Possible causes of amphi-Atlantic distribution of Orchestia gammarellus (Pallas, 1776) (Crustacea, Amphipoda, Talitridae) in the North Atlantic: a review

Hypotheses concerning the modern distribution of Orchestia gammarellus (Crustacea, Amphipoda, Talitridae) and its causes in the North Atlantic are discussed. The synanthropic dispersal hypothesis of Henzler and Ingólfsson (2008) considers O. gammarellus as originating on the eastern shore of the North Atlantic and being transported by humans to Iceland and the western Atlantic shore (Newfoundland and the Maritime Provinces of Canada). The Eocene and natural dispersal hypothesis of Myers and Lowry (2020) proposes a geologically earlier origin of O. gammarellus when the west and east shores of the North Atlantic were still connected. Present day amphi-Atlantic distribution was explained by vicariance, with the vicariant event causing separation of O. gammarellus being continental drift drawing apart the west and east shores of the North Atlantic. A post-glacial natural dispersal hypothesis proposed herein, involves transport on ice floes or in driftwood from European shores to Iceland and the Atlantic Provinces of Canada. The small genetic distances amongst populations found by Henzler and Ingólfsson (2008) at the COI gene are inconsistent with the Eocene vicariance hypothesis. On evolutionary grounds, we question Myers and Lowry's (2020) designation of the Icelandic and Canadian populations as a new species of Orchestia. Existing molecular and morphological data are insufficient to distinguish between human-aided dispersal and natural rafting.