Introduction
Jainus Mizelle, Kritsky & Crane, Reference Mizelle, Kritsky and Crane1968 (Dactylogyridae: Ancyrocephalinae) was established in the original description by Mizelle et al. (Reference Mizelle, Kritsky and Crane1968) with the description of Jainus jainus Mizelle, Kritsky & Crane, Reference Mizelle, Kritsky and Crane1968, the type species of the genus. To date, Jainus comprises 13 valid species, all parasitizing characiform fishes. Species of Jainus have been reported exclusively from the Neotropical region (Abdallah et al. Reference Abdallah, Azevedo and Luque2012; Cohen et al. Reference Cohen, Kohn and Boeger2012; Cruces et al. Reference Cruces, Santillan, Silvera, Morey, Rubin and Chero2024; Karling et al. Reference Karling, Bellay, Takemoto and Pavanelli2011; Kritsky and Leiby Reference Kritsky and Leiby1972; Kritsky et al. Reference Kritsky, Thatcher and Kayton1980; Morey et al. Reference Morey, Viana, Chota and Chero2025; Yamada et al. Reference Yamada, Müller, Zago, Yamada, Ebert, Franceschini and Silva2023).
Knowledge of the diversity of Jainus has progressively increased through the description of additional species, including Jainus hexops Kritsky & Leiby, Reference Kritsky and Leiby1972, Jainus amazonensis Kritsky, Thatcher & Kayton, Reference Kritsky, Thatcher and Kayton1980, Jainus piava Karling, Bellay, Takemoto & Pavanelli, Reference Karling, Bellay, Takemoto and Pavanelli2011, Jainus iocensis Cohen, Kohn & Boeger, Reference Cohen, Kohn and Boeger2012, and Jainus leporini Abdallah, Azevedo & Luque, Reference Abdallah, Azevedo and Luque2012. More recently, Jainus beccus Yamada, Yamada, Chero, Cruces, Bellay & Karling, 2023, Jainus ornatus Yamada, Yamada, Chero, Cruces, Bellay & Karling, 2023, and Jainus radixelongatus Yamada, Yamada, Chero, Cruces, Bellay & Karling, 2023 were described based on both morphological and molecular data (Yamada et al. Reference Yamada, Müller, Zago, Yamada, Ebert, Franceschini and Silva2023). Subsequently, Jainus peruensis Cruces, Morey & Luque, 2024, Jainus iquitensis Morey, Cruces & Luque, Reference Morey, Viana, Chota and Chero2025, Jainus loretoensis Morey, Cruces & Luque, Reference Morey, Viana, Chota and Chero2025, and Jainus sardinae Morey, Cruces & Luque, Reference Morey, Viana, Chota and Chero2025 were described from the Peruvian Amazon (Cruces et al. Reference Cruces, Santillan, Silvera, Morey, Rubin and Chero2024; Morey et al. Reference Morey, Viana, Chota and Chero2025). Together, these studies have increased current knowledge of the diversity, host associations, and evolutionary relationships of the genus.
Within Triportheidae, Jainus is currently represented by three species described from Triportheus angulatus (Spix & Agassiz, 1829) in the Peruvian Amazon: J. iquitensis, J. loretoensis, and J. sardinae (Morey et al. Reference Morey, Viana, Chota and Chero2025). Agoniates halecinus Müller & Troschel, 1845, the host species examined in the present study, also belongs to Triportheidae, a family for which the diversity of monopisthocotylans remains poorly documented compared with that of other characiform hosts.
Partial LSU rDNA and mitochondrial COI were selected as molecular markers widely used in dactylogyrid systematics, enabling direct comparisons with previously published datasets and providing complementary evidence for species delimitation and phylogenetic reconstruction.
Therefore, the present study describes a new species of Jainus parasitizing A. halecinus, representing the first record of a species of this genus in this host genus. In addition, molecular analyses were used to evaluate its phylogenetic position within Jainus and related dactylogyrid taxa.
Materials and methods
Host and parasite sampling
In March 2025, five individuals of A. halecinus were obtained from the Pindaíba River, located in the municipality of Cocalinho, Mato Grosso state, Brazil (14°31′39.9″S, 51°41′38.9″W), through fishing using a cast net. Host specimens were identified to the species level based on morphological characters. The sampling was carried out under authorization from the Brazilian Institute of Environment and Renewable Natural Resources (IBAMA; SISBIO licence #60640-1). Fish were euthanized by severing the spinal cord and examined for parasites under a stereomicroscope. All experimental procedures were performed in accordance with the ethical standards established by the Ethics Committee on Animal Use (CEUA) of the University Center of Vale do Araguaia, Barra do Garças, Mato Grosso, Brazil (CEUA protocol #010/2025). Monopisthocotylan parasites were isolated from the gills; specimens intended for morphological characterization were fixed in formalin preheated to approximately 60–70°C, whereas those designated for molecular analyses were preserved in absolute ethanol. For the examination of sclerotized structures, selected specimens were mounted in Hoyer’s medium (Eiras et al. Reference Eiras, Takemoto and Pavanelli2000). Additional specimens were stained with carmine, mounted in Canada balsam, and examined to observe the internal organs. Prevalence and mean intensity were determined following the methods proposed by Bush et al. (Reference Bush, Lafferty, Lotz and Shostak1997).
Morphometric analyses were carried out using a microscope equipped with differential interference contrast or phase-contrast optics and a computerized image analysis system (Leica Application Suite V3; Leica Microsystems, Wetzlar, Germany). Measurements are presented in micrometres (μm), correspond to the maximum dimensions of the structures examined, and follow the morphometric scheme of Yamada et al. (Reference Yamada, Müller, Zago, Yamada, Ebert, Franceschini and Silva2023). Hook pairs were numbered according to the convention proposed by Mizelle (Reference Mizelle1936). Illustrations were prepared using a camera lucida attached to the microscope and refined based on photomicrographs of the specimens. Type and voucher specimens of the monopisthocotylans were deposited in the Helminthological Collection of the Instituto Oswaldo Cruz (CHIOC), Rio de Janeiro state, Brazil, whereas hologenophore preparations were deposited in the Helminthological Collection of the Institute of Biosciences (CHIBB), UNESP, Botucatu, São Paulo state, Brazil. Additionally, all material was registered in compliance with Brazilian legislation in the National System for the Management of Genetic Heritage and Associated Traditional Knowledge (SISGEN) under registration code #A7B400A.
Molecular and phylogenetic analyses
For molecular analyses, the collected material was initially screened to confirm the identity of the parasite species present in the samples based on morphological examination. Selected specimens identified as the target species were processed following a hologenophore approach sensu Pleijel et al. (Reference Pleijel, Jondelius, Norlinder, Nygren, Oxelman, Schander, Sundberg and Thollesson2008), in which the posterior portion of the body was used for DNA extraction, while the anterior region containing the male copulatory complex was mounted on permanent slides in Hoyer’s or Gray & Wess medium for subsequent morphological confirmation. These slides, together with additional mounted specimens, will be deposited in the Helminthological Collection of the Institute of Biosciences (CHIBB), UNESP, Botucatu, São Paulo state, Brazil.
Total genomic DNA was isolated using the DNeasy® Blood and Tissue Kit (Qiagen) according to the manufacturer’s protocol. Partial fragments of LSU rDNA and COI mtDNA were amplified through conventional PCR assays.
Amplification of LSU rDNA was performed using primers 382F (5′–AGC TGG TGG AGT CAA GCT TC–3′) and 1289R (5′–TGC TCA CGT TTG ACG ATC GA–3′) (Yamada et al. Reference Yamada, Müller, Zago, Yamada, Ebert, Franceschini and Silva2023) under the following cycling conditions: initial denaturation at 95°C for 5 min; 40 cycles consisting of 95°C for 30 s, 56°C for 30 s, and 72°C for 2 min; followed by a final extension at 72°C for 10 min. Amplification of COI mtDNA employed primers COI Mono 5 (5′–TAA TWG GTG GKT TTG GTA A–3′) and COI Mono 3 (5′–TAA TGC ATM GGA AAA AAA CA–3′) (Plaisance et al. Reference Plaisance, Rousset, Morand and Littlewood2008), using the following profile: 94°C for 3 min; 40 cycles of 94°C for 30 s, 47°C for 30 s, and 72°C for 1 min 30 s; and a final extension step at 72°C for 7 min.
PCR products were examined on 1% agarose gels stained with GelRed™. Successful amplicons were purified with Agencourt AMPure XP magnetic beads (Beckman Coulter, Inc.) and sequenced in both directions using the same primers applied in the amplification reactions, employing the BigDye Terminator v.3.1 Cycle Sequencing Kit on an ABI3730xl Genetic Analyzer (Applied Biosystems). Consensus sequences were assembled and manually edited in Sequencher v.5.2 (Gene Codes).
The newly generated LSU rDNA and COI mtDNA sequences were aligned with homologous sequences of Dactylogyridae available in GenBank using MUSCLE implemented in Geneious v.7.1.3 (Kearse et al. Reference Kearse, Moir, Wilson, Sones-Havas, Cheung, Sturrock, Buxton, Cooper, Markowitz, Duran, Thierer, Ashton, Meintjes and Drummond2012). Terminal regions containing ambiguous positions were trimmed prior to analyses. For the LSU rDNA dataset, 29 Dactylogyridae sequences were retrieved from GenBank and combined with the two newly generated sequences from the present study. Sequences of Murraytrema pricei Bychowsky & Nagibina, 1977, Pseudorhabdosynochus epinepheli (Yamaguti, 1938) Kritsky & Beverley-Burton, 1986, and Pseudorhabdosynochus lantauensis (Beverley-Burton & Suriano, 1981) Kritsky & Beverley-Burton, 1986 were included as outgroups, resulting in a final LSU rDNA alignment comprising 34 sequences and 842 bp (Supplementary Information 1). For the COI mtDNA dataset, 16 Dactylogyridae sequences were retrieved from GenBank and combined with the two newly generated sequences from the present study. The sequence of Acanthocotyle gurgesiella Ñacari, Sepúlveda, Escribano & Oliva, 2017 was included as the outgroup, resulting in a final COI mtDNA alignment comprising 19 sequences.
The most appropriate nucleotide substitution model was determined with jModelTest 2.1.6 (Darriba et al. Reference Darriba, Taboada, Doallo and Posada2012) through the CIPRES Science Gateway (Miller et al. Reference Miller, Pfeiffer and Schwartz2010) under the Akaike Information Criterion (AIC), with GTR + I + G selected for both molecular markers. Because RAxML does not implement the GTR + I + G model as a separate substitution model, maximum likelihood (ML) phylogenetic reconstructions were inferred independently for each marker using RAxML-HPC v.8.2.12 (Stamatakis Reference Stamatakis2014) on the ACCESS platform under the GTRGAMMA model, whereas Bayesian inference (BI) analyses were performed using MrBayes v.3.2.7 (Ronquist et al. Reference Ronquist, Teslenko, van der Mark, Ayres, Darling, Höhna, Larget, Liu, Suchard and Huelsenbeck2012) on the ACCESS platform. Bayesian analyses consisted of 1 × 106 generations with trees sampled every 100 generations, and the first 25% of the sampled trees were discarded as burn-in. Convergence among runs was evaluated using the diagnostics available in MrBayes, including stabilization of likelihood scores and the average standard deviation of split frequencies. Posterior probabilities (pp) ≥0.95 were interpreted as strong support. Nodal support for ML analyses was estimated using 1,000 bootstrap pseudoreplicates, with values ≥70% considered robust. For the COI mtDNA analyses, all codon positions were included and analysed as a single partition, thus avoiding unnecessary model overparameterization. COI sequences were translated using the flatworm mitochondrial genetic code (translation table 14) and checked for unexpected stop codons and indels. Pairwise genetic distances were estimated as uncorrected p-distances in MEGA12 (Kumar et al. Reference Kumar, Stecher, Suleski, Sanderford, Sharma and Tamura2024).
Phylogenetic trees were visualized and edited in FigTree v.1.4.4 (Rambaut Reference Rambaut2020) and finalized graphically using CorelDraw X8.
Results
Dactylogyridea Bychowsky, 1937
Dactylogyridae Bychowsky, 1937
Jainus Mizelle, Kritsky & Crane, Reference Mizelle, Kritsky and Crane1968
Jainus sensai n. sp. (Fig. 1A–I; Fig. 2A–F)
Schematic drawings of Jainus sensai n. sp. from Agoniates halecinus Müller & Troschel, 1845. (A) Whole mount, ventral view; (B) copulatory complex; (C) ventral anchor; (D) dorsal anchor; (E) ventral bar; (F) dorsal bar; (G) hook pairs II, III, IV, VI, and VII; (H) hook pair V; and (I) hook pair I. Holotype (CHIOC 41115a).
Figure 1. Long description
The image consists of nine panels labeled A through I.
Panel A occupies the left half, showing a whole mount ventral view of the parasite. It has an elongated body with a tapered anterior end containing four eye spots and a posterior haptor with various hooks. A large central region contains numerous circular egg-like structures. A scale bar indicates 100 micrometers.
Panel B shows the copulatory complex with a coiled tube and a dark, curved accessory piece. Scale bar is 20 micrometers.
Panel C and D display the ventral and dorsal anchors respectively. Both are hook-shaped with distinct roots and points. Scale bars are 10 micrometers.
Panel E shows the T-shaped ventral bar, and Panel F shows the elongated, slightly curved dorsal bar. Scale bar for E is 10 micrometers.
Panels G, H, and I illustrate different hook pairs. G shows hook pairs II, III, IV, VI, and VII. H shows hook pair V. I shows hook pair I. Each hook consists of a curved blade and a thin filament. A 10 micrometer scale bar applies to these three panels.
Photomicrographs of Jainus sensai n. sp. from Agoniates halecinus Müller & Troschel, 1845. (A) Dorsal anchor and dorsal bar; (B) ventral anchor and ventral bar; (C) hook pair V; (D) hook pair I; (E) hook pairs VI and VII; and (F) copulatory complex. Scale bars: (A, B) = 20 μm; (C–F) = 10 μm.
Figure 2. Long description
A collection of six phase-contrast photomicrographs labeled A through F.
* Panel A: At the top left, a 20 mu m scale bar sits beside a horizontal dorsal bar connecting two curved dorsal anchors with broad bases and sharp, inward-pointing tips.
* Panel B: Below A, a 20 mu m scale bar accompanies a complex arrangement of a ventral bar and two ventral anchors with more intricate, branched proximal ends.
* Panel C: At the bottom left, a 10 mu m scale bar highlights hook pair V, appearing as small, thin, curved sclerites.
* Panel D: At the top right, a 10 mu m scale bar shows hook pair I, two isolated small hooks with a distinct handle and sickle shape.
* Panel E: In the middle right, a 10 mu m scale bar shows hook pairs VI and VII, which are clustered and show elongated shafts.
* Panel F: At the bottom right, a 10 mu m scale bar identifies the copulatory complex, a large, vertically oriented structure with a coiled base and a tubular accessory piece extending upward.
Type-host: Agoniates halecinus Müller & Troschel, 1845 (Characiformes: Triportheidae).
Type-locality: Pindaíba River (14°31′39.9″S, 51°41′38.9″W), Araguaia Basin, municipality of Cocalinho, Mato Grosso state, Brazil.
Type-material: Holotype, CHIOC 41115a; paratypes, CHIOC 41115b–d, and CHIOC 41116a–b; hologenophores, CHIBB 1001-1002; specimens mounted in Canada balsam, CHIBB 1003-1004.
Site of infection: Gills.
Prevalence: five of five examined hosts (100%).
Mean intensity of infection: 6.0 ± 1.0 (5–7), with a total of 30 specimens collected.
ZooBank registration: The Life Science Identifier (LSID) for Jainus sensai n. sp. is urn:lsid.org:act:0359E237-7E87-4DD1-B604-4A8E8A469698. The publication is also registered in ZooBank: urn:lsid.org:pub:63174F32-5516-48C4-B21F-FDCC836634A5.
Etymology: The specific epithet sensai is based on the Japanese word 繊細 (せんさい, sensai), meaning ‘delicate’ or ‘soft’, referring to the weakly sclerotized extensions of both roots of the ventral anchor and the weakly sclerotized median process of the ventral bar. The name is treated as a noun in apposition and is therefore indeclinable.
Description
(Based on 17 specimens: 10 mounted in Hoyer’s medium and seven stained with carmine mounted in Canada balsam.) Body elongated, fusiform, 759.1 (664–867, n = 10) long and 422.7 (304–495, n = 10) wide near midlength. Cephalic lobes well developed, especially the median lobe. Three pairs of cephalic organs observed. Four eyespots present, similar in size; distance between eyespots of anterior pair equal to that of posterior pair. Accessory granules distributed throughout trunk. Pharynx subspherical, 18.6 (18.2–19.4, n = 3) long, 18.3 (18–19, n = 3) wide. Peduncle short. Haptor subovate, 113.1 (81–170, n = 10) long, 168.5 (110–197, n = 10) wide. Anchors dissimilar in shape and size. Ventral anchor 16.5 (13–18, n = 10) long, 9.5 (6–11, n = 10) wide, robust, with elongate shaft and curved point; superficial root flattened and deep root elongate, with weakly sclerotized extensions; base with slight protuberance. Superficial root 6.5 (5–7, n = 10) long; deep root 11.2 (9–15, n = 10) long. Dorsal anchor 37.5 (33–41, n = 10) long, 16.8 (14–20, n = 10) wide, with base bifurcate; superficial and deep roots present, superficial root elongated and deep root reduced; shaft slender and curved. Ventral bar 3.25 (2–4, n = 10) long, 13 (10–14, n = 10) wide, elongate, delicate, slightly curved, bearing a weakly sclerotized median process measuring 16.35 (12–19, n = 10) long from its insertion on the ventral bar. Dorsal bar 6.3 (5–7, n = 10) long, 44.8 (38–51, n = 10) wide, elongated, with slightly expanded ends. Seven hook pairs (I–VII), numbered according to Mizelle (Reference Mizelle1936), dissimilar in morphology. Hook pair I 11.4 (10–12, n = 10) long, slender and elongated with FH loop. Hook pair V 9.6 (8–11, n = 5) long, with FH loop and slender thumb. Remaining hook pairs 9.5 (8–10, n = 10) long, with FH loop, thicker thumb, rounded base, and slightly curved point. Male copulatory organ (MCO) 29.2 (23–35, n = 10) long, 15.4 (12–17, n = 10) wide, comprising a slender, J-shaped tube, with its proximal portion forming a single counterclockwise ring and its distal portion extending through the accessory piece. Accessory piece 14.65 (13–19, n = 10) long, flattened and laminar, with pointed extremities and an opening in its distal portion through which the copulatory tube passes, bearing an elongate, strongly curved projection tapering to a pointed tip (Figs. 1 and 2). Testis subspherical, dorsal to germarium. Prostatic reservoir and prostatic glands not observed. Germarium elongate; oviduct, ootype, uterus, vagina, and seminal receptacle not observed. Vitellaria densely distributed throughout trunk. Egg not observed.
Remarks
Jainus sensai n. sp. most closely resembles J. iocensis and J. ornatus in having ventral anchors with superficial and deep roots bearing weakly sclerotized extensions. However, it differs from both congeners in the greater length and slender morphology of these extensions. Jainus sensai n. sp. also resembles J. loretoensis and J. radixelongatus in having a J-shaped, coiled MCO, but differs in the configuration of the copulatory complex. As in J. radixelongatus, the MCO of J. sensai n. sp. forms a counterclockwise ring, whereas in J. loretoensis the ring is clockwise. In J. sensai n. sp., the distal portion of the copulatory tube extends through an opening in the accessory piece. The accessory piece is flattened and laminar, with pointed extremities and an elongate, strongly curved projection tapering to a pointed tip, differing from the two-subunit accessory piece of J. radixelongatus and the distally bifurcated accessory piece with a median branch of J. loretoensis. In J. sensai n. sp., the accessory piece is connected to the base of the MCO by a delicate ligament. The new species further differs from its congeners in having a weakly sclerotized median process on the ventral bar, whereas the median process, when present in other species of Jainus, has not been described as weakly sclerotized. Additionally, J. sensai n. sp. shares with J. hexops the presence of dissimilar hooks but differs in having an elongated hook pair I, hook pair V with a slender thumb, and the remaining hook pairs with thicker thumbs and rounded bases, whereas J. hexops has hooks predominantly characterized by a narrow shank and erect thumb, with dilation of the shank in some hook pairs.
Molecular analyses
Partial LSU rDNA and COI mtDNA sequences were successfully obtained from specimens of J. sensai n. sp. The LSU rDNA sequences (GenBank accession numbers PX687974 and PX687975) were 1,330 and 1,367 bp long, respectively, whereas the COI mtDNA sequences (PX685598 and PX685599) were 942 and 951 bp long, respectively.
Bayesian inference (BI) and maximum likelihood (ML) analyses of the LSU rDNA dataset recovered phylograms with similar topologies and generally congruent clades (Fig. 3). In both analyses, the two sequences of J. sensai n. sp. clustered together as a distinct lineage within Jainus, in association with J. piava and J. radixelongatus (pp = 0.73; bootstrap = 33).
Phylogram with Bayesian inference topology for the partial LSU rDNA gene showing the position of Jainus sensai n. sp. among other closely related monopisthocotylans. Numbers before nodes correspond to support for posterior probabilities (pp) and bootstrap values (bv), given as pp/bv. GenBank® accession numbers follow species names. The new sequences are presented in bold. The branch length scale bar indicates the number of substitutions per site.
Figure 3. Long description
A Bayesian inference phylogram based on partial L S U r D N A sequences. The tree originates from a root on the left with a support value of 1 forward slash 100.
Top section (Characiformes, green background):
- Acestrorhamphidae: Includes Cacatuocotyle chajuli O Q 8 8 8 6 9 6, C. papilionis M G 8 3 2 8 8 9, and Diaphorocleidus species.
- Serrasalmidae: Rhinoxenus euryxenus O R 0 6 6 2 2 7 and R. paranaensis O R 0 6 6 2 2 6.
- Anostomidae: A large clade of Urocleidoides species including U. curvocuspidis, U. paradoxus, U. digitabulum, U. sinus, U. vanini, and U. atilaiamarinoi.
- Triportheidae: Contains Jainus radixelongatus O Q 8 4 3 0 1 8, J. piava O Q 8 4 3 0 1 9, and the new species Jainus sensai n. sp. P X 6 8 7 9 7 4 and P X 6 8 7 9 7 5 in bold text.
Middle section (Gymnotiformes, yellow background):
- Gymnotidae: Urocleidoides gymnotus, U. carapus, and U. uncinus.
Lower section (Siluriformes, blue background):
- Doradidae: Cosmetocleithrum gussevi, C. gigas, and C. laciniatum.
- Loricariidae: Demidospermus spirophallus and D. anus.
- Pimelodidae: Nanayella fluctuatrium, N. aculeatrium, Boegeriella conica, and B. ophiocirrus.
Bottom section (Cichliformes, purple background):
- Cichlidae: Sciadicleithrum mexicanum, S. splendidae, and Parasciadicleithrum octofasciatum.
Final section (Outgroup, grey background):
- Includes Pseudorhabdosynochus epinepheli, P. lantauensis, and Murraytrema pricei.
A scale bar at the bottom indicates 0.2 substitutions per site. Support values (p p forward slash b v) are provided at each node, such as 1 forward slash 100 or 0.99 forward slash 91.
The BI and ML analyses of the COI mtDNA dataset also recovered phylograms with similar topologies and generally congruent clades (Fig. 4). In both analyses, the two sequences of J. sensai n. sp. clustered together as a distinct lineage within Jainus, recovered as sister to the clade formed by J. piava and J. radixelongatus (pp = 0.41; bootstrap = 19).
Phylogram with Bayesian inference topology for the partial COI mtDNA gene showing the position of Jainus sensai n. sp. among other closely related monopisthocotylans. Numbers before nodes correspond to support for posterior probabilities (pp) and bootstrap values (bv), given as pp/bv. GenBank® accession numbers follow species names. The new sequences are presented in bold. The branch length scale bar indicates the number of substitutions per site.
Figure 4. Long description
The phylogram is organized into color-coded horizontal bands representing different orders.
* Top section (light blue and teal): Serrasalmidae and Heptapteridae within Siluriformes. Includes Anacanthorus species and Aphanoblastella travassosi.
* Second section (dark green): Anostomidae and Triportheidae within Characiformes. This section contains the new species Jainus sensai n. sp. (accession numbers P X 6 8 5 5 9 8 and P X 6 8 5 5 9 9) in bold, clustered with Jainus piava, Jainus beccus, and Jainus radixelongatus.
* Third section (yellow): Gymnotidae and Hypopomidae within Gymnotiformes, featuring Urocleidoides species.
* Fourth section (medium green): Acestrorhamphidae within Characiformes, featuring Diaphorocleidus species.
* Fifth section (purple): Cichlidae within Cichliformes, featuring Cichlidogyrus species and Sciadicleithrum mexicanum.
* Bottom: Acanthocotyle gurgesiella serves as the outgroup.
Nodes are labeled with posterior probabilities and bootstrap values (p p forward slash b v). A scale bar at the bottom indicates 0.2 substitutions per site.
The two sequences of J. sensai n. sp. showed identical divergence values in comparisons with the other Jainus species. For the partial LSU rDNA sequences, genetic divergence was 20% from J. piava and 21% from J. radixelongatus. For the partial COI mtDNA sequences, divergence was 32% from J. piava and 28% from J. radixelongatus.
Discussion
The phylogenetic relationships of J. sensai n. sp. within Jainus received support values below the thresholds adopted in this study, which may be related, at least in part, to the limited molecular sampling currently available for the genus. Most described species of Jainus still lack molecular data, and the available sequences used for comparison are from species associated with anostomid hosts, whereas J. sensai n. sp. was found in a triportheid host. Recent studies have also increased knowledge of Jainus associated with triportheid fishes, with three species described from T. angulatus (Morey et al. Reference Morey, Viana, Chota and Chero2025). However, molecular sequences for these species are not yet available, and they could therefore not be included in the present phylogenetic analyses. Host association is an important component of monopisthocotylan diversification, and previous phylogenetic analyses have recovered dactylogyrid lineages associated with particular host groups, including patterns related to host order and family (Yamada et al. Reference Yamada, Müller, Zago, Yamada, Ebert, Franceschini and Silva2023). Accordingly, the current representation of Jainus in molecular datasets may not encompass the diversity of host associations known for the genus. The genetic distance results also require cautious interpretation. The LSU rDNA distance matrix showed overlap between congeneric and intergeneric distances, indicating that this marker should not be used alone as evidence of taxonomic differentiation. Morphological and molecular data therefore provide complementary information for species differentiation and for understanding relationships within Jainus.
Morphological variation within Jainus is especially evident in the haptoral and copulatory structures. The ventral anchor remains one of the main characters associated with the genus, although the relative development and shape of its roots vary among species. Recent descriptions have revealed considerable variation in these structures, including modifications of the superficial and deep roots (Morey et al. Reference Morey, Viana, Chota and Chero2025). The condition observed in J. sensai n. sp., with both roots bearing elongate, weakly sclerotized extensions, adds to the morphological variation currently known within the genus while remaining consistent with the general configuration of the ventral anchor in Jainus. Variation is also evident in the copulatory complex, particularly in the number and direction of MCO coils, the connection between the MCO and accessory piece, and the morphology of the accessory piece. Jainus amazonensis, J. leporini, and J. radixelongatus show markedly different configurations of these structures (Abdallah et al. Reference Abdallah, Azevedo and Luque2012; Kritsky et al. Reference Kritsky, Thatcher and Kayton1980; Yamada et al. Reference Yamada, Müller, Zago, Yamada, Ebert, Franceschini and Silva2023), and the copulatory complex of J. sensai n. sp. further extends the range of morphological configurations currently known within Jainus.
The occurrence of J. sensai n. sp. in A. halecinus represents the first record of Jainus in the host genus Agoniates and expands the known host range of the parasite genus within Triportheidae. Considering the diversity of Triportheidae and the limited number of its species investigated for monopisthocotylan parasites, the diversity and host associations of these parasites within the family may be underestimated in Neotropical freshwater systems. The occurrence of Jainus in another triportheid genus further indicates that additional host associations may remain undocumented within this fish family.
Supplementary material
The supplementary material for this article can be found at http://doi.org/10.1017/S0022149X2610248X.
Availability of data and material
All data will be available for consultation. Whenever necessary, they should be requested from the corresponding author. The specimens used for this work will be available in specialized scientific collections.
Acknowledgements
We would like to thank the members of LAPAS (Laboratory of Parasitology of Wild Animals, Institute of Biosciences, São Paulo State University – UNESP, Botucatu), Rodrigo Narciso, Julio Aguiar, and Philippe Alves, for their invaluable assistance during fieldwork and for their continuous support. We also acknowledge the non-governmental organization for the preservation of Brazilian wild animals (ONG PAS) and the Graduate Program in Veterinary Sciences at the Federal University of Uberlândia (PPGCVET/UFU) for their assistance in the fish collection.
Authors’ contribution
All authors contributed to the study’s conception and design. Material preparation, data collection, and analysis were performed by M.M.O-P. and R.J.S. The first version of the manuscript was written by M.M.O-P. and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.
Financial support
We thank the agencies for the financial support to this study: FAPESP – São Paulo Research Foundation [R.J.S. #2020/05412-9; M.M.O-P. #2025/07772-6] and CNPq – National Council for Scientific and Technological Development [R.J.S. #311635/2021–0].
Competing interests
On behalf of all authors, the corresponding author states that there is no conflict of interest.
Ethical standards
All applicable international, national, and/or institutional guidelines for the use and care of animals were followed.
