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

Abstract

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.

Keywords: Aglaophenia; Epiphytes; Hydroid; Hydrozoa; Sargasso Sea; Sargassum.

Figure 1. Morphological differences between Sargassum forms as described in Parr (1939) and Schell, Goodwin & Siuda (2015).

S. fluitans have thorny stems whereas S. natans have smooth stems. Bladder and blade attributes differ widely among forms. (A) For S. fluitans III, thorns on stem are present, blades are wide, bladders are devoid of spines, and bladders are oblong. (B) For S. natans I, stem is smooth, blades are narrow, spines are present on bladders, and bladders are spherical (C) For S. natans VIII (photo credit: Janet Bering), stem is smooth, blades are wide, bladders are devoid of spines, and bladders are spherical.

Figure 2. Aglaophenia latecarinata hydroids.

(A) Isolated Aglaophenia latecarinata specimen. (B) A colony of the epiphytic hydroid species, Aglaophenia latecarinata, attached to Sargassum stem. Arrow points to stolon connecting genetically identical units of a single colony.

Figure 3. Cruise tracks.

Each cruise is represented by a different color. Cruise labels are Sea Education Association cruise numbers. C259 and C263 took place in 2015, C266 took place in 2016, C273 took place in 2017, and C277 and C279 took place in 2018.

Figure 4. Geographic distribution of hydroid samples by Sargassum form.

Lines indicate boundaries between oceanographic regions (Gulf Stream, North Sargasso Sea, South Sargasso Sea, Tropical Atlantic, and Caribbean) as described in the text. Each circle represents a single station and circle size corresponds to the number of Sargassum samples collected at each station. Yellow indicates stations where only S. fluitans III was collected, green indicates stations where only S. natans VIII was collected, and maroon indicates stations where both S. fluitans III and S. natans VIII were collected.

Figure 5. Haplotype network of Aglaophenia latecarinata sequences.

Circle size reflects the number of individuals possessing a given haplotype (n). Yellow indicates hydroids found on S. fluitans III and green indicates hydroids found on S. natans VIII.

Figure 6. Maximum likelihood phylogeny of the Aglaopheniidae.

Support for nodes is indicated by ML bootstrap values before the slash (only those above 50 are shown) and Bayesian posterior probabilities after the slash (only those above 0.5 are shown). A. latecarinata sequences are highlighted in red.