Crossing the Atlantic: The tropical macroalga Caulerpa ashmeadii Harvey 1858 as a recent settler in Porto Santo Island (Madeira archipelago, North-eastern Atlantic)

The green macroalga Caulerpa ashmeadii was recently found in the North-eastern Atlantic Ocean, on the southern coast of Porto Santo Island (Madeira archipelago). Caulerpa ashmeadii was detected in a subtidal soft-bottom forming scattered patches, mostly among the green calcareous macroalgae Halimeda incrassata and Penicillus capitatus, within a depth range of 18–26 m. Some isolated patches were also observed in bare sand. The species is currently distributed over an estimated area of approximately 0.22 km2. Caulerpa ashmeadii may represent a recent settler (Nov. 2020), although the hypothesis of the species being present elsewhere in Porto Santo prior to our discovery cannot be discarded. Alternatively, the presence of C. ashmeadii in Porto Santo may indicate the influence of climate change on the composition and structure of benthic communities in the Webbnesia region, which further stresses the importance of conducting more intensive biological and environmental monitoring programmes. Overall, this new record constitutes an exceptional eastern expansion of this marine species by more than 4200 km, comprising its first register in the Eastern Atlantic Ocean.