Europe. Until this introduction, the GOM had only one species of sponge feeding nudibranch, Cadlina laevis , which is a trophic specialist on one genus of sponge. Unlike this endemic, stenotrophic feeding dorid, the introduction of another large sized, sponge predator known to feed on diverse sponge species, that occur from the intertidal to at least 25 m in depth has the potential to significantly impact community structure over a wide variety of hard bottom habitats.