Alien vs native species: testing for holdfast preference of the seahorse Hippocampus reidi Ginsburg, 1933

Abstract The seahorse Hippocampus reidi is a cryptic, benthic-associated fish species commonly found along the coastal rocky reefs of Rio de Janeiro, Brazil. In these environments, native populations now coexist with the invasive sun coral Tubastraea spp., yet the ecological implications of this interaction remain poorly understood. This study aimed to evaluate the holdfast preference of H . reidi by experimentally testing the seahorse’s selection between the invasive coral and native macroalgae ( Sargassum sp. and Spyridia sp.), as well as to assess potential behavioral alterations or physical impacts resulting from contact with the coral. Sixty trials were conducted, comprising 20 replicates for each condition, including two control groups. The results showed that H . reidi significantly preferred macroalgal substrates over Tubastraea spp., aligning with its natural habitat preferences. No physical injuries were observed after contact with the sun coral, suggesting a possible tolerance of H . reidi to the presence of this invasive species. These findings contribute to the understanding of species interactions in invaded rocky reef ecosystems and highlight the importance of monitoring behavioral responses of native fauna to invasive species.