Mobile fauna communities associated with the three common holopelagic Sargassum morphotypes in the western North Atlantic, Caribbean Sea, and Gulf of Mexico

Holopelagic Sargassum is an oasis of life in the North Atlantic, supporting sessile and mobile fauna that attract ecologically and economically important migratory fauna. Presently, two species and three common morphotypes of pelagic Sargassum are recognized, with distinct but overlapping distributions and varying structural complexity. S. fluitans III and S. natans I display higher order branching; the former’s denser foliation creates more complex habitat. In contrast, the previously rare S. natans VIII is structurally simple. Differences in mobile fauna diversity between Sargassum morphotypes may vary their ecological value as a habitat. In 2015/2016, 373 dip net samples of the algae were collected in the north equatorial Atlantic, Antilles Current, Caribbean basin, Gulf of Mexico, Gulf Stream, and Sargasso Sea. 20,975 individual mobile fauna, from 32 taxa (species and groups of species), were recorded. Abundance of individuals and taxon richness both increased with Sargassum sample size. S. fluitans III supported more taxa per gram than S. natans VIII. Sargassum in the Sargasso Sea supported greater abundance of individuals than most regions as well as displayed the highest cumulative taxon richness; conversely, Sargassum in the Tropical Atlantic hosted the fewest taxa. Most assemblages were comprised of a few dominant and many rare taxa. Just four taxa were shared across all regions while eleven were shared across all three Sargassum morphotypes. This study is the most comprehensive of its kind to consider influences of Sargassum species/morphotype and geographic area on mobile fauna assemblage, which is particularly relevant as the algae’s regional distributions shift. Recent increases in central Atlantic Sargassum abundance and Caribbean beaching events, dominated by the structurally-simple and fauna-poor S. natans VIII, emphasize the need to better understand the ecological value and role of the various morphotypes.