Grandidierella Japonica Stephensen, 1938 (amphipoda: Aoridae) in the Northwest Atlantic Ocean

The aorid amphipod Grandidierella japonica Stephensen, 1938, originally described from northern Japan, is reported for the first time from the Gulf of Maine and Long Island Sound in the Northwest Atlantic Ocean. It was discovered among grab samples of eelgrass (Zostera marina Linnaeus, 1753) beds taken in Casco Bay, Maine in summer 2018 and has been found intertidally in Long Island Sound since 2013 along Connecticut shores. It occurs in habitats like its native range but also in rocky areas and tidepools in bedrock. The presence of adults of both sexes, ovigcrous females, and immature stages in all collections indicates the species is established. Among males, some morphological characteristics of gnathopod 1 vary with increasing total body length: the number of accessory carpal teeth increases, the carpus shape (length/width ratio) changes, and numbers of stridulating ridges increase. Maine and Connecticut specimens are distinguished from each other by the shape of male gnathopod 1 basis. Morphological variation, temporal differences in discovery, and separation by the biogeographic barrier Cape Cod suggests Maine and Connecticut populations originate from separate introductions.