Introduction

Hydroids are known from all marine habitats worldwide, spanning from shallow to abyssal waters, pole to pole (Calder 1998; Fernandez et al. 2020). Their wide distribution and relatively high abundance are supposed to be related to various factors, including asexual reproduction, a wide variety of life cycles, high phenotypic plasticity, and the diversity of biological traits (Cornelius 1995a, b; Cunha et al. 2016, 2022; Di Camillo et al. 2017; Fernandez et al. 2023). Some hydroid species form extensive three-dimensional “forests” for a wide range of organisms, from viruses to vertebrates (Gomes-Pereira and Tempera 2016; Di Camillo et al. 2017). However, the few diagnostic characters of hydroids have also led to taxonomic challenges, such as species with perplexing distribution records and puzzled phylogenetic affinities inconsistent with some presumed diagnostic characteristics (Maronna et al. 2016; Cunha et al. 2017).

Faunal knowledge of hydroids is highly heterogeneous across the world. Some coastal areas have been well sampled in important faunal studies (e.g., Millard 1975; Migotto 1996; Calder 1988, 1991, 1997, 1998; Schuchert 2003; Vervoort and Watson 2003; Gil and Ramil 2021), while others remain unexplored or uninvestigated. The South Atlantic is one such region (Fernandez et al. 2020), and several of these neglected areas occur along the extensive Brazilian coastline (Oliveira et al. 2016), with its wide variety of habitats and microclimates. This knowledge deficit makes it difficult to conserve, manage, and monitor the biodiversity of these areas (Amaral and Jablonski 2005). For example, only a small portion of the 342 hydroid species known from Brazil was recorded in the Northeast region (Oliveira et al. 2016; Mendonça et al. 2022), and this knowledge is derived from a few faunal studies (Maÿal 1983; Pires et al. 1992; Calder and Maÿal 1998; Kelmo and Santa-Isabel 1998; Shimabukuro et al. 2006; Mendonça et al. 2022). The situation is even worse in the North region, for which there are no published records of non-calcareous benthic hydroids on the Amazon continental shelf, with the northernmost record being in Ceará State (Oliveira et al. 2016). This knowledge gap contrasts with the great potential biodiversity for this region, given the geological and biological characteristics of the reefs along the North and Northeast coast of Brazil (Carneiro et al. 2022; Vale et al. 2022), which are expected to harbor a rich hydroid diversity.

This study aims to address this gap by accessing new collections of materials, shedding light on the diversity of hydroid communities along the North and Northeast coast of Brazil. This effort contributes to our knowledge of benthic hydrozoans in this extensive area of the western South American Atlantic.

Material and Methods

The study area comprised the Amazonia and Northeast Brazil marine ecoregions (Spalding et al. 2007), from Amapá (northernmost Brazilian state, 4.500000; -50.050000) to Alagoas State (-9.218975; -35.0564197). We studied material from 224 stations distributed along the continental shelf and the slope, from the intertidal zone to 3,800 m (Supplementary material). These stations were located within the sedimentary basins of Amazonas, Pará-Maranhão, Barreirinhas, Ceará, Potiguar, Paraíba, and Sergipe/Alagoas (Fig. 1).

Fig. 1
figure 1

Study area from Amapá (4º30′0"N 50º2′60"W) to Alagoas State (9º13′8.31"S 35º3′23.112"W) along the Brazilian coastline, comprising the 224 stations (black dots) distributed across the continental shelf and slope, from 0 to 3,800 m in depth

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Hydroid specimens were collected through various methods, including Van Veen sampling, dredging, or manually by SCUBA diving, as part of different projects aiming to characterize the benthic megafauna and abiotic parameters of Brazilian marine ecosystems over the last five decades. These collections were conducted without the presence of Hydrozoa specialists, potentially impacting the quality of collection, sorting, and preservation of some specimens. The 738 studied specimens are deposited in the Cnidarian collection at the Oceanography Museum Prof. Petrônio Alves Coelho at the Federal University of Pernambuco. We calculated the frequency of species occurrence across all stations as the percentage of each species’ records in relation to the totality of sampled states; and the frequency of occurrence within the sampled states as the percentage of states with records of each species among the nine states sampled. Canonical correspondence analysis using four metrics (Wilks 1935; Pillai 1956; Anderson 1984; Muller & Peterson 1984; Rao 2002) was performed to evaluate potential correlations between the independent variable, bathymetry, and the dependent variables (number of records and richness). We used the “igraph” package (Csardi and Nepusz 2006) to identify species groupings by state.

The specimens were preserved in 70% ethanol and were examined by using stereomicroscopes and compound microscopes. Materials were primarily identified based on the original species descriptions and relevant literature, with terminology following Millard (1975), Cornelius (1995a, b), and Bouillon et al. (2006). We have adopted the classification of Leptothecata suggested by Maronna et al. (2016), while for anthoathecates we have followed the classification available at WoRMS (2023).

Results

A total of 739 hydroid specimens were recorded along 3,382 km of coastline across nine states in the North and Northeast regions of Brazil, from Amapá to Alagoas. Of the 224 surveyed stations, approximately 86% (n = 193) were located on the continental shelf within depths of 0–100 m, with only 31 stations distributed on the slope.

We are aware of the caveat that the collection methods varied, and this likely influenced different richness results for each station, as well as the collection effort for each point. However, our intention was to compare to the best of our knowledge the hydroid communities in different geographic areas, and we do this conservatively, without assuming that these biases indicate significant absolute richness. The coast of Pernambuco state was the most thoroughly surveyed, with 48 stations (21%), followed by Pará (39 stations / 17%), and Maranhão (34 stations / 15%) (Table 1). These three states accounted for the highest number of collected hydroid specimens, with Pará having 191 records, Maranhão with 165 records, and Pernambuco with 93 records (Fig. 2). However, the highest species richness was found in Rio Grande do Norte, with 39 species (representing 66% of total richness), followed by Maranhão with 34 species (58% of richness), and Pará with 33 species (56% of richness) (Fig. 2). The reef habitats of the study area were found to provide favorable conditions for a diverse hydroid habitat. Some stations exhibited great richness, with up to 26 collected species. The results of the Canonical correspondence analysis showed no correlation between bathymetry and the number of species and records, with all metrics yielding similar significance value (p = 0.8164557).

Table 1 List of hydroids species collected along the North and Northeast Brazilian coasts during various collection campaigns. The table includes information on the Brazilian states where the species were collected, the frequencies at sampled stations and states, the depth range in the study area, previous records for South America (based on Oliveira et al. 2016), new records for different areas, the fertile or non-fertile condition of the specimens at the time of collection, hydroids that release free medusa or medusoid stages in their life cycles; and whether these species has a circumglobal distribution. (Amapá—AP, Pará—PA, Maranhão—MA, Piauí—PI, Ceará—CE, Rio Grande do Norte—RN, Paraíba—PB, Pernambuco—PE, Alagoas—AL, Fernando de Noronha Archipelago—FN)
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Fig. 2
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Overview of the number of records, species richness, and the number (X axis) of sampled stations for the North and Northeast Brazilian states sampled (Y axis)

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Based on the grouping analysis within the study area, Rio Grande do Norte was characterized as having the highest number of exclusive species, followed by Paraíba, Maranhão, Pará, and Ceará, respectively. In contrast, Amapá, Piauí, Pernambuco and Alagoas did not exhibit species exclusive to these states. Furthermore, most of the species occurring in Amapá, Pará, Ceará, and Maranhão (states mainly located in the center of the diagram) were found in multiple states, illustrating species sharing between these regions (Fig. 3).

Fig. 3
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Species grouping by state. Red dots: names of states; blue dots: species name. Red arrows indicate the states where each species occurs

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We found a total of 59 distinct hydroid species belonging to 32 genera and 15 families (Table 1 and Figs. 4 and 5). Of these species, only two are “anthoathecates”, while the rest belong to Leptothecata. Of these species, 25 are new records for the Northern coast, seven species for the North-eastern coast, two are new records for Brazil, eight for the South Atlantic, and two for the Atlantic Ocean (Table 1). However, eight morphotypes could not be identified to the species level due to damaged or fragmented colonies with limited distinguishing features and no reproductive structures. Out of the 59 morphotypes studied, 21 were fertile at the time of collection, and only four are known to produce free medusa or medusoid stages; the other 38 are species with fixed gonophores.

Fig. 4
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Overview of the number (X axis) of records and species richness for each family collected (Y axis) in the study area

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Fig. 5
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Overview of the number (X axis) of records and species richness for each genus collected (Y axis) in the study area

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Regarding the distribution of the hydroids, 54% were circumglobal, while 29% were restricted to the Atlantic Ocean and adjacent seas, and 15% were recorded in both the Atlantic and another ocean (Table 1). It is worth noting that 80% (12) of the fifteen species of Sertulariidae recorded here have a circumglobal distribution. Conversely, among the species with distributions restricted to the Atlantic, we recorded all the three species of Thyroscyphidae and five out of the ten species of Aglaopheniidae.

The families Aglaopheniidae (192 colonies, comprising 26% of the total number of colonies), Thyroscyphidae (189, 25.5%, with Thyroscyphus ramosus, one of the three species of the family, being the most common), and Sertulariidae (147, or 20%) exhibited the highest number of recorded specimens (Fig. 4). Sertulariidae and Aglaopheniidae exhibited the highest richness (15 and 12 species, respectively). The most prevalent genera were Thyroscyphus (137 colonies) and Aglaophenia (131), primarily due to the presence of T. ramosus (122 colonies; frequency 16.2%) and Aglaophenia rhynchocarpa (67; frequency 9%), both of which were the most common species and the only species found in all states sampled from Amapá to Alagoas (Table 1). This was followed by Sertularelloides cylindritheca (52; frequency 7.0%), Hincksella formosa (51; frequency 6.7%), Aglaophenia latecarinata (46; frequency 6.2%), and Amphisbetia distans (42; frequency 5.6%), all of which exhibited a wide distribution in the study area, occurring respectively in 44.4%, 66.7% and 66.7% of the sampled states. Conversely, several species had isolated records, indicating a restricted distribution in the sampled area (Table 1).

Hydroids were collected from the intertidal zone to a depth of 3,888 m (Fig. 6). Among the observed diversity, 15 species had a bathymetric distribution range ≥ 300 m, with Eudendrium carneum, A. distans, H. formosa, S. cylindritheca, and T. ramosus recorded from 0 to 3,888 m (= eurybathic; see Table 1). On the other hand, species such as Parawrightia robusta and Gymnangium sinuosum had a limited bathymetric range, occurring at depths of only 39–43 m and 62–75 m, respectively (= stenobathic; see Table 1). Some hydroids were collected only once and, therefore, have a single depth record. The number of records and species richness declined with increasing depth. The range between 0–50 m had the highest number of records (387 records; 52% of the total number of records), as well as the greatest species richness (41 species; 70% of the total number of species), followed by the 51–100 m range with 239 records (32%) and 33 species (56%), and the 101–200 m range with 40 records (5%) and 29 species (49%) (Fig. 6). Only 16 species were collected in regions deeper than 201 m. Most species occurred only at the continental shelf (0–100 m) or on the upper slope (101–200 m). Conversely, nine species occurred exclusively on the slope at various depths (Table 1). Additionally, we extended the known bathymetry of 26 species in South America to deeper reefs in the mesophotic zone and the deep sea.

Fig. 6
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Overview of the number (X axis) of records, species richness, and number of sampled stations per depth range (Y axis) in the study area

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Thirty-six morphotypes of epizoic hydroids colonized biogenic substrates (Table 2), displaying a higher frequency in substrates such as sponges, other hydroids, and algae, but were also observed on mollusks, corals, bryozoans, and polychaete tubes. Sponges were the most frequent used basibiont, serving as substrates for 15 hydroid species. Oceanapia bartschi (Laubenfels, 1934) was the most frequently colonized sponge substrate, hosting 11 hydroid species. Nonetheless, the most frequent association was observed among hydroids themselves, with 19 species observed attached to other hydroids. Hydroids with larger and more robust colonies, such as T. ramosus and S. cylindritheca, were the most frequent substrates for other hydroids. Thyroscyphus ramosus was observed as a basibiont ​​of 12 hydroid morphotypes. Another important cnidarian basibiont was the octocoral Nicella spicula, hosting eight different hydroid species (Table 2).

Table 2 Substrates for hydroid attachment observed in this study with their respective bathymetric range. (Algae; Por: Porifera; Cni: Cnidaria; Mol: Mollusca; Pol: Polychaeta; Cho: Chordata)
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The majority of the epizoic hydroids presented small-sized colonies and were substrate-generalists, as the same species was observed on various distinct basibionts. For example, the hydroid H. formosa thrived on nine distinct substrates, while Aglaophenia species were also observed on a wide variety of substrates (Table 2). However, it is important to acknowledge that certain epizoic hydroids observed in this study were exclusively found on a single substrate. Nevertheless, we must refrain to conclusively state that they are substrate-specific, as this outcome could be influenced by sampling bias.

Discussion

This study represents the most comprehensive hydroid survey in Brazil to date in terms of the number of specimens studied, the length of the coastline covered, and bathymetric range covered (Oliveira et al. 2016). The richness of hydroids observed in this study enhances our understanding of the poorly studied benthic biodiversity in underexplored areas of Brazil (Aued et al. 2018). It particularly sheds light on the Northern and North-eastern coastlines, where research has been historically limited. Furthermore, it expands our knowledge of hydrozoans in the South Atlantic by documenting new occurrences and broadening the geographic and bathymetric range of most species (Table 3). The identification of 33 species along the Northern coast represents a substantial increase of 513% in the known hydroid species for this region, which previously only documented four recorded benthic calcified hydroid species. Similarly, among the 57 morphotypes identified in the Northeast, 20 are new records, contributing to a total of 144 known species in the area (Oliveira et al. 2016; Mendonça et al. 2022). It is worth noting that the Brazilian hydroid fauna exhibits greater species richness along the Southeastern and Southern coasts (216 species reported) due to greater sampling effort carried out in these areas (Oliveira et al. 2016).

Table 3 Updated distribution of hydroid species recorded in this study along the Brazilian coast. The new records are shown in blue, while the previous records based on Oliveira et al. (2016) and Mendonça et al. (2022) are highlighted in orange. (AP: Amapá; PA: Pará; MA: Maranhão; PI: Piauí; CE: Ceará; RN: Rio Grande do Norte; PB: Paraíba; PE: Pernambuco; AL: Alagoas; SE: Sergipe; BA: Bahia; ES: Espírito Santo; RJ: Rio de Janeiro; SP: São Paulo; PR: Paraná; SC: Santa Catarina; RS: Rio Grande do Sul; FN: Fernando de Noronha Archipelago; RA: Rocas Atoll; SA: São Pedro and São Paulo Archipelago; TR: Trindade Island)
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In general, we observed that in states with the highest number of stations, there was a greater number of specimens collected and, consequently, greater richness. However, the highest richness was found in the Potiguar Basin area in Rio Grande do Norte State, which is characterized by rich hydroid and reef organism ecosystems, despite accounting for only the fifth-highest number of sampled stations. The benthos of the Potiguar Basin has recently become the focus of attention (Larrazábal-Filho et al. 2015, 2018; Cavalcanti et al. 2018; Dias et al. 2019) and represents one of the areas with the highest richness of deep-sea coral species in Brazil, offering substrates such as rocks, sponges and dead scleractinians (Cordeiro et al. 2020). These authors have observed the presence and abundance of several mesophotic scleractinians from depths from 102 to 108 m, often associated with sponges and gorgonian species of the genus Nicella, as well as other octocoral species. In our study, eight hydroid species were found to be epizoic on Nicella (Table 2). Black corals and octocorals form dense assemblages, creating complex habitats for associated species (Roberts et al. 2006). These dense assemblages, along with the proliferation of microhabitats, may explain the high number of hydroid species observed at some stations in the Potiguar Basin. For example, there was the occurrence of 26 hydroid species at a depth of 102 m during a single season, accompanied by the presence of 18 coral species at the same station (Cordeiro et al. 2020). This suggests favorable conditions for hydroid microhabitats and a rich diversity of other reef organisms.

The low number of anthoathecate hydroids is likely due to unfavorable conditions for collection, sorting, and poor specimen preservation. Anthoathecates are generally delicate animals due to the absence of perisarc protecting soft parts that are important diagnostic characters for the taxonomy of the group (Petersen 1990), and the absence of experts in the group during collection makes them more susceptible to improper processing and suboptimal preservation conditions, basically destroying or overlooking the materials. Additionally, the lack of important taxonomic characters poses challenges in identifying the specimens (Maggioni et al. 2018). Therefore, the richness of anthoathecate species must be considered underestimated at best.

The ability of hydroids to adapt to different conditions, including temperature and depth, as well as the organizational diversity of their colonies and life cycles, likely enables them to occur in all oceans (Leclère et al. 2007). In this study, slightly over half of the collected species exhibited a circumglobal distribution. This may support the great dispersal capacity of hydroids through rafting and advective life stages (Cornelius 1992). Interestingly, only four of the identified species in this study release medusae or medusoid stages (Table 1), indicating that life stages may not play a decisive role in wider distribution. Another explanation for wide distributions is an insufficient taxonomic knowledge at the species level (Cunha et al. 2020, 2022). This taxonomic uncertainty is also influenced by the lack or scarcity of diagnostic taxonomic characteristics and the reliance on plastic characteristics based on trophosomal and gonosomal morphology, as well as the limited understanding of many of their life cycles (Lindner et al. 2011). Indeed, some of these widely distributed species may represent species complexes, such as those occurring in the families Clytiidae and Obeliidae (Cunha et al. 2017) and some species of Sertulariidae (Moura et al. 2011).

The large number of Sertulariidae species collected in this study and their circumglobal distribution lend support to the wide distribution observed for most species of this family elsewhere (Bouillon et al. 2006). Conversely, 15 species found in this study are exclusively recorded in the Atlantic Ocean, including three Thyroscyphidae species and half of the Aglaopheniidae species. Assuming a general high dispersal capacity of hydroids and the plasticity of their life cycles may not be applicable in this case (Cornelius 1990). Interestingly, this distribution pattern of endemicity could be explained by the general system of ocean currents across the Atlantic (Miranda et al. 2015), which could play a role in the distribution of planktonic hydrozoans (Rodriguez et al. 2017), or by some historical factors influencing the distribution of certain biota (e.g., Miranda et al. 2021). Lastly, the distribution of the hydroid species could also be driven by trait-enviroment relationships, offering an ecological perspective to this conundrum (Fernandez et al. 2023).

The hydroid T. ramosus was the most common and widely distributed species in the study area, playing an important role in the reef ecosystems by providing substrate for the establishment of other hydrozoans’ larvae and forming associations with various reef organisms such as algae, sponges, mollusks, and polychaetes. This species has a broad distribution in Brazilian waters and is one of the most frequently observed and conspicuous hydroids in reef ecosystems, with robust colonies (Shimabukuro and Marques 2006), reaching heights exceeding 25 cm (this study). Due to their large and branched colonies, they provide substrate for the attachment and use of several organisms, particularly other hydroids (Migotto 1996). The fact that T. ramosus and A. rhynchocarpa were the two most frequent species at the sampled stations corroborates their wide distribution in the Western Atlantic, extending from Florida, USA (Calder 2013) to Santa Catarina, Brazil (Oliveira et al. 2016), including the Gulf of Mexico (Calder and Cairns 2009), the Caribbean (Galea 2008), and the West African coast (Vervoort 1959). Therefore, the records from the Amazon shelf, located in the northernmost part of the Brazilian coast, fill the knowledge gap regarding the distribution of these species in the Western Atlantic.

Richness of hydroid species and the total number of records decrease with increasing depth. The highest richness and number of records were found between 0 and 50 m (accounting for 69.5% of richness), consistent with previous studies (Calder 1998; Genzano et al. 2009; Ajala-Batista et al. 2020). This trend was followed by the 51–100 m range (representing 56% of richness), and the 101–200 m range (49% of richness). Although 86% of the sampled stations are located on the continental shelf at depths ranging from 0 to 100 m, the inclusion of stations on the slope during this study greatly expanded the bathymetric range for several species (Table 1). Knowledge of hydroid diversity in mesophotic and deep-sea reefs in Brazil is still incipient, and further studies in these areas are needed to fully understand their actual diversity, particularly considering that the range between 101 and 200 m is recognized as one of the richest in terms of hydroid fauna (Calder 1998). In Brazil, few records have been obtained for depths up to 375 m (Fernandez et al. 2020), and most existing records were collected at depths up to 90 m (Oliveira et al. 2016). These records were obtained during oceanographic expeditions such as HMS Challenger and Vega (Allman 1883, 58 m; Jäderholm 1903, 56 m, respectively) or through systematic studies on hydroid fauna in specific areas along the Brazilian coast (e.g., Maÿal 1983, up to 80 m; Migotto 1996, up to 25 m; Grohmann et al. 2003, up to 90 m).

Overall, approximately 1/4 of the hydroids in the study area displayed eurybathic characteristics, occurring from the intertidal zone up to 3,888 m, while 3/4 were stenobathic. These findings diverge from previous studies, which observed that 59% of Atlantic hydroid records occurring from 50 m were eurybathic (Fernandez and Marques 2018). Some species recorded up to 90 m on the continental shelf of southeastern Brazil have also been suggested as eurybathic (Grohmann et al. 2003), including species also found on the Amazon shelf, such as Plumularia margaretta, Lytocarpia tridentata, and Sertularelloides cylindritheca. The reduced number of eurybathic species in our study is likely due to sampling bias, as many species were only collected a few times, making an accurate analysis of their bathymetric range challenging. In the study area, hydroids such as E. carneum, A. rhynchocarpa, A. distans, H. formosa, S. cylindritheca, and T. ramosus are frequently found at different depths and occur from the intertidal zone to 3,888 m (Table 1). These species exhibit more restricted ranges in the western South Atlantic (Table 1) and, while common in shallow waters, they also have records in the deep North Atlantic (e.g., E. carneum up to 137 m, Fraser 1944; A. rhynchocarpa up to 365 m, Vervoort 1968; T. ramosus up to 457 m, Nutting 1915).

In addition to the 33 epibiotic hydroid species, a greater number may have been recorded if some hydroids from the analyzed collections had not detached from their original substrates during sample collection and sorting. Sponges and hydroids were the main substrates in the sampled reefs. Sponges possess complex canal systems that maintain continuous water flow, providing an ideal habitat for numerous hydroid species that can live on the surface or inside of sponge (Puce et al. 2005). Among the basibiont sponges on the Amazon shelf, the sponge O. bartschi, with its arborescent shape and prominent oscular tubes that facilitate continuous water flow, resulted as the most commonly inhabited by hydroids. Of the eleven hydroid species found on O. bartschi in this study, eight are also present in the Caribbean Sea, where this sponge also occurs (Rützler et al. 2014). This suggests that O. bartschi serves as an important substrate, promoting connectivity between the hydroid fauna of the Brazilian coast and that of the Caribbean Sea. The observed associations between hydroids and sponges were not species-specific, indicating that the selection of sponge substrates is also influenced by random larval settlement, as the hydroid hydrorhiza grew superficially on the sponges.

Conversely, hydroids serve as suitable substrates for smaller hydroid species, with epizoics benefiting from the protection offered by the host species' cnida against predators (Gili and Hughes 1995). In this study, nineteen out of the 33 epibiotic species occurred on other hydroids, a pattern well documented in the literature (e.g. Vervoort 1968; Calder 1991; Migotto 1996; Oliveira et al. 2016). Associations between hydroids are a successful strategy in areas characterized by high diversity and intense competition for space (Genzano and Rodriguez 1998). The erect, large, and sturdy colonies of hydroids form three-dimensional “forests” comparable to those created by gorgonians, fostering biotic interactions with various organisms, including other hydroid species (Di Camillo et al. 2017). This highlights the importance of hydroids as a key component of shallow and mesophotic areas. The majority of epibiont hydroids observed had small colonies measuring up to 3.0 mm (e.g., A. latecarinata, H. formosa, A. distans – Migotto 1996), suggesting that size is a limiting factor in the selection of basibionts by hydroid larvae. Some hydroids with larger colonies, such as Synthecium tubithecum, Sertularella diaphana, T. ramosus, and S. cylindritheca (Calder 1991; Migotto 1996), served as substrates for smaller bryozoans and other smaller hydroids, suggesting that the size of both the basibiont and epibiont colonies plays a role in substrate selection. Larger hydroid colonies facilitate the settlement of larvae of smaller species, as well as the settlement of non-larval propagules and overlapping vegetative growth. For instance, the hydroid T. ramosus was observed serving as a substrate for 12 different hydroid species, benefiting from settling larvae on stable substrates with ample space for colony development in a reef environment with strong hydrodynamics. Large colonies are common basibionts for many taxa because they provide additional surface area above the substrate, especially in the basal portions where organisms are particularly abundant (Hughes 1975; Garcia et al. 2009).

The results of our study suggest the larval settlement of hydroid larvae on the colonies of other hydroids occurs randomly, indicating that hydroids can be classified as substrate-generalists. Most species were observed on more than one basibiont, and records of these species on various other organisms exist in the literature (e.g., Vervoort 2006; Calder 2019). Aglaophenia species, in particular, demonstrate a generalist nature and grow on diverse facultative substrates (Calder 1997). In addition to being observed on five other hydroid species, two sponges, and a macroalgae in this study, Aglaophenia latecarinata has been previously observed as an epibiont on other substrates (e.g., Sargassum spp., Migotto 1996 and Calder 2013; sponge and bivalve shells, Galea 2010).