Holopelagic Sargassum blooms in the tropical North Atlantic since 2011 are composed of two species, Sargassum natans and S. fluitans, and three morphotypes: S. natans VIII, S. natans I and S. fluitans III. The distinct morphology and the variations in space and time of the proportion of these three morphotypes suggest that they may have different physiology. For the first time, we have quantified the growth rates of these three morphotypes through in situ 9-day experiments on the coast of Martinique Island (French West Indies). Despite the non-optimal conditions for growth for these pelagic species and the short time of the experiment, we have observed that Sargassum fluitans III was growing faster (approximately twice as fast) than S. natans VIII and S. natans I. Sargassum natans I exhibited the slowest growth. The differences in tissue composition (CNP and CN natural isotopes) of morphotypes point to a greater benefit for S. fluitans III from the coastal localization of our experiment than for the two S. natans morphotypes, and suggest that S. natans I had achieved its last growth further offshore before our experiment. These contrasting growth performances are consistent with the dominance of S. fluitans III in recent observations in the Caribbean region and along the path from the Sargassum belt. This also makes this last morphotype the best candidate for cultivation. Making the distinction between the growth performances of morphotypes may improve the current predictive models about dispersal of these species.