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.

Epibiont hydroids on beachcast Sargassum in the Mexican Caribbean

“…ovatum (Collins) W.R. Taylor; Ss, Sargassum stenophyllum (undefined authority); Ssw, Sargassum swartzii C.Agardh; St, Sargassum thunbergii (Merten ex Roth) Kuntze; Sto, Sargassum tortile (C. Agardh) C. Agardh. (1) Parr (1939) , (2) Nishihira (1965) , (3) Ryland (1974) , (4) Calder (1995) , (5) Morri & Bianchi (1999) , (6) Haddad & Chiaverini (2000) , (7) Moazzam & Moazzam (2006) , (8) Oliveira, Marques & Migotto (2006) , (9) Cunha & Jacobucci (2010) , (10) Oliveira & Marques (2011) and (11) Govindarajan et al (2019) . x , presence.…”

“…From all species of hydroids recorded on Sargassum here and in other cited studies, three are most frequent: A. latecarinata, C. noliformis and O. dichotoma (Table 2). Parr (1939) and Govindarajan et al (2019) mentioned that A. latecarinata is often found on S. fluitans III and S. natans VIII.…”

“…Of prime importance amongst invertebrate epibionts on species of the phaeophycean genus Sargassum are hydrozoans and bryozoans ( Niermann, 1986 ; Morris & Mogelberg, 1973 ; Smith, Burns & Carpenter, 1973 ; Cunha & Jacobucci, 2010 ; Oliveira, Marques & Migotto, 2006 ; Oliveira & Marques, 2011 ). To date, the number of hydroid species (~50) occurring on benthic species of Sargassum ( Nishihira, 1965 ; Shepherd & Watson, 1970 ; Morri & Bianchi, 1999 ; Haddad & Chiaverini, 2000 ; Cunha & Jacobucci, 2010 ) is higher than hydroid species (~18) growing on pelagic species of Sargassum ( Parr, 1939 ; Ryland, 1974 ; Calder, 1995 ; Mendoza-Becerril, Simões & Genzano, 2018 ; Govindarajan et al, 2019 ). On thalli of pelagic Sargassum , 14 species of hydroids have been reported in the Sargasso Sea, offshore in the southern Gulf of Mexico, and in the tropical (excluding the Mexican Caribbean) and subtropical North Atlantic Ocean ( Calder, 1995 ; Mendoza-Becerril, Simões & Genzano, 2018 ; Govindarajan et al, 2019 ), with five species being most frequent.…”

“…To date, the number of hydroid species (~50) occurring on benthic species of Sargassum ( Nishihira, 1965 ; Shepherd & Watson, 1970 ; Morri & Bianchi, 1999 ; Haddad & Chiaverini, 2000 ; Cunha & Jacobucci, 2010 ) is higher than hydroid species (~18) growing on pelagic species of Sargassum ( Parr, 1939 ; Ryland, 1974 ; Calder, 1995 ; Mendoza-Becerril, Simões & Genzano, 2018 ; Govindarajan et al, 2019 ). On thalli of pelagic Sargassum , 14 species of hydroids have been reported in the Sargasso Sea, offshore in the southern Gulf of Mexico, and in the tropical (excluding the Mexican Caribbean) and subtropical North Atlantic Ocean ( Calder, 1995 ; Mendoza-Becerril, Simões & Genzano, 2018 ; Govindarajan et al, 2019 ), with five species being most frequent. However, in the Mexican Caribbean, no such studies have yet been undertaken, either for native or non-native species of Sargassum .…”

Consistent genetic divergence observed among pelagic Sargassum morphotypes in the western North Atlantic

“…Substantial changes in density and range of pelagic Sargassum morphotypes imply a large‐scale shift in the dynamics of the North Atlantic (Djakouré et al, 2017; Johns et al, 2020). If each morphotype hosts a different faunal community and possibly represents different levels of ecological value (Calder, 1995; Govindarajan et al, 2019; Martin et al, 2021), accurate identification of pelagic Sargassum is essential and can be facilitated by genetic differentiation of morphotypes.…”