A survey of epibiont hydrozoans on Sargassum .

The brown alga provides a natural substrate occupied by hydrozoans in shallow marine waters. A global count in 2007 listed 39 epibiotic species of Hydrozoa growing on , but more studies have been published since, therefore, an update is timely, particularly due to the increased abundance of in the Caribbean. This review, based on a recent literature survey and new records from Mexico, includes 133 publications of epibiotic hydrozoans on spanning 220 years, from 1802 to 2022. A total of 131 hydrozoan species were recorded on 26 species of , most belonging to the subclass Hydroidolina (130), with only one record of a trachyline medusa (, subclass Trachylinae). Most publications centered on the Tropical Atlantic, where the greatest number of hydrozoan species (67 species) were recorded. All hydrozoan species possess a hydrorhiza, except one hydromedusae species that attach to via adhesive tentacles. Most of the hydrozoan species associated with exhibited a benthic life cycle (93 species) and are comprised of erect, branched colonies (67 species) and large hydrothecae (69 species). Although the number of studies of epibiotic hydrozoans on has increased since the mid-20th century, nevertheless hydrozoan richness has not reached an asymptote. Therefore, more sampling of species would likely identify more hydrozoan species associated with , especially among benthic , and might help reveal potential biogeographical and ecological patterns between and hydrozoan epibionts.