Morphological identification of two invading ascidians: new records of Ascidiella aspersa (Müller, 1776) from Nova Scotia and Diplosoma listerianum (Milne-Edwards, 1841) from New Brunswick and Quebec

We report new observations of two invasive ascidian species, the solitary Ascidiella aspersa and the colonial Diplosoma listerianum, from Eastern Canada, which include some records from new sites. Because these species share many superficial characteristics with native and non-native species in the region, identifications based on detailed taxonomic studies of internal structures-and not only on external features-are essential and can complement molecular methods. In this report, specimens of A. aspersa collected from Nova Scotia in 2013 and D. listerianum from New Brunswick and Quebec in 2017 were described morphologically to confirm species identity. Specimens of both were collected one year after initial discoveries in Nova Scotia and New Brunswick, respectively. Specimens of D. listerianum from Quebec were collected after a six-year hiatus. This report contributes a new record of A. aspersa 30 km (by sea) north of the site of the first record in Canada. It also confirms the first record of D. listerianum and documents its rapid spread in New Brunswick. This report is the first time that A. aspersa from Eastern Canada and D. listerianum from New Brunswick and the second time that D. listerianum from Quebec was explicitly identified based on an examination of internal features. Species confirmation of these easily overlooked or misidentified taxa contributes a time-stamp on current invasion fronts and suggests that the risk of further introductions and spread of these invasive species is present at un-invaded sites throughout Eastern Canada.