The distribution and mitochondrial genotype of the hydroid Aglaophenia latecarinata is correlated with its pelagic Sargassum substrate type in the tropical and subtropical western Atlantic Ocean

The pelagic brown macroalga Sargassum supports rich biological communities in the tropical and subtropical Atlantic region, including a variety of epiphytic invertebrates that grow on the Sargassum itself. The thecate hydroid Aglaophenia latecarinata is commonly found growing on some, but not all, Sargassum forms. In this study, we examined the relationship between A. latecarinata and its pelagic Sargassum substrate across a broad geographic area over the course of 4 years (2015–2018). The distribution of the most common Sargassum forms that we observed (Sargassum fluitans III and S. natans VIII) was consistent with the existence of distinct source regions for each. We found that A. latecarinata hydroids were abundant on both S. natans VIII and S. fluitans III, and also noted a rare observation of A. latecarinata on S. natans I. For the hydroids on S. natans VIII and S. fluitans III, hydroid mitochondrial genotype was strongly correlated with the Sargassum substrate form. We found significant population genetic structure in the hydroids, which was also consistent with the distributional patterns of the Sargassum forms. These results suggest that hydroid settlement on the Sargassum occurs in type-specific Sargassum source regions. Hydroid species identification is challenging and cryptic speciation is common in the Aglaopheniidae. Therefore, to confirm our identification of A. latecarinata, we conducted a phylogenetic analysis that showed 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). A nominal A. latecarinata sequence from a second Brazilian location (Alagoas) likely belongs to a different species.

Quantitative Estimation of Epiphytic Invertebrate Populations

Quantitative comparisons were made of the relative accuracy, precision, and cost of various population estimation techniques for epiphytic invertebrates. Quadrat clipping was often more accurate than the Gerking, Macan, Minto, or KUG samplers, yielding population estimates an average of sevenfold greater where differences were found. All quadrat sizes (112 cm2 to 1 m2) usually yielded equal estimates of epiphytic invertebrate populations. The spatial variance of replicate epiphytic invertebrate samples increased with population density and decreased with the size of sampler employed (R2 = 0.94; n = 497). All samplers yielded equivalent levels of sampling precision. A method providing provisional estimates of the requisite number of replicate samples for a given level of precision is presented. Greater replication is required at low invertebrate population density or when small samplers are used. The most cost effective sampler size for the phytofauna is about 500 cm2. Improved sampling design can result in up to fivefold reductions in sampling effort. Taxa examined include Acari, Amphipoda, Chironomidae, Cladocera, Copepoda, Gastropoda, Hirudinea, Lepidoptera, Nematoda, Oligochaeta, Ostracoda, Trichoptera, and Turbellaria.