A novel myxosporean parasite Myxobolus klamathellus n. sp. (Cnidaria: Myxosporea) from native blue chub (Gila coerulea) in Klamath Lake, Oregon

Description of Myxobolus hoabinhensis n. sp. (Myxosporea: Myxobolidae), infecting the trunk muscles of goldfish Carassius auratus (Linnaeus, 1758) (Cypriniformes: Cyprinidae) in northern Vietnam

During a survey of myxosporean parasites of freshwater fishes in northern Vietnam, myxospores resembling those of the genus Myxobolus (Myxosporea: Myxobolidae) were found in the trunk muscle of 6 out of 35 specimens (17.14%) of wild goldfish Carassius auratus (Cypriniformes: Cyprinidae). The mature spores were 12.0 ± 0.4 (11.4 - 12.6) µm long, 8.5 ± 0.2 (7.9 - 9.0) µm wide and 6.1 ± 0.2 (5.8 - 6.3) µm thick, containing two pyriform-shaped polar capsules unequal in size. The larger polar capsule was 7.6 ± 0.3 (7.1 - 8.4) µm long and 3.5 ± 0.1 (3.3 - 3.8) µm wide, and the smaller polar capsule was 6.2 ± 0.3 (5.5 - 6.7) µm long and 2.9 ± 0.2 (2.6 - 3.4) µm wide. Each polar capsule contained a polar filament with 3-5 coils. A phylogenetic analysis based on the small subunit rDNA (SSU rDNA) sequence revealed that this Myxobolus species forms a distinct branch in the phylogenetic tree sister to Myxobolus artus and Myxobolus cyprini, with DNA sequence similarity at 97.6% to M. artus and 97.5% to M. cyprini. A combination of the morphological characteristics and molecular data suggest that this is an undescribed species, and we propose the name Myxobolus hoabinhensis n. sp.

Synopsis of the species of Myxobolus (Cnidaria, Myxozoa, Myxosporea) described between 2014 and 2020

A synopsis of the species of Myxobolus Bütschli, 1882 (Cnidaria, Myxosporea, Myxobolidae) described from 2014 up till now is presented. It includes 122 nominal species described all over the world. For each of the species, the most relevant morphological and morphometric data, as well as data are provided related to the location in the host, type host and type locality. The GenBank accession numbers are provided whenever possible, and the spores were redrawn based on the original descriptions. The bibliography includes all the papers containing the species descriptions.

Novel and known myxobolids (Cnidaria, Myxozoa) infecting Chondrostoma angorense (Cypriniformes: Leuciscidae) in Turkey

Turkey has more than 200 endemic freshwater fish species, one of which is the Ankara nase, Chondrostoma angorense Elvira, 1987 (Cypriniformes: Leuciscidae), a food fish in northern Turkey. Like most endemic fish species in Turkey, its myxosporean parasite fauna (Cnidaria: Myxosporea) are not yet described. We surveyed twenty C. angorense from Lâdik Lake in northern Turkey, and identified two myxosporean parasites from gills of these fish: Myxobolus arrabonensis Cech, Borzák, Molnár, Székely, 2015, and a co-infection of a novel species, Myxobolus polati sp. nov. We characterized both infections based on myxospore morphology, morphometry, tissue tropism, small subunit ribosomal DNA sequence and phylogenetic analysis. Plasmodia of both species were observed in gills, but had distinct tropism: M. arrabonensis is an intrafilamental vascular type, and M. polati sp. nov. is an intralamellar vascular type. We identified M. arrabonensis on the basis of myxospore characters and 100% similarity to the type DNA sequence from the closely-related host C. nasus. The small subunit ribosomal DNA sequence of M. polati sp. nov. (1946 base pairs; GenBank Accession number MH392318) had a maximum similarity of 98% with any Myxobolus sp. from other Eurasian cypriniforms. Phylogenetic analysis revealed that M. polati sp. nov. is most closely related to gill-infecting Myxobolus diversicapsularis from Rutilus rutilus (L.). The present study is the first record of myxosporean species infecting C. angorense comprising a novel species, M. polati sp. nov. and a known species M. arrabonensis.

Myxozoan hidden diversity: the case of Myxobolus pseudodispar Gorbunova, 1936

Myxobolus pseudodispar Gorbunova, 1936 (Myxozoa) was originally described as a parasite of common roach, Rutilus rutilus (Linnaeus), with developing stages in muscles and spores disseminated in macrophage centres of different organs and tissues. Later, this parasite was described from several other cyprinids, but with relatively large intraspecific differences based on SSU rDNA gene sequences. Within our long-term study on myxozoan biodiversity, we performed a broad microscopic and molecular screening of various freshwater fish species (over 450 specimens, 36 species) from different localities. We investigated the cryptic species status of M. pseudodispar. Our analysis revealed four new unique SSU rDNA sequences of M. pseudodispar as well as an infection in new fish host species. Myxobolus pseudodispar sequence analysis showed clear phylogenetic grouping according to fish host criterion forming 13 well-recognised clades. Using 1% SSU rDNA-based genetic distance criterion, at least ten new species of Myxobolus Bütschli, 1882 may be recognised in the group of M. pseudodispar sequences. Our analysis showed the paraphyletic character of M. pseudodispar sequences and the statistical tests rejected hypothetical tree topology with the monophyletic status of the M. pseudodispar group. Myxobolus pseudodispar represents a species complex and it is a typical example of myxozoan hidden diversity phenomenon confirming myxozoans as an evolutionary very successful group of parasites with a great ability to adapt to a new hosts with subsequent speciation events.

Myxobolus spp. (Cnidaria: Myxozoa) in introduced yellow perch Perca flavescens (Mitchill, 1814)

We surveyed introduced yellow perch Perca flavescens (Mitchill, 1814) from the Willamette River, OR, USA, to determine if these fish have co-introduced myxosporean parasites. Mature parasite myxospores were observed in brains of 3/19 fish, and were morphologically and molecularly consistent with Myxobolus neurophilus (Guilford 1963), a parasite known from yellow perch in their native range. We identified another Myxobolus species from the gill filaments of 1/22 fish. The spores from the gill filaments were oval-shaped, 11.7 (10.7-12.3) μm long × 8.6 (7.7-9.0) μm wide × 5.2 (4.6-5.6) μm thick, with two oval-shaped polar capsules 5.7 (5.1-6.5) μm × 2.7 (2.4-3.2) μm, each containing a polar tubule with 8-9 turns. Small-subunit ribosomal DNA sequences from each of four plasmodia were identical, and 4.0% different (over 1800 nucleotides) from the closest known myxosporeans. Interestingly, these sequences had overlapping peaks in their chromatograms, which suggested that DNA from multiple species was present. Hence, we isolated and sequenced three individual myxospores and found that they too had mixed chromatograms, which indicated presence of at least two sequence types of small-subunit ribosomal DNA in each spore (GenBank accession MK592012, MK592013), a rare character among described myxosporeans. The spore morphology, morphometry, tissue tropism, and DNA sequence supported a diagnosis of a novel species, Myxobolus doubleae n. sp. This parasite is unknown from yellow perch in its native range, despite extensive historical surveys, which suggests that introduced yellow perch might have acquired an endemic Myxobolus species via spillback from another fish host.

Morphological and molecular characterization of myxobolids (Cnidaria, Myxozoa) infecting cypriniforms (Actinopterygii, Teleostei) endemic to the Iberian Peninsula

The Iberian Peninsula provides a unique freshwater ecosystem for native and endemic cypriniforms to thrive. Despite cypriniforms being hosts to multiple myxobolids worldwide, little research has been performed in this geographic location. In this study, the examination of three Iberian endemic cypriniforms showed that myxosporean richness in the Iberian Peninsula is underestimated, with three new and one known myxobolid species being reported based on morphological and molecular data (SSU). Myxobolus arcasii n. sp. is described from the kidney and gonads of the "bermejuela" Achondrostoma arcasii, M. duriensis n. sp. from the gills of the Northern straight-mouth nase Pseudochondrostoma duriense, and Thelohanellus paludicus n. sp. from the intestine of the Southern Iberian spined-loach Cobitis paludica. Myxobolus pseudodispar Gorbunova, 1936 is further reported from several organs of P. duriense, and from the spleen of A. arcasii. The occurrence of M. pseudodispar in endemic Iberian species reveals that host-shift followed its co-introduction with central European leuciscids into this geographic location. Several other myxobolids originally described from barbels in central Europe have also been reported from the Iberian endemic cypriniform Luciobarbus bocagei. Nonetheless, except for M. musculi, the identification of these myxobolids in L. bocagei is here shown to be dubious and require molecular confirmation. Phylogenetic analyses reveal M. arcasii n. sp. and M. duriensis n. sp. clustering within different lineages of leuciscid-infecting species, showing that myxobolids entered Leuciscidae as hosts multiple times during their evolution. Constituting the first myxobolid reported from the subfamily Cobitinae, Thelohanellus paludicus n. sp. stands alone in the tree topology.

Morphological and molecular characterization of the muscle-infecting myxosporean Myxobolus xinyangensis sp. nov. from Abbottina rivularis in China

During an environmental assessment on the Huang River in Xinyang City (Henan Province, China), a novel Myxobolus species (Myxozoa: Myxobolidae) was found infecting the trunk muscle of Chinese false gudgeon Abbottina rivularis Basilewsky, 1855 (Gobioninae, Cyprinidae). Plasmodia of the new myxozoan, nominated herein as Myxobolus xinyangensis sp. nov., are round and yellowish, symmetrically and bilaterally located dorsal to the openings of the 2 opercula, and measure about 4.5 mm in diameter. The mature myxospores are orbicular in frontal view and fusiform in sutural view, with slightly tapered anterior end and rounded posterior end, and measure 9.4 ± 0.5 (8.7-10.6) µm long, 8.6 ± 0.6 (7.3-9.5) µm wide and 6.4 ± 0.3 (5.8-7.1) µm thick (mean ± SD, range). The ratio of spore length to spore width is close to 1. Two slightly unequal pyriform polar capsules, with tapering anterior ends and rounded posterior ends, measure 5.6 ± 0.67 (4.3-6.8) µm long and 3.0 ± 0.3 (2.4-3.6) µm wide, present as a figure 8 in the anterior part of spores and tightly converge at the top end of spores. Polar filament coils show 4 to 5 turns and are situated perpendicularly to the longitudinal axis of the polar capsules. No intercapsular appendix or sutural folds at the posterior end of spores were observed. The obtained partial small subunit ribosomal DNA sequence did not match any available data in GenBank and showed the highest sequence identity (93%) with 2 cyprinid trunk muscle-infecting Myxobolus species, M. pseudodispar and M. klamathellus. Phylogenetic analysis clearly showed that M. xinyangensis sp. nov. clustered within a cyprinid trunk muscle-infecting Myxobolus subclade at the basal position, but as an independent branch which was a possible reflection of its distinct myxospore morphology. This is the first record of infection of Myxobolus species in the trunk muscle of Abbotina fish.

Morphological, histological, and molecular description of Myxobolus ompok n. sp. (Myxosporea: Myxobolidae), a kidney myxozoan from Pabdah catfish Ompok pabda (Hamilton, 1822) (Siluriformes: Siluridae) in India

In a parasitological survey of freshwater fishes near Meerut, Uttar Pradesh, India, myxozoan infections in Pabdah catfish Ompok pabda Ham. (Siluriformes: Siluridae) were found. Round plasmodia and scattered spores of Myxobolus ompok n. sp. were found in the kidney of the host. Plasmodia measuring 150-200 μm were located in the renal interstitium. Spores of Myxobolus ompok n. sp. were elongated pyriform, 13.6-14.4 (14.8 ± 0.42) μm long, 5.6-6.4 (6.5 ± 0.33) μm wide, and 5.2-6.4 (5.9 ± 0.43) μm thick with two equal polar capsules measuring 8.0-8.5 (8.2 ± 0.2) μm in length and 1.5-2.4 (1.8 ± 0.33) in width having six filamental turns. Both the morphology and DNA analysis of the 18S rRNA gene revealed that Myxobolus ompok n. sp. is distinct from previously described species of Myxobolus and shares no significant similarity with any other Myxobolus deposited in the GenBank database. Phylogenetic analysis inferred that this species showed the closest similarity to Myxobolus miyarii (KT001495). This is the first record of any Myxobolus sp. from O. pabda in India.

Myxozoan polar tubules display structural and functional variation

BACKGROUND

Myxozoa is a speciose group of endoparasitic cnidarians that can cause severe ecological and economic effects. Although highly reduced compared to free-living cnidarians, myxozoans have retained the phylum-defining stinging organelles, known as cnidae or polar capsules, which are essential to initiating host infection. To explore the adaptations of myxozoan polar capsules, we compared the structure, firing process and content release mechanism of polar tubules in myxospores of three Myxobolus species including M. cerebralis, the causative agent of whirling disease.

RESULTS

We found novel functions and morphologies in myxozoan polar tubules. High-speed video analysis of the firing process of capsules from the three Myxobolus species showed that all polar tubules rapidly extended and then contracted, an elasticity phenomenon that is unknown in free-living cnidarians. Interestingly, the duration of the tubule release differed among the three species by more than two orders of magnitude, ranging from 0.35 to 10 s. By dye-labeling the polar capsules prior to firing, we discovered that two of the species could release their entire capsule content, a delivery process not previously known from myxozoans. Having the role of content delivery and not simply anchoring suggests that cytotoxic or proteolytic compounds may be present in the capsule. Moreover, while free-living cnidarians inject most of the toxic content through the distal tip of the tubule, our video and ultrastructure analyses of the myxozoan tubules revealed patterns of double spirals of nodules and pores along parts of the tubules, and showed that the distal tip of the tubules was sealed. This helical pattern and distribution of openings may minimize the tubule mechanical weakness and improve resistance to the stress impose by firing. The finding that myxozoan tubule characteristics are very different from those of free-living cnidarians is suggestive of their adaptation to parasitic life.

CONCLUSIONS

These findings show that myxozoan polar tubules have more functions than previously assumed, and provide insight into their evolution from free-living ancestors.