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Host-parasite associations have historically been considered compelling examples of coevolution and useful in examining cospeciation. However, modern molecular methods have revealed more complex dynamics than previously assumed, with host-switching events appearing commonly across taxa and challenging traditional views of strict coevolution in host-parasite relationships. Monogenean parasites are considered highly host-specific and have long served as models for probing evolution of host-parasite associations, particularly in differentiating geographic and phylogenetic patterns of parasite diversification. We investigated the phylogeographic patterns of monogenean ectoparasites associated with four species of characin fishes across Panama, Nicaragua, and Mexico. We hypothesize that parasite diversity and community structure are more strongly correlated with host species (suggesting cospeciation) than with geographic location (indicative of allopatric speciation). We found high genetic differentiation among parasites and their hosts across different locations. However, while geography explained the genetic structure of both host fishes and parasites, the observed patterns were neither congruent nor parallel. Parasite community structure and genetic similarity were consistently better explained by geographic location than by host species identity, although both factors played a significant role. Contrary to our predictions, we found no evidence of cospeciation. Instead, the diversification of these monogenean parasites appears to be primarily driven by their ability to switch hosts. At this taxonomical scale, host-switching is mediated by the geographical proximity of potential hosts, underscoring the importance of spatial factors in parasite evolution.

Based on molecular data three major clades have been recognized within Bilateria: Deuterostomia, Ecdysozoa, and Spiralia. Within Spiralia, small-sized and simply organized animals such as flatworms, gastrotrichs, and gnathostomulids have recently been grouped together as Platyzoa. However, the representation of putative platyzoans was low in the respective molecular phylogenetic studies, in terms of both, taxon number and sequence data. Furthermore, increased substitution rates in platyzoan taxa raised the possibility that monophyletic Platyzoa represents an artifact due to long-branch attraction. In order to overcome such problems, we employed a phylogenomic approach, thereby substantially increasing 1) the number of sampled species within Platyzoa and 2) species-specific sequence coverage in data sets of up to 82,162 amino acid positions. Using established and new measures (long-branch score), we disentangled phylogenetic signal from misleading effects such as long-branch attraction. In doing so, our phylogenomic analyses did not recover a monophyletic origin of platyzoan taxa that, instead, appeared paraphyletic with respect to the other spiralians. Platyhelminthes and Gastrotricha formed a monophylum, which we name Rouphozoa. To the exclusion of Gnathifera, Rouphozoa and all other spiralians represent a monophyletic group, which we name Platytrochozoa. Platyzoan paraphyly suggests that the last common ancestor of Spiralia was a simple-bodied organism lacking coelomic cavities, segmentation, and complex brain structures, and that more complex animals such as annelids evolved from such a simply organized ancestor. This conclusion contradicts alternative evolutionary scenarios proposing an annelid-like ancestor of Bilateria and Spiralia and several independent events of secondary reduction.

The Neodermata is a group of parasitic flatworms that includes the classes Trematoda, Cestoda, and Monogenea. Understanding the phylogenetic relationships within the Neodermata has been a longstanding challenge. Molecular studies utilizing different datasets have produced variable results, leading to differing evolutionary hypotheses. Resolving the phylogenetic relationships requires careful consideration of the molecular targets and sequences used. In this study, our objective was to investigate the topological variability of phylogenetic trees by examining different mitochondrial genes, molecular datasets (nucleotides and amino acids), as well as the 18S and 28S nuclear rRNA genes, and three software packages used for phylogenetic analysis. To evaluate the utility of different markers, we constructed 96 unilocus trees and nine multilocus trees. Our findings revealed that each gene provided unique information and resulted in different topologies depending on the sequences used, with only few mitochondrial genes indicating the monophyly of the Monogenea. Multilocus analyses mitochondrial and mitochondrial + 18S + 28S produced a consistent topology, supporting the monophyly of each of the four major neodermatan lineages (Cestoda, Trematoda, Monopisthocotylea, and Polyopisthocotylea). Notably, the monophyly of the Polyopisthocotylea and Cestoda consistently appeared in the different analyses. Conversely, we observed discrepancies between results obtained from mitochondrial genes and nuclear genes. This study contributes to our understanding of the phylogeny of the Neodermata by examining the topological variability of phylogenetic trees using both mitochondrial and nuclear genes. Our results emphasize that carefully selected molecular markers and multilocus approaches are crucial for illuminating the complex evolutionary history within the Neodermata.

Fish are exposed to various parasites such as monogeneans which are flatworms that preferentially affect the gills and skin of these hosts. This study represents the first investigation concerning the distribution of monogenean infracommunities of Enteromius guirali in Cameroon. A total of 100 fish were collected from Cesala River and preserved in 8% formalin solution. Parasite community consisted of Dactylogyrus mendehei and D. nyongensis . These monogenean species adopted an aggregated distribution. Their mean intensities and prevalence increased with host size. Both sides of E. guirali were similarly infested, and no preference for gill arches was recorded for each parasite species. Posterior hemibranch was always the most parasitized, showing the highest parasitic load. Median sector and distal filamentous zone were highly colonized by both monogenean species. This study contributes to understand that the large gill surface offered by great hosts, heterogeneity, bilateral symmetrically of the host’s gill system on one hand, and the morphology of haptoral parts and reproduction of parasites, on the other hand, are the main factors which determine the distribution of studied component community.

Microstomidae (Platyhelminthes: Macrostomorpha) diversity has been almost entirely ignored within recent years, likely due to inconsistent and often old taxonomic literature and a general rarity of sexually mature collected specimens. Herein, we reconstruct the phylogenetic relationships of the group using both previously published and new 18S and CO1 gene sequences. We present some taxonomic revisions of Microstomidae and further describe 8 new species of Microstomum based on both molecular and morphological evidence. Finally, we briefly review the morphological taxonomy of each species and provide a key to aid in future research and identification that is not dependent on reproductive morphology. Our goal is to clarify the taxonomy and facilitate future research into an otherwise very understudied group of tiny (but important) flatworms.

Studies of forces driving interlineage variability in the evolutionary rates (both sequence and architecture) of mitochondrial genomes often produce contradictory results. Flatworms (Platyhelminthes) exhibit the fastest-evolving mitogenomic sequences among all bilaterian phyla. To test the effects of multiple factors previously associated with different aspects of mitogenomic evolution, we used mitogenomes of 223 flatworm species, phylogenetic multilevel regression models, and causal inference. Thermic host environment (endothermic vs. ectothermic) had nonsignificant impacts on both sequence evolution and mitogenomic size. Mitogenomic gene order rearrangements (GORR) were mostly positively correlated with mitogenomic size (R2 ≈ 20–30%). Longevity was not (negatively) correlated with sequence evolution in flatworms. The predominantly free-living “turbellaria” exhibited much shorter branches and faster-evolving mitogenomic architecture than parasitic Neodermata. As a result, “parasitism” had a strong explanatory power on the branch length variability (>90%), and there was a negative correlation between GORR and branch length. However, the stem branch of Neodermata comprised 63.6% of the total average branch length. This evolutionary period was also marked by a high rate of gene order rearrangements in the ancestral Neodermata. We discuss how this period of rapid evolution deep in the evolutionary history may have decoupled sequence evolution rates from longevity and GORR, and overestimated the explanatory power of “parasitism”. This study shows that impacts of variables often vary across lineages, and stresses the importance accounting for the episodic nature of evolutionary patterns in studies of mitogenomic evolution.

Aggregation is a fundamental feature of parasite distribution in the host population, but the biological implications of the aggregation indices used to describe the relationships between the populations of parasites and hosts are not evident. It is speculated that the form of distribution in each case is predicated on the host’s varying resistance to the infection, which is hard to control, making it difficult to adequately interpret the index values. This paper examines several cases from trout farms in Russian Karelia to explore the monogenean Gyrodactylus spp. infection in rainbow trout of varying ages. The genetic homogeneity of cage-reared fish and the direct life cycle of the helminths make the relationship between the species more lucid than in natural host-parasite systems. The results give no ground to speak of any specific patterns: as well as in the natural systems, the infection rates in trout vary widely, i.e., the helminth distribution has not become more uniform; the observed distributions in all cases are adequately approximated by the negative binomial model; the positive abundance-occupancy relationships (AORs) and abundance-variance relationships (AVRs) common for parasitic systems apply to the basic infection parameters. The form of the negative binomial distribution is shaped by two parameters— k and θ , the former being a metric of the infection variability, which depends on the host’s individual resistance, and the latter representing the parasites’ reproduction and establishment success rates. A rise in the parameter k indicates increased aggregation and a higher parameter θ points to a more uniform frequency distribution. These parameters can be used as a representative tool for monitoring the parasite communities in salmonid fishes, including in aquaculture.

Dactylogyrids are flatworms of ecological and economic significance, parasitizing fish worldwide. In recent years, there has been a surge in the description of Neotropical dactylogyrids, particularly those infecting siluriform fishes. While these studies have contributed to the organization of some genera and refined species boundaries through integrative taxonomy, certain groups within the family, such as Demidospermus , remain taxonomically unstable. This study focuses on Demidospermus , aiming to reclassify species of uncertain status into appropriate genera and establish a morphological framework to support future evolutionary analyses and taxonomic revisions within the Demidospermus -like species group. Supported by morphological phylogenetic analysis, we propose the new genera Rhabdolachosus n. gen., Martorellius n. gen., Magnanchistrius n. gen., and Sicohencotyle n. gen., along with the resurrection of Omothecium Kritsky, Thatcher & Boeger, 1987, and Paramphocleithrium Suriano & Incorvaia, 1995. Additionally, two new species are described: Sicohencotyle antoniomaiai n. gen. n. sp. and Ameloblastella sakulocirra n. sp. Also, Demidospermus centromochi Mendoza-Franco & Scholz, 2009 is classified as sedis mutabilis , while D. annulus Marcotegui & Martorelli, 2011, D. brevicirrus Mendoza-Palmero et al. , 2012, D. cornicinus Kritsky & Gutierrez, 1998, D. idolus Kritsky & Gutierrez, 1998, D. armostus Kritsky & Gutierrez, 1998, D. mortenthaleri Mendoza-Palmero et al. , 2012, D. osteomystax Tavernari et al. , 2010, D. tocantinensis Cohen et al. , 2020, D. doncellae Morey et al. , 2024, D. bifurcatus Justo, Martins & Cohen, 2024, D. juruaensis Justo, Martins & Cohen, 2024, and D. takemotoi Justo, Martins & Cohen, 2024 are considered incertae sedis . Lastly, Urocleidoides amazonensis Mizelle & Kritsky, 1969 remains classified as incertae sedis . Les Dactylogyridae sont des vers plats d’importance écologique et économique, parasitant les poissons du monde entier. Ces dernières années, on a assisté à une forte augmentation de la description de Dactylogyridae néotropicaux, en particulier ceux qui infectent les poissons siluriformes. Si ces études ont contribué à l’organisation de certains genres et à l’affinement des frontières entre espèces grâce à une taxonomie intégrative, certains groupes de la famille, comme Demidospermus , demeurent taxonomiquement instables. Cette étude se concentre sur Demidospermus et vise à reclasser les espèces au statut incertain dans des genres appropriés et à établir un cadre morphologique pour étayer les futures analyses évolutives et révisions taxonomiques au sein du groupe d’espèces apparentées à Demidospermus . En nous appuyant sur une analyse phylogénétique morphologique, nous proposons les nouveaux genres Rhabdolachosus n. gen., Martorellius n. gen., Magnanchistrius n. gen. et Sicohencotyle n. gen., ainsi que la résurrection d’ Omothecium Kritsky, Thatcher & Boeger, 1987 et de Paramphocleithrium Suriano & Incorvaia, 1995. De plus, deux nouvelles espèces sont décrites : Sicohencotyle antoniomaiai n. gen. n. sp. et Ameloblastella sakulocirra n. sp. Demidospermus centromochi Mendoza-Franco & Scholz, 2009 est classé comme sedis mutabilis , tandis que D. annulus Marcotegui & Martorelli, 2011, D. brevicirrus Mendoza-Palmero et al. , 2012, D. cornicinus Kritsky & Gutierrez, 1998, D. idolus Kritsky & Gutierrez, 1998, D. armostus Kritsky & Gutierrez, 1998, D. mortenthaleri Mendoza-Palmero et al. , 2012, D. osteomystax Tavernari et al. , 2010, D. tocantinensis Cohen et al. , 2020, D. doncellae Morey et al. , 2024, D. bifurcatus Justo, Martins & Cohen, 2024, D. juruaensis Justo, Martins & Cohen, 2024 et D. takemotoi Justo, Martins & Cohen, 2024, sont considérés comme incertae sedis . Urocleidoides amazonensis Mizelle & Kritsky, 1969 reste classé comme incertae sedis .

The pindani, Chindongo socolofi (Cichliformes, Cichlidae) is a popular freshwater ornamental fish from Lake Malawi in Africa. Although identifying parasites associated with the global ornamental fish trade is critical for developing biosecurity practices, little is known about the parasite fauna of C. socolofi . Therefore, this study sought to determine what monogenean parasites C. socolofi harbours in India. Adult specimens of this host species were collected from various aquarium shops across the country between 2020 and 2022, and their gills were subjected to parasitological examination. Monogeneans were detected in five host specimens (22.7%) with low mean intensities (6.2 ± 3.8). They were identified as Cichlidogyrus tilapiae (Monogenea: Dactylogyridae) based on the presence of the following morphometric characteristics: two pairs of anchors, two auricles on the dorsal bar, a V-shaped ventral bar, and an accessory piece with a folded rim and a bent bifurcated tip. The morphological identification was confirmed by the sequence analysis of the specimen’s 18S-ITS1 gene regions and 28S rRNA genes to C. tilapiae from Paratilapia polleni (Cichliformes, Cichlidae) in Madagascar. This article is the first report on a species of Cichlidogyrus in India, found in aquarium shops, contributing to the growing list of known freshwater monogeneans that are being distributed globally via the ornamental fish trade. Additionally, it adds a new host species ( C. socolofi ) and geographic location (India, within aquarium trade) to the existing knowledge of C. tilapiae , a widespread and often co-introduced tropical fish parasite. Le pindani, Chindongo socolofi (Cichliformes, Cichlidae), est un poisson d’ornement d’eau douce populaire originaire du lac Malawi, en Afrique. Bien que l’identification des parasites associés au commerce mondial des poissons d’ornement soit essentielle au développement de pratiques de biosécurité, on sait peu de choses sur la faune parasitaire de C. socolofi . Par conséquent, cette étude visait à déterminer quels monogènes parasites C. socolofi héberge en Inde. Des spécimens adultes de cette espèce hôte ont été collectés dans divers magasins d’aquariophilie du pays entre 2020 et 2022, et leurs branchies ont été soumises à un examen parasitologique. Des monogènes ont été détectés chez cinq spécimens hôtes (22,7 %) avec de faibles intensités moyennes (6,2 ± 3,8). Ils ont été identifiés comme Cichlidogyrus tilapiae (Monogenea : Dactylogyridae) grâce à la présence des caractéristiques morphométriques suivantes : deux paires d’ancres, deux auricules sur la barre dorsale, une barre ventrale en V et une pièce accessoire avec un bord recourbé et une extrémité bifurquée et courbée. L’identification morphologique a été confirmée par l’analyse de la séquence des régions du gène 18S-ITS1 et des gènes de l’ARNr 28S du spécimen de C. tilapiae de Paratilapia polleni (Cichliformes, Cichlidae) à Madagascar. Cet article est le premier rapport sur la présence d’une espèce de Cichlidogyrus en Inde, dans des aquariums, et contribue à compléter la liste croissante des monogènes d’eau douce connus et introduits dans le monde entier via le commerce des poissons d’ornement. De plus, il ajoute une nouvelle espèce hôte ( C. socolofi ) et une nouvelle localisation géographique (l’Inde, dans des magasins d’aquariophilie) aux connaissances existantes sur C. tilapiae , un parasite répandu et souvent co-introduit chez les poissons tropicaux.

The parasites infecting invasive carps in North America (all Cypriniformes: Xenocyprididae: grass carp, Ctenopharyngodon idella [Valenciennes, 1844]; silver carp, Hypophthalmichthys molitrix [Valenciennes, 1844]; bighead carp, Hypophthalmichthys nobilis [Richardson, 1845]; and black carp, Mylopharyngodon piceus [Richardson, 1846]) are little studied, and no parasite has been reported from silver carp there. We herein surveyed silver carp from Barkley Reservoir and Cheatham Reservoir (Cumberland River, Tennessee; June and December 2021) and the White River (Arkansas; May 2022) and collected numerous monogenoid specimens infecting the pores on the outer face of the gill raker plate. We heat-killed, formalin-fixed, and routinely stained some specimens for morphology and preserved others in 95% ethanol for DNA extraction and sequencing of the large subunit ribosomal DNA (28S). We identified our specimens as Dactylogyrus cf. skrjabini because they had a dorsal anchor deep root that is much longer than the superficial root, an approximately parallel penis and accessory piece, and a relatively large marginal hook pair V. No type specimen of Dactylogyrus skrjabiniAkhmerov, 1954 (type host and locality is silver carp, Amur River, Russia) is publicly available, but we borrowed several vouchers (NSMT-Pl 6393) that infected the gill rakers of silver carp captured in the Watarase River, Japan. The original description of D. skrjabini was highly stylized and diagrammatical, differing from the specimens we studied from North America and Japan by the dorsal anchor having a superficial root and shaft that comprise a strongly C-shaped hook (the superficial root curves toward the dorsal anchor point) (vs. superficial root straight, at ∼45° angle to deep root and directed away from the dorsal anchor point), a single, much reduced transverse bar that is narrow for its entire breadth (vs. dorsal and ventral transverse bars robust and broad, having an irregular outline), an accessory piece that lacks digitiform projections (vs. accessory piece with 4 digitiform projections), and an accessory piece that lacks a half cardioid-shaped process (vs. accessory piece having a half cardioid-shaped process). Our 28S sequences (generated from 4 specimens of D. cf. skrjabini: 2 from Tennessee [763 base pairs (bp)] and 2 from Arkansas [776 bp]) were identical to 1 ascribed to D. skrjabini from Japan. The present study is the first verifiable and credible report of a parasite from silver carp in North America and the first nucleotide information for a parasite from silver carp in North America.