Abstract: Geographically isolated populations of the same species often encounter distinct environmental conditions that shape their survival strategies. These local pressures can drive the emergence of unique phenotypic traits that enhance adaptability. With accelerating ocean warming, such selective pressures are intensifying. In northern Iberian Peninsula, marine forests have undergone marked declines, yet the warm-temperate red alga Gelidium corneum remains a dominant canopy species in subtidal rocky habitats. However, recent studies report biomass declines, particularly in the easternmost region. This variability makes G. corneum a good model for investigating adaptive processes in macroalgae populations. Then, we selected three G. corneum populations along the northern Spanish coastline, spanning a west-east summer temperature gradient. We monitored the propagation process, from fragment reattachment to the development of new individuals, revealing phenotypic differences in the latter stage. Overall, populations adapted to warmer conditions showed greater growth capacity both in their natural habitat and when translocated to colder regions. Notably, shoot elongation was greater at the eastern site with colder waters, across all populations, with overall mean values (± SE) of 0.24 ± 0.01 cm (n = 469) for the western, 0.51 ± 0.02 cm (n = 1123) for the central, and 0.47 ± 0.01 cm (n = 2637) for the eastern populations. These results suggest local adaptation may influence G. corneum's thermal tolerance and resilience to varying environmental conditions. In conclusion, we found evidence of phenotypic plasticity in G. corneum's vegetative propagation across populations, with the warm-adapted population offering a promising avenue for species persistence.
