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Parasites are often hidden in their hosts and exhibit patchy spatial distributions. This makes them relatively difficult to detect and sample. Consequently we have poor knowledge of parasite diversities, distributions, and extinction. We evaluate our general understanding of parasite diversity and highlight the enormous bias in research on parasites such as helminths and arthropods that infect vertebrate hosts. We then focus on Myxozoa as an exemplary case for demonstrating uncharted parasite diversity. Myxozoans are a poorly recognized but speciose clade of endoparasitic cnidarians with complex life cycles that have radiated to exploit freshwater, marine, and terrestrial hosts by adopting strategies convergent to those of parasitic protists. Myxozoans are estimated to represent some 20% of described cnidarian species—greatly outnumbering the combined species richness of scyphozoans, cubozoans, and staurozoans. We summarize limited understanding of myxozoan diversification and geographical distributions, and highlight gaps in knowledge and approaches for measuring myxozoan diversity. We close by reviewing methods and problems in estimating parasite extinction and concerns about extinction risks in view of the fundamental roles parasites play in ecosystem dynamics and in driving host evolutionary trajectories.

The present study consists of a detailed phylogenetic analysis of myxosporeans of the Myxobolus and Henneguya genera, including sequences from 12 Myxobolus/Henneguya species, parasites of South American pimelodids, bryconids and characids. Maximum likelihood and maximum parsimony analyses, based on 18 S rDNA gene sequences, showed that the strongest evolutionary signal is the phylogenetic affinity of the fish hosts, with clustering mainly occurring according to the order and/or family of the host. Of the 12 South American species studied here, six are newly described infecting fish from the Brazilian Pantanal wetland. Henneguya maculosus n. sp. and Myxobolus flavus n. sp. were found infecting both Pseudoplatystoma corruscans and Pseudoplatystoma reticulatum; Myxobolus aureus n. sp. and Myxobolus pantanalis n. sp. were observed parasitizing Salminus brasiliensis and Myxobolus umidus n. sp. and Myxobolus piraputangae n. sp. were detected infecting Brycon hilarii.

Cnidaria, the sister group to Bilateria, is a highly diverse group of animals in terms of morphology, lifecycles, ecology, and development. How this diversity originated and evolved is not well understood because phylogenetic relationships among major cnidarian lineages are unclear, and recent studies present contrasting phylogenetic hypotheses. Here, we use transcriptome data from 15 newly-sequenced species in combination with 26 publicly available genomes and transcriptomes to assess phylogenetic relationships among major cnidarian lineages. Phylogenetic analyses using different partition schemes and models of molecular evolution, as well as topology tests for alternative phylogenetic relationships, support the monophyly of Medusozoa, Anthozoa, Octocorallia, Hydrozoa, and a clade consisting of Staurozoa, Cubozoa, and Scyphozoa. Support for the monophyly of Hexacorallia is weak due to the equivocal position of Ceriantharia. Taken together, these results further resolve deep cnidarian relationships, largely support traditional phylogenetic views on relationships, and provide a historical framework for studying the evolutionary processes involved in one of the most ancient animal radiations.

A new species of Myxobolidae, Henneguya cardii n. sp., is described infecting the European seabass Dicentrarchus labrax , a fish of high commercial value intensively cultivated in southern Europe. Henneguya cardii n. sp. was found in the bulbus arteriosus and spleen with a prevalence of infection of 13.5%. In the heart, it forms irregular whitish plasmodia measuring 1 mm in size. Mature myxospores are broadly obovoid in frontal view and ellipsoidal in lateral view, with 2 equal caudal appendages. Polar capsules are ovoid and symmetric, with 3–4 polar tubule coils. Myxospores measure 10.2 ± 0.6 (8.8–11.6) μm in length, 8.0 ± 0.7 (5.3–8.8) μm in width and 5.6 ± 0.2 (5.1–6.4) μm in thickness. Caudal appendages are 36.6 ± 3.2 (27.4–42.9) μm long. Total spore length is 47.6 ± 3.2 (41.2–53.2) μm. Polar capsules measure 4.0 ± 0.2 (3.4–4.6) by 2.2 ± 0.1 (1.9–2.6) μm. Small subunit ribosomal RNA-based tree topologies position H. cardii n. sp. within a lineage of marine myxobolids that is mostly comprised of other Henneguya species. Host-relatedness is reinforced as the main evolutionary driver for myxobolids, with the positioning of H. cardii n. sp. further suggesting tissue tropism as another important evolutionary driver for marine heart infecting Henneguya . Nonetheless, the inner complexity of this lineage suggests that identification of the evolutionary patterns driving its phylogeny will require discovery of the true diversity of marine myxobolids.

The Amazon basin has the largest number of fish in the world, and among the most common fishes of the Neotropical region, the threespot ( Leporinus friderici ) is cited, which in relation to its microparasitic fauna, has described only 1 species of the genus Henneguya , Henneguya friderici . The Myxozoa class is considered an obligate parasite, being morphologically characterized by spores formed by valves connected by a suture line. This study describes a new species of Henneguya sp. in the Amazon region for L. friderici . This parasite was found in the host's pyloric caeca and caudal kidney, with mature spores with a total spore length of 38.4 ± 2.5 (35.9–40.9) μ m; the spore body 14.4 ± 1.1 (13.3–15.5) μ m and 7.3 ± 0.6 (6.7–7.9) μ m wide. Regarding its 2 polar capsules, they had a length of 5.1 ± 0.4 (4.7–5.5) μ m and a width of 2.0 ± 0.1 (1.9–2.1) μ m in the same pear-shaped, and each polar capsule contained 9–11 turns. Morphological and phylogenetic analyses denote that this is a new species of the genus Henneguya .

Up to this time, only five myxosporean species have been documented from fishes of the Barbonymus genus. Due to a limited number of myxozoan studies conducted in Southeast Asia, particularly in Malaysia, the diversity of this parasite group remains largely undiscovered. In this study, a comprehensive parasitology survey was conducted, revealing nine myxozoan parasites, including five different Myxobolus spp., three Thelohanellus sp. and one Myxidium sp. Using morphological and molecular data, we describe here five new species: Myxobolus gonionoti n. sp., found in the gill filaments; Myxobolus barbonymi n. sp., and Myxobolus faizahae n. sp. found in the muscle cells; Thelohanellus gonionoti n. sp. found in the fins; and Thelohanellus barbonymi n. sp. found in the gill arches. Additionally, we identified spores of the previously described Myxobolus dykovae in the gill lamellae of B. schwanefeldii and Thelohanellus zahrahae in the gill filaments of B. gonionotus . Furthermore, two undescribed species were documented solely based on morphology and morphometrics, a Myxobolus sp. from the muscle cells of B. schwanefeldii and a Myxidium sp. from the gallbladder of B. gonionotus .

The Celestún Lagoon is located on the coast of the Yucatán Peninsula in southern Mexico. The peripheral margins of the lagoon are exposed to a constant influx of water from freshwater springs, where a wide diversity of freshwater fishes occurs. Simultaneously, these freshwater springs are mixing with marine water from the sea and brackish water from the lagoon. Gambusia yucatana is an endemic poeciliid freshwater fish of this coastal region, with no current parasitological records. In December 2023, 24 specimens from two freshwater springs near the Celestún Lagoon were examined for myxozoan infection. Disporic plasmodia containing mature myxospores floating freely within the bile were detected in all fish. The myxospores were ellipsoidal in the valvular and sutural views, measuring 9.5 ± 0.6 μm length and 6.5 ± 0.5 μm width. Two pyriform polar capsules discharging on opposite sides, 3.2 ± 0.4 μm length and 2.3 ± 0.2 μm width, and 5–6 polar filament coils. Polar tubule 31.3 μm long and coiled 5–6 times. The partial sequences of 18S rDNA and 28S rDNA displayed a similarity of ≤ 96.4% and ≤ 85.2% to all other available sequences from the genus Ellipsomyxa . Phylogenetic analyses revealed that the novel species named Ellipsomyxa prima n. sp. is closely related to other congeneric Ellipsomyxa parasites. This study describes the first parasite in G . yucatana and the first Ellipsomyxa species reported in a fish of the order Cyprinodontiformes and the family Poeciliidae.

Four new actinospore types belonging to the sphaeractinomyxon collective group (Cnidaria, Myxosporea) are described from the coelomic cavity of a marine Baltidrilus sp. (Oligochaeta, Naididae) inhabiting a northern Portuguese estuary. Host identification supports the usage of marine oligochaetes, namely of the family Naididae Ehrenberg, 1828, as definitive hosts for myxosporeans inhabiting estuarine/marine environments. The absence of mixed infections in the host specimens analysed is suggested to reflect the influence of host-, parasite- and environmental-related factors regulating myxosporean–annelid interactions. Molecular analyses matched the SSU rDNA sequences of three of the four new types with those of mugiliform-infecting Myxobolus spp., namely Myxobolus mugiliensis and a Myxobolus sp. from flathead grey mullet Mugil cephalus, and Myxobolus labrosus from thicklip grey mullet Chelon labrosus. These results directly link, for the first time, the sphaeractinomyxon collective group to a myxospore counterpart, further confirming their previously hypothesized specific involvement in the life cycle of myxobolids that infect mullets. Acknowledging this life cycle relationship, the functionality of the sphaeractinomyxon morphotype is suggested to have been decisive for the evolutionary hyperdiversification of the genus Myxobolus in mullets. Unlike other actinospore morphotypes, sphaeractinomyxon lack valvular processes, which implies a limited capability for buoyancy. Considering the benthic-feeding nature of mullets, this feature is most likely crucial in promoting successful transmission to the vertebrate host.

A novel myxosporean species was identified. The species formed spherical to ellipsoidal pseudocysts within the alimentary tract wall of a yellowfin seabream Acanthopagrus latus fished in the East China Sea. Histological examination confirmed that pseudocysts were localized within the submucosal layer of the stomach wall. Round to ellipsoidal myxospores exhibited two posterior caudal appendages, consistent with the morphological characteristics of the genus Henneguya . The myxospore body measured 9.6 ± 0.5 (8.6–10.6) μm in length, 7.3 ± 0.4 (6.8–7.9) μm in width, and 6.0 ± 0.2 (5.5–6.4) μm in thickness. Two equal pyriform polar capsules were observed, measuring 3.5 ± 0.3 (2.9–4.4) μm × 1.9 ± 0.2 (1.4–2.2) μm. Pairwise comparison referring to small subunit ribosomal DNA sequence revealed a highest identity of 94.19% with Henneguya yokoyamai Li et al. , 2012, supporting the classification of the specimens as a new species, Henneguya cystigena n. sp. Phylogenetic analyses demonstrated intermixed groupings of myxobolid species, highlighting persistent discrepancies between traditional morphological taxonomy and increasingly refined molecular phylogeny. To the best of our knowledge, this study represents the first description of a Henneguya species parasitizing a marine fish in the East China Sea near mainland China. Une nouvelle espèce de Myxosporea a été identifiée, qui formait des pseudokystes sphériques à ellipsoïdaux dans la paroi du tube digestif d’un Pagre à nageoires jaunes, Acanthopagrus latus , pêché en mer de Chine orientale. L’examen histologique a confirmé la localisation des pseudokystes dans la couche sous-muqueuse de la paroi stomacale. Les myxospores rondes à ellipsoïdales présentaient deux appendices caudaux postérieurs, compatibles avec les caractéristiques morphologiques du genre Henneguya . Le corps de la myxospore mesurait 9,6 ± 0,5 (8,6–10,6) μm de longueur, 7,3 ± 0,4 (6,8–7,9) μm de largeur et 6,0 ± 0,2 (5,5–6,4) μm d’épaisseur. Deux capsules polaires piriformes égales ont été observées, mesurant 3,5 ± 0,3 (2,9–4,4) μm × 1,9 ± 0,2 (1,4–2,2) μm. La comparaison par paires des séquences de la petite sous-unité de l’ADN ribosomique a révélé que l’identité la plus élevée était 94,19% avec Henneguya yokoyamai Li et al. , 2012, ce qui soutient la classification des spécimens comme une nouvelle espèce, Henneguya cystigena n. sp. Les analyses phylogénétiques ont mis en évidence des regroupements mixtes d’espèces de Myxobolidae, soulignant des divergences persistantes entre la taxonomie morphologique traditionnelle et une phylogénie moléculaire de plus en plus affinée. À notre connaissance, cette étude constitue la première description d’une espèce d’ Henneguya parasitant un poisson marin en mer de Chine orientale, près de la Chine continentale.

Members of the genus Ortholinea are among the worldwide distributed myxozoan parasites that mainly infect marine fish. In this study, a new myxosporean species, Ortholinea hamsiensis n. sp., was isolated from the urinary bladder of European anchovy Engraulis engrasicolus collected from the Sinop coasts of the Black Sea. The prevalence and density values of infection were 1.4% and 1–5 individuals in the field of view (1 + ), respectively. Mature myxospores are subspherical with slight tapering down to the less pronounced tip in the frontal view and subspherical in the sutural view. Myxospores measured 9.1 ± 0.25 (8.8–9.9) μm in length, 9.2 ± 0.11 (8.9–9.4) μm in thickness, and 8.4 ± 0.33 (8.2-9.1) μm in width. Two polar capsules equal in size measured 3.1 ± 0.11 (3.0–3.3) μm in length and 2.7 ± 0.11 (2.6–2.9) μm in width. The polar tubule had 3–4 coils. Along with morphological peculiarities, the results of the 18S rDNA also revealed it to be a new species for science compared to the other species of the genus. In this study, another myxosporean species O. gobiusi was also detected in round goby Neogobius melanostomus with a prevalence of infection value of 4.8% and a density of 1–5 individuals in the field of view (1 + ). The present study also provided the first data of 18S rDNA of O. gobiusi from N. melanostomus and type species of the genus O. divergens from Gobius niger and the phylogenetic relationships of these species with other Ortholinea species have been revealed.