Abstract

The genus Sargassum harbors the only holopelagic species of macroalgae in the world, S. fluitans and S. natans. Distributed throughout the Caribbean, Gulf of Mexico and North Atlantic, both species exhibit a wide range of morphologi-cal variation (Winge 1923, Parr 1939). Historically, common morphological variants of Sargassum included S. fluitans III and S. natans I, with greatest abundances in the Sargasso Sea and Gulf of Mexico. A third, previously-rare form, morpho-logically described as S. natans VIII, has recently inundated beaches in the tropical Atlantic and Caribbean, resulting in devastating ecological and economic impacts. These unprecedented accumulations reveal shifting patterns of Sargassum distribution, abundance and diversity in the Atlantic (Gower et al. 2013, Johnson et al. 2013, Schell et al. 2015). Molecular tools help resolve the genetic and evolutionary underpinnings of changes in Sargassum biodiversity while furthering nuanced identification of Sargassum morphological variants. Sea Education Association has amassed decades of Sargassum observations and specimens on repeat cruise tracks in the North Atlantic and Caribbean. Here, we developed molecular tools to assess genetic divergence and differentiate among Sargassum forms collected at sea.

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