Sargassum species as hydrozoans substrates: Key patterns of association or just availability?

The genus Sargassum comprises both benthic and holopelagic species, serving as basibionts for numerous marine organisms, including frequent epibiotic occurrences of hydrozoans (Cnidaria). Several aspects of the epibiotic interaction between hydrozoans and Sargassum species remain insufficiently explored. This includes patterns such as the hydrozoan community’s composition and abundance across sections and structures of Sargassum. To address these gaps, we conducted an investigation aiming to uncover potential differences in the species composition and abundance of hydrozoan epibionts on benthic and holopelagic Sargassum species (BSS and HSS, respectively) from the Atlantic and Pacific Oceans of Mexico. Additionally, we sought to characterize hydrozoan epibionts preferences for specific sections and structures of Sargassum. Our analysis identified 18 hydrozoan epibiont species on Sargassum, belonging to Leptothecata (88.9%) and anthoathecate (11.1%), with one new register for Sargassum species. Each Sargassum species harbored a distinct hydrozoan epibiont species with a marked higher mean total coverage. The total percentage coverage of hydrozoans on BSS exhibited less variability than HSS. The basal section of BSS hosted the highest number of species. Concerning Sargassum structures, a greater species richness was found on the axis and the blades. Our study also demonstrates that differences in hydrozoan species primarily depend on the geographical distribution of Sargassum species.