Type-Specific Source Regions for Pelagic Sargassum Indicated by Mitochondrial Genotype of the Epiphytic Hydroid Aglaophenia latecarinata

Holopelagic Sargassum algae supports floating biological hotspots far from land in the tropical and subtropical Atlantic Ocean. A variety of epiphytic invertebrates, including the hydroid Aglaophenia latecarinata, are among the many animals supported by Sargassum. In 2011, severe Sargassum inundations began occurring along Caribbean, Gulf of Mexico, Brazilian, and West African coasts, causing significant negative economic impacts. These inundations are comprised of mixed assemblages of Sargassum morphotypes, including the previously rare S. natans VIII. We examined the relationship between A. latecarinata and its pelagic Sargassum substrates across a broad geographic area from 2015-2018. We found that the distribution of the most commonly observed morphotypes (S. fluitans III and S. natans VIII) suggested the existence of distinct source regions for each. We sequenced the 16S mitochondrial DNA of 139 A. latecarinata hydroids originating from individual Sargassum fragments and found that mitochondrial genotype was strongly correlated with Sargassum substrate form. Further, A. latecarinata exhibited significant population genetic structure, consistent with the distribution of the Sargassum forms. Our results suggest that hydroid settlement may occur in Sargassum-type-specific source regions. We also compared our A. latecarinata sequences to 48 other aglaopheniid sequences on Genbank. We found that while the genus Aglaophenia was not monophyletic, all A. latecarinata haplotypes associated with pelagic Sargassum belonged to the same clade and were likely the same species as previously published sequences from Florida, Central America, and one location in Brazil (São Sebastião). These findings have implications for understanding Sargassum-type-specific source regions and modeling the mechanisms driving recent episodic Sargassum inundations, as well as for understanding Sargassum-associated faunal community composition and the ecological value of each Sargassum morphotype.