Biodiversity on the move: Epibiotic communities associated with pelagic sargassum in the northeast Atlantic.

Between late 2023 and early 2024, massive amounts of rafting Sargassum, made up of two co-occurring species (Sargassum natans and Sargassum fluitans; class Phaeophyceae) were recorded in the Azores and Madeira archipelagos (NE Atlantic). This phenomenon provided a rare opportunity to investigate their epibiont assemblages. Offshore algal samples were collected around São Miguel (Azores) and Madeira islands in February 2024. Sargassum fluitans III and S. natans VIII were the dominant species, supporting diverse communities of bryozoans, hydroids, crustaceans, and molluscs. A total of 15 motile and 10 sessile epibiotic species were identified. Epibiotic assemblages differed significantly in species abundance and composition between the two archipelagos, suggesting the influence of distinct local environmental conditions and ocean currents, ultimately affecting which species thrive in each location. Notably, S. fluitans III, typically the dominant morphotype linked to the Great Atlantic Sargassum Belt (GASB), was most prevalent, suggesting that the influxes in the Azores and Madeira may have originated from this region rather than the Sargasso Sea. However, the timing of sightings, the lack of known direct currents connecting GASB to both archipelagos and the proximity of the Sargasso Sea, indicate that the exact origin remains debatable. This study provides a novel geographic perspective for examining the structure and composition of epibiont communities associated with rafting Sargassum.