An unexpected journey – the arctic deep-sea halicreatid trachymedusa Botrynema brucei ellinorae off Florida: a reassessment under an integrative taxonomic approach

The study of the trachymedusa Botrynema has a long history of research, encompassing over 120 years of exploration in the deep sea. Two distinct morphotypes are recognized within Botrynema: one with a characteristic apical knob and another without it. Both morphotypes are present in the subspecies B. brucei ellinorae, while only specimens with a knob are known for the remainder of B. brucei. Specimens with a knob have been reported across all oceans and latitudes, whereas specimens without a knob are only known from Arctic and Subarctic regions. In this study, we use historical records, molecular tools and phylogenetic analyses to challenge the widely accepted notion of a cosmopolitan distribution for B. brucei as traditionally understood. We propose a range expansion to the subtropical western Atlantic Ocean for B. brucei ellinorae based on molecular data and hypothesize the existence of a mesopelagic soft barrier in the North Atlantic region as a plausible explanation to emerging biogeographical patterns revealed in this study.