Abstract

Pelagic Sargassum is comprised of two species distinguished by stem characteristics. S. fluitans has thorny stems and S. natans has smooth stems (Parr 1939). Each species is further divided into multiple morphological forms, although S. fluitans III Parr and S. natans I Parr historically dominated abundance by 90% or more (Parr 1939). Until recently, pelagic Sargassum observations indicated abundance maxima in the Sargasso Sea, at the center of the North Atlantic gyre, and the Gulf of Mexico (Parr 1939, Gower and King 2011, Hu et al. 2016). The Caribbean region, Brazil, and the west African coast have experienced episodic inundations of pelagic Sargassum since 2011. Coincident with the recent abun-dance increase in the Caribbean, there has been a shift in pelagic Sargassum diversity. In the western tropical Atlantic, eastern Caribbean and Antilles Current (north of Puerto Rico) during late 2014, Schell et al. (2015) documented mixed assemblages of pelagic Sargassum overwhelmingly dominated by the previously rare S. natans VIII Parr. However, with broad leaves and bladders often lacking spines, S. natans VIII Parr was mistaken for S. fluitans III Parr in some initial reports from the region. To compound the confusion associated with identification, Aglaophenia latecarinata, a hydroid epibiont that was historically exclusive to and dominant on S. fluitans III Parr (Calder 1995), was also dominant on S. natans VIII Parr.

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