The First Report and Description of a New Species of Rhinebothrium from a Dasyatid Stingray from the Brazilian Northeastern Coast with a Review of the Distribution of the Genus throughout Endemic Marine Ecoregions

Currently, the order Rhinebothriidea Healy, Caira, Jensen, Webster, and Littlewood, 2009, includes 166 species, 25 genera, and 5 families (Ruhnke et al., 2017; Trevisan and Marques, 2017; Trevisan et al., 2017; Benmeslem et al., 2018; Boudaya et al., 2018; Dedrick et al., 2018; Herzog and Jensen, 2018; Coleman et al., 2019a, 2019b; Trevisan and Caira, 2020). Members of the order are endoparasites of batoids from marine and freshwater environments of tropical and sub-tropical regions of the world (Golestaninasab and Malek, 2015; Caira et al., 2016; Ruhnke, et al., 2017). Despite recent efforts to discover and describe new species of rhinebothriideans (Menoret and Ivanov, 2011; Marques and Reyda, 2015; Trevisan and Marques, 2017; Coleman et al., 2019a, 2019b; Trevisan and Caira, 2020), there are strong indications that the diversity of the order remains poorly known.

Rhinebothriideans are considered to be host-specific in general (Reyda and Marques, 2011; Marques and Reyda, 2015; Ruhnke et al., 2017; Trevisan and Marques, 2017), and, on average, their hosts are known to be parasitized by multiple species in the order. Still, many potential hosts have not been examined for rhinebothriideans throughout the world (Ruhnke et al., 2017). Hence, as new hosts and regions of the world are sampled, there is the expectation of the discovery of new taxa within Rhinebothriidea.

RhinebothriumLinton, 1890, is the most speciose genus of the order composed of 51 valid species worldwide (Ruhnke et al., 2017; Trevisan and Marques, 2017; Coleman et al., 2019a, 2019b; Trevisan and Caira, 2020). However, phylogenetic studies have shown the non-monophyletic status the genus, suggesting that Rhinebothrium should be revised in the near future (Healy et al., 2009; Reyda and Marques, 2011, Caira et al., 2014; Ruhnke et al., 2015; Marques and Caira, 2016). Be that as it may, cestodes assigned to this genus have been reported from all tropical and sub-tropical oceans, in addition to all major river systems of South America (Healy et al., 2009; Reyda and Marques, 2011, Caira et al., 2014; Ruhnke et al., 2015; Marques and Caira, 2016; Trevisan and Marques, 2017; Coleman et al., 2019a; Trevisan and Caira, 2020). Most of the diversity of Rhinebothrium has been described from batoids of the family Dasyatidae Jordan and Gilbert (28 species), but the genus has also been reported from other myliobatiformes families including Myliobatidae Bonaparte (2 species); Potamotryogonidae Garman (17 species); Urolophidae Müller and Henle (1 species); Urotrygonidae McEachran, Dunn, and Miyake (5 species); and from 3 families of Rhinopristiformes (Glaucostegidae Last, Séret and Naylor with 2 species, Rhinobatidae Bonaparte and Trygonorrhinidae Last, Séret and Naylor each with 1 species of Rhinebothrium) and with 3 species of Rhinebothrium from Rajiformes in the Arhynchobatidae Fowler (Caira et al., 2016; Ruhnke et al., 2017).

Many of the endemic marine regions, known to be inhabited by various species of batoids, remain to be surveyed for tapeworms—including the Brazilian coast. To date, only 2 species of Rhinebothrium have been reported from Brazilian coastal waters: Rhinebothrium fulbrightiReyda and Marques, 2011, and Rhinebothrium jaimeiMarques and Reyda, 2015. Both were described from Potamotrygon orbignyi (Castelnau)—a Neotropical freshwater stingray that occasionally occurs in the estuarine waters of Marajó Bay, in Colares, Pará State, Brazil (Reyda and Marques, 2011). In total, 70 species of batoids have been reported from Brazilian coastal waters (Rosa and Gadig, 2014). They include 11 species of dasyatids (Rosa and Gadig, 2014; Caira et al., 2016; Fricke et al., 2020), from which 9 have never been examined for Rhinebothrium in this biogeographical area.

Here, we present the first report of a species of Rhinebothrium, which was found in the longnose stingray Hypanus guttatus (Bloch and Schneider) from the northeast coast of Brazil. In addition to describing this new species, we use the patterns of infection and distribution for species of Rhinebothrium to predict the expected diversity within the genus for unsurveyed hosts and endemic marine ecoregions of the world.

The spiral intestines of a total of 10 specimens of Hypanus guttatus collected from the coast of Maceió-AL in September of 2010 (9°40′25″S, 35°44′07″W) were collected following the guidelines of the permit issued to Fernando P. L. Marques by ICMBio/IBAMA No. 20964. Each stingray was euthanized and subjected to necropsy to remove the spiral intestine, which was opened with a mid-ventral incision, washed with saline solution, and fixed in a 4% seawater-buffered formalin solution or 95% ethanol and shaken for approximately 2 min. Five spiral intestines and washes of their cavity were fixed in 95% ethanol and stored at −20 C, while the other 5 were fixed in buffered formalin and transferred to 70% ethanol after 3 days for long-term storage. No specimens of Rhinebothrium were found in ethanol-fixed samples. The specimens of Rhinebothrium examined here were selected from the spiral intestines fixed in formalin using a stereomicroscope.

Specimens prepared as whole mounts for light microscopy were hydrated in a graded alcohol series, stained with Delafield's or Mayer's Hematoxylin, destained in a 1% acid (HCl) ethanol solution, alkalized in a 1% basic (NaOH) ethanol solution, dehydrated in graded alcohol series, cleared in methyl salicylate, and mounted in Canada balsam on glass slides under coverslips. Photographic documentation was performed using an Olympus SC30 camera with the AnalySIS getIT software (Olympus Soft Images Solutions) connected to an Olympus BX51 microscope (Olympus, North Rhine-Westphalia, Germany). Image processing and morphometric data acquisition were done with the assistance of the programs Fiji/ImageJ (Schindelin et al., 2012) and WormBox (Vellutini and Marques 2014), respectively. Only complete specimens with mature (i.e., with open genital pores) or further developed proglottids (e.g., with atrophied testes or vas deferens-filled) were measured. Measurements of reproductive structures were mainly obtained from terminal proglottids. Testes dimensions were obtained from subterminal proglottids in specimens with atrophied testes in terminal proglottids. Measurements are presented as ranges followed in parentheses by the number of specimens from which the measurements were obtained. All measurements are in micrometers unless otherwise indicated. Repeated measurements for the number and dimensions of testes and vitelline follicles were averaged for individuals. Line drawings were prepared with the aid of a drawing tube attached to an Olympus BX51 microscope.

Specimens selected for scanning electron microscopy (SEM), had their scolex and posterior portion of the strobila removed. Their scoleces were carefully cleaned with a fine brush to remove host tissue and mucus. Then, they were hydrated in progressive ethanol series, transferred to 1% osmium tetroxide overnight, dehydrated in progressive ethanol series, and placed in hexamethyldisilizane (HMDS). Scoleces were then allowed to air-dry overnight and were subsequently mounted on carbon tape on aluminum stubs, sputter-coated with gold/palladium, and examined with an FEI Quanta 600 FEG scanning electron microscope. The strobila of each worm used for SEM was prepared as a whole mount voucher, as described above. The terminology used to describe the microtriches follows Chervy (2009).

For histological sections, the posterior region of the strobila of each specimen was removed, embedded in paraffin, and sectioned in 7 μm intervals using a LEICA RM 2025 retracting rotary microtome (Leica Microsystems, Baden-Württemberg, Germany). Sections were mounted on glass slides, flooded with distilled water, dried for 5 minutes on a slide warmer, and later transferred to an oven at 30 C for 30 min. Cross-sections were then stained with Mayer's hematoxylin and counterstained with eosin, dehydrated in a progressive ethanol series, cleared in xylene, and mounted in Entellan (Merck). The anterior portion of each worm sectioned was prepared as a whole mount as described above and retained as a voucher.

Museum abbreviations are as follows: HWML, Harold W. Manter Laboratory, University of Nebraska, Lincoln, Nebraska; LRP, Lawrence R. Penner Parasitology Collection, Department of Ecology and Evolutionary Biology, University of Connecticut, Storrs, Connecticut; MZUSP, Museu de Zoologia da Universidade de São Paulo, Universidade de São Paulo, São Paulo, SP, Brazil.

Worms acraspedote, euapolytic, 1.7–4 mm (n = 41) long, composed of 12–32 (n = 41) proglottids (Fig. 1; Table I). Scolex composed of 4 bilobed stalked bothridia, slightly constricted at center with muscular rims (Figs. 1, 2A, 3A). Bothridia 210–359 (n = 32) long by 81–115 (n = 26) wide, divided by 19–24 (n = 16) transverse septa and 1 medial longitudinal septum into 39–49 (n = 16) loculi; anterior-most loculus single, 11–26 (n = 17) long by 20–49 (n = 17) wide; posterior-most loculi double (Figs. 1, 2A, 3A). Medial longitudinal septum extending from the posterior margin of anterior loculus to posterior margin of the bothridium. Cephalic peduncle 25–48 (n = 24) long by 26–49 (n = 24) wide (Figs. 1A, 2A). Proximal bothridial surfaces near rim with acicular filitriches and papilliform filitriches (Fig. 3B), remaining proximal bothridial surfaces with acicular filitriches only (Fig. 3C). Distal bothridial surfaces near center of bothridia and near transverse septa, both with acicular and capilliform filitriches (Fig. 3D, E).

Immature proglottids wider than long, becoming longer than wide with maturity (Fig. 1). Mature proglottid 244–510 (n = 32) long by 46–93 (n = 32) wide, 3–6 (n = 38) in number (Fig. 2C). Some terminal proglottids with pronounced uterus extending entire length of proglottid (Fig. 2B, C) and anterior portion of the proglottid filled with parenchymatous tissue and atrophied testes (Fig. 2C). Testes 4–5 (n = 40) in number, oval, arranged in 2 irregular columns in anterior half of proglottid, 15–30 (n = 29) long by 16–29 (n = 29) wide (Figs. 2B, 4A). Cirrus sac in anterior half of proglottid, pyriform, 39–66 (n = 21) long by 35–62 (n = 21) wide, tilted slightly posterior, containing coiled cirrus; cirrus armed with spinitriches (Fig. 2D). Genital pores 43–59% (n = 30) of proglottid length from posterior end, irregularly alternating (Figs. 1, 2B, C). Vagina thick-walled, slightly enlarged, narrowing in the distal part, weakly sinuous, extending from ootype along medial line of proglottid with the anteromedial portion adjacent to the cirrus sac (Fig. 2D). Vaginal sphincter absent. Ovary near posterior end of proglottid, bilobed in dorso-ventral view (Fig. 2B, C), tetra-lobed in cross-section (Fig. 4B), slightly asymmetrical, 73–129 (n = 10) long by 22–44 (n = 9) wide at isthmus (Fig. 2B, C). Uterus medial extending from near anterior edge of proglottid to the posterior margin of the proglottid (Fig. 2B, C). Vitelline follicles arranged in 2 lateral bands; each band consisting of dorsal and ventral columns of follicles; bands extending the length of proglottid, 9–23 (n = 20) long by 7–18 (n = 20) wide (Fig. 2B, C). Detached gravid proglottids and eggs not observed.

Hypanus guttatus (Bloch and Schneider) Myliobatiformes: Dasyatidae.

Coast of Maceió, Alagoas, Brazil (9°40′25″S, 35°44′07″W).

Spiral intestine.

MZUSP 7982 (Holotype), 7983a–7983n (Paratypes: 14 complete worms), 7979 (1 SEM voucher), and 7980a–7980e, 7981a–7981f (vouchers from histological sections); HWML 216325 (Paratypes: 13 complete worms); and LRP 10215–10227 (Paratypes: 13 complete worms).

20% (2 of 10 spiral intestines).

urn:lsid:zoobank.org:act:28DC82BF-204C-42A9-A586-26F123A66657.

The species is named in honor of Graciliano Ramos, a renowned Brazilian writer born in the State of Alagoas.

Rhinebothrium ramosi can be distinguished from the 51 other members of the genus in its possession of a unique combination of morphological characters that include 4–5 testes, a uterus that extends throughout the length of proglottid, and posterior-most bothridial loculi arranged as a pair. Rhinebothrium ramosi exhibits a testes number that overlaps with 17 of those species (see Table I). Within this set of 17 species, only 2 have been reported for the Tropical Atlantic Ocean: Rhinebothrium margaritenseMayes and Brooks, 1981, and Rhinebothrium reydaiTrevisan and Marques, 2017 from Styracura schmardae (Werner). Both species can be differentiated from R. ramosi by having single posterior-most bothridial loculi rather than arranged as a pair (see Table I). Rhinebothrium ramosi further differs from R. margaritense by having fewer proglottids (12–32 vs. 75–100, respectively) and fewer loculi (39–49 vs. 53–55, respectively). Ten additional species (Rhinebothrium kruppi, Rhinebothrium chollaense, Rhinebothrium dasyatidis, Rhinebothrium lintoni, Rhinebothrium bunburyense, Rhinebothrium fungiforme, Rhinebothrium urolophi, Rhinebothrium leopardensis, Rhinebothrium vandiemeni, and Rhinebothrium walga) also have a single posterior-most bothridial locus differing from R. ramosi (Table I). Within this group, R. bunburyense, R. fungiforme, R. leopardensis, R. lintoni, R. urolophi, and R. vandiemeni can be also distinguished from the new species by possessing more bothridial loculi (see Table I). There are only 4 species that share with R. ramosi the presence of posterior-most bothridial loculi arranged as a pair: Rhinebothrium corbatai, Rhinebothrium mistyae, and Rhinebothrium paratrygoni (restricted to Neotropical freshwater habitats) and Rhinebothrium maccallumi reported from Temperate Northern Atlantic (Table I). These 4 species have a larger number of proglottids in comparison to R. ramosi. In addition, all freshwater species also have more bothridial loculi (Table I). Finally, Rhinebothrium taeniuri reported from the Western Indo-Pacific can be differentiated from R. ramosi by its greater length (5.1–5.7 vs. 1.7–4, respectively) and fewer loculi (18–22 vs. 39–49, respectively; Table I).

The discovery of a new species of Rhinebothrium from the northeastern Brazilian coast brings the total number of valid species of Rhinebothrium to 52. A close look at the known host association and distribution of these 52 species of Rhinebothrium reveals that there are sampling gaps and that the diversity of this genus is underestimated (see Fig. 5; Table II). Species of Rhinebothrium have been reported from 8 of the 12 marine biogeographical realms of the world (sensu Spalding et al., 2007): the exceptions are the Arctic, Temperate Southern Africa, the Southern Ocean, and Tropical Eastern Pacific realms (Fig. 5). However, even for the biogeographical realms from which species of Rhinebothrium have been reported, the number of species known is strikingly low. For instance, there are only 2 species of Rhinebothrium—Rhinebothrium monodiEuzet, 1954, from Senegal and R. taeniuri from Egypt—reported for the entire coast of Africa. Four biogeographical realms have been assigned to the African coast (Fig. 5), which is inhabited by a great diversity of batoid fishes (Last et al., 2016; Nelson et al., 2016). African batoids are likely to host new species of tapeworms, including rhinebothriids such as Rhinebothrium. The same is true for the Eastern Indo-Pacific from which only Rhinebothrium devaneyiBrooks and Deardorff, 1988, from Marshall Island and Rhinebothrium hawaiienseCornford, 1974, from Hawaii have been described. Therefore, the worldwide biogeographical distribution of the genus suggests that most of the diversity of the genus remains to be described.

The Brazilian coast is comprised of 3 biogeographical provinces (Fig. 5)—North Brazil Shelf, Tropical Southwestern Atlantic, and Warm Temperate Southwestern Atlantic—which are collectively divided into 8 ecoregions (see Spalding et al., 2007). These potential areas of endemism are known to be inhabited by 70 species of batoids (Rosa and Gadig, 2014; Fricke at al., 2020), many of which have not been examined for cestodes. Therefore, this region of the world has the potential to have many undescribed species.

The expectation of finding new species of Rhinebothrium throughout the world, in general, and on the Brazilian coast, in particular, can also be justified if we consider the pattern of infection of this group of tapeworms. The genus is characterized by species that tend to be restricted to both biogeographical areas and host species (Menoret and Ivanov, 2011; Ruhnke et al., 2017). Freshwater species (e.g., Rhinebothrium copianullumReyda, 2008, and R. paratrygoni), however, appear to be exceptions to the strict host specificity (i.e., oioxenous; sensu Euzet and Combes, 1980) commonly reported for marine members of this genus (Reyda and Marques, 2011). Nonetheless, the common pattern for marine lineages is to find 1, in some cases more, species of Rhinebothrium per species of batoid.

In total, species of Rhinebothrium are known to parasitize 9 families of batoids—specifically, the Dasyatidae, Myliobatidae, Potamotrygonidae, Urolophidae, Urotrygonidae, Glaucostegidae, Rhinobatidae, Trygonorrinidae, and Arhynchobatidae. However, most species in these families have never been examined for Rhinebothrium (Table II). As an example, Dasyatidae—the most diverse family cited above—is comprised of 97 species, but only 19 have been previously examined for Rhinebothrium. Twenty-eight species of Rhinebothrium have been described from these 19 species of dasyatids, averaging ∼1.5 species of Rhinebothrium per host species (Table II). Within this group of batoids, considering only those genera for which Rhinebothrium have been reported to date (60), there remain 41 species to be examined for cestodes, which could potentially host 60 undescribed species of Rhinebothrium globally and 12 for the Brazilian coast (Table II), most of them in dasyatids of the genus Hypanus Lesueur (Table II). Expanding our estimation for the global diversity of Rhinebothrium in Myliobatiformes, we would expect 119 species yet to be discovered. Our total estimation considering all major batoid taxa for which species of this genus have been described is 160 species globally, within which 25 should be found on the Brazilian coast (Table II). Finally, our rough estimations do not consider the observation that batoid species that are widely distributed, especially across biogeographical regions, tend to host a higher diversity of rhinebothriideans (see Trevisan et al., 2017).

The estimated diversity of Rhinebothrium is congruent with what has been expected for Rhinebothriidea in general. According to Ruhnke et al. (2017), more than three-quarters of the diversity of the order remains undescribed. These authors estimated that the order may include as many as 800 species, ∼90% of which parasitize Myliobatiformes (424) and Rajiformes (309). A considerable component of this potential diversity resides in unexplored biogeographical regions, such as the Brazilian coast, and unexamined elasmobranchs/batoids species. Hence, we reiterate the point made by Ruhnke et al. (2017) that studies in unsurveyed biogeographical areas and elasmobranchs are encouraged if we are to achieve a better understanding of the diversity of Rhinebothriidea, in general, and Rhinebothrium, in particular, throughout the world.

We thank Sabrina Outeda-Jorge (University of São Paulo—USP, São Paulo, Brazil), for her help during the sampling trip to Alagoas. We are also grateful for the logistical support provided by Dr. Vandick da Silva Batista, Dr. Nidia Noemi Fabré, and their students (Universidade Federal de Alagoas, Maceió, AL, Brazil) during our collecting trip. We thank Lilian Sakai, Natalia Luchetti, and Yu Golfetti (USP) for their help with specimen preparation. Special thanks go to Sheila Schuindt, Enio Mattos, and Phillip Lenktaitis also from USP for their assistance with SEM. We thank Janine N. Caira (University of Connecticut, Storrs, Connecticut) for comments on earlier versions of the manuscript. This study was financed by FAPESP—Fundação de Amparo a Pesquisa do Estado de São Paulo (No. 2017/11063-4, 2017/20544-6, 2018/03534-0) and partially supported by the CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior, Brasil, No. 001) and the CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico, Brasil, No. 620182/2008-3).