Myxozoan communities parasitizing Notropis hudsonius (Cyprinidae) at selected localities on the St. Lawrence River, Quebec: possible effects of urban effluents.

Late summer myxozoan infra- and component communities parasitizing 73 Notropis hudsonius at 5 sites on the St. Lawrence River upstream and downstream from the island of Montreal are described from study of histological sections of individual fish. Community membership included Myxobolus sp. A (intracellular in striated muscle fibers), Myxobolus sp. B (intracellular in striated muscle fibers), Myxobolus sp. C (brain), and Thelohanellus notatus (Mavor, 1916) Kudo, 1929 (loose connective tissue), all of which are histozoic myxobolids displaying strict tissue specificity for trophozoite development. Mean infracommunity richness in fish at the separate localities was estimated to be 0.4 +/- 0.5 to 1.3 +/- 0.7, with a maximum richness of 3 in any 1 fish. Component community richness in host samples was 2 to 4. It is argued that these are relatively high levels of diversity for freshwater fish parasites but that the values are probably conservative because of the study of only portions of individual fish. The percentage of fish infected with myxozoans of any species and infracommunity richness was significantly greater below the island of Montreal compared with above it. It is suspected that increased oligochaete populations at these sites, resulting from sewage-caused organic enrichment of sediments, may have accounted for the observed increased prevalence of infection.     

The myxozoan fauna of spottail shiner in the Great Lakes basin: membership, richness, and geographical distribution

Spottail shiner (Notropis hudsonius) from localities in each of the Great Lakes plus some nearby waterbodies, i.e., the St. Lawrence River, and the Chester River, Maryland, were examined for myxozoan parasites. A total of 10 species was found, including 7 histozoic (Myxobolus sp.; M. algonquinensis Xiao and Desser, 1997; M. bartai Salim and Desser, 2000; M. xiaoi Salim and Desser, 2000; M. fanthami Landsberg and Lom, 1991; M. hendricksoni Mitchell, Seymour, and Gamble, 1985; Thelohanellus notatus Mavor, 1916) and 3 coelozoic (Chloromyxum sp., Zschokkella sp., Sphaerospora sp.) representatives. Infracommunity richness varied from 0 to 5 species per fish; mean infracommunity richness varied from 0 to 2.5 species. Component community richness varied from 0 to 7. Significant positive correlations were observed between mean and maximum infracommunity richness and component community richness. Similarly, maximum prevalence of each species at any 1 site was positively correlated with geographic range as measured by number of localities where a parasite species was encountered. Individual species occurred independently of each other. Representative histozoic and coelozoic species displayed similarly widespread distributions from Wisconsin to Maryland, but overall, histozoic species were dominant members within component communities. The study concludes that, under the present taxonomic paradigm, species parasitizing spottail shiner appear to be part of a larger network that cycles, in varying degrees, through certain other cyprinid and catostomid fish. The challenge of future research is to determine whether each parasite species constitutes single or multiple genetically isolated populations.     

Recent Advances in Our Knowledge of the Myxozoa

In the last few years two factors have helped to significantly advance our understanding of the Myxozoa. First, the phenomenal increase in fin fish aquaculture in the 1990s has lead to the increased importance of these parasites; in turn this has lead to intensified research efforts, which have increased knowledge of the development, diagnosis. and pathogenesis of myxozoans. The hallmark discovery in the 1980s that the life cycle of Myxobolus cerebralis requires development of an actinosporean stage in the oligochaete. Tubifex tubifex, led to the elucidation of the life cycles of several other myxozoans. Also, the life cycle and taxonomy of the enigmatic PKX myxozoan has been resolved: it is the alternate stage of the unusual myxozoan, Tetracapsula bryosalmonae, from bryozoans. The 18S rDNA gene of many species has been sequenced, and here we add 22 new sequences to the data set. Phylogenetic analyses using all these sequences indicate that: 1) the Myxozoa are closely related to Cnidaria (also supported by morphological data); 2) marine taxa at the genus level branch separately from genera that usually infect freshwater fishes; 3) taxa cluster more by development and tissue location than by spore morphology; 4) the tetracapsulids branched off early in myxozoan evolution, perhaps reflected by their having bryozoan, rather than annelid hosts; 5) the morphology of actinosporeans offers little information for determining their myxosporean counterparts (assuming that they exist); and 6) the marine actinosporeans from Australia appear to form a clade within the platysporinid myxosporeans. Ribosomal DNA sequences have also enabled development of diagnostic tests for myxozoans. PCR and in situ hybridisation tests based on rDNA sequences have been developed for Myxobolus cerebralis, Ceratomyxa shasta, Kudoa spp., and Tetracapsula bryosalmonae (PKX). Lectin-based and antibody tests have also been developed for certain myxozoans, such as PKX and C. shasta. We also review important diseases caused by myxozoans, which are emerging or re-emerging. Epizootics of whirling disease in wild rainbow trout (Oncorhynchus mykiss) have recently been reported throughout the Rocky Mountain states of the USA. With a dramatic increase in aquaculture of fishes using marine netpens, several marine myxozoans have been recognized or elevated in status as pathological agents. Kudoa thyrsites infections have caused severe post-harvest myoliquefaction in pen-reared Atlantic salmon (Salmo salar), and Ceratomyxa spp., Sphaerospora spp., and Myxidium leei cause disease in pen-reared sea bass (Dicentrarchus labrax) and sea bream species (family Sparidae) in Mediterranean countries.     

Parasite fauna and food contents of Notothenia c.f. angustata Hutton, 1875 (Pisces: Nototheniidae) from two intertidal localities of the Arauco Gulf,Chile

We studied the parasite fauna and food contents of 38 juvenile of Notothenia c.f. angustata specimens captured in the intertidal of Island Santa María and Maule, in the Arauco Gulf, Chile. Trophic habits of Notothenia c.f. angustata were not different between localities, which was mainly algae. A total 11 parasitic taxa among were found with the digenean Lecitasther macrocotyle, cestode larvae Tetraphyllidea and Diphyllidea and nematode Ascarophis sp. being the dominant taxa. The abundance of parasites was similar in the two localities, although the infracommunity richness was higher in fish from Maule. The most abundant parasite was L. macrocotyle in hosts from Isla Santa María, while those from Maule they were the Diphyllidea larvae and nematodes Ascarophis sp. The body length of the hosts was not a good predictor of the variations in abundance and richness at infracommunities, probably because of the early ontogenetic stage of fishes.     

Impact of municipal effluents and hydrological regime on myxozoan parasite communities of fish

Increased productivity from sewage effluents can enhance species richness locally. Results from a study of spottail shiners (Notropis hudsonius) in 1999 showed that prevalence and the mean number of myxozoan parasite species per host were higher downstream of the wastewater outflow from the Island of Montreal than upstream in the St. Lawrence River, Quebec, Canada. This was attributed to organic enrichment of the sediments which presumably lead to increased densities of oligochaetes, the alternate hosts, downstream of Montreal. Spottail shiners subsequently were collected every August/early September in 2001–2004 to examine the stability and repeatability of these patterns. Prevalence and mean number of myxozoan species per fish typically were again higher downstream of the sewage source each year compared to upstream, although there was no significant difference in these measurements pooled across years between localities immediately upstream and downstream of the effluent outflow. Density of the oligochaete Limnodrilus hoffmeistereri, a common alternate host of myxozoans, was much higher at two downstream localities than at an upstream one. At a larger spatial scale, mean myxozoan infracommunity richness across sites in the St. Lawrence River was negatively correlated with mean water levels measured in the 3 months prior to fish sampling. Results suggest that on a local scale, variations in prevalence and diversity among localities are influenced by municipal effluents, but that at a landscape scale annual variations across sites are affected by the hydrological regime and climate. In effect, water level fluctuation had a landscape-wide impact that was superimposed over pollution-induced local variations.     

Morphological,ultrastructural, and molecular analysis of a new species of Myxobolus (Cnidaria,Myxosporea) parasitizing Apareiodon piracicabae (Characiformes,Parodontidae) from Brazil

About forty-two species of Myxobolus have been previously described to parasitize characiform fishes in South America. Here, we described a new myxozoan species, Myxobolus parodontidis n. sp., in the gills of Apareiodon piracicabae (Characiformes, Parodontidae) from the streams of the Middle Paranapanema River basin, Brazil. The proposed new species is supported by a combination of morphological, ultrastructural, and molecular characterization (small subunit ribosomal DNA). Thirteen specimens of A. piracicabae were analyzed and 30.8% was infected by Myxobolus parodontidis n. sp. The myxospores was classified as intralamellar asymmetric type. A few aberrant myxospores with three polar capsules were observed: the spore length and width were the same of normal myxospores, but the polar capsules had smaller sizes. Ultrastructural analysis showed that the plasmodial membrane of Myxobolus parodontidis n. sp. was in direct contact with the host tissue and a connective capsule surrounding the plasmodium was not observed. The phylogenetic analysis showed that the main influence in the clustering of species of myxobolids seems to be related to the phylogenetic relationships established among fish hosts, mainly at the level of family and order. This is the first record of a myxozoan species parasitizing parodontid fish, contributing to the knowledge of the biodiversity of myxozoans from Brazil.     

Host-parasite relationships of longnose dace, Rhinichthys cataractae, from the Ford River, Michigan.

A total of 1,115 longnose dace, Rhinichthys cataractae (family Cyprinidae), were examined for parasites from May 1983 through October 1986 from 3 localities in the Ford River in Michigan's Upper Peninsula. Thirteen parasite species (1 Monogenea, 2 Digenea, 2 Cestoda, 4 Nematoda, 1 Acanthocephala, 3 Protozoa) infected dace. The parasite faunas of dace, taxonomically and in species number, were similar between localities. Posthodiplostomum minimum minimum, Neascus sp., and Rhabdochona canadensis were the most common helminths infecting dace from each locality. The first 2 species did not exhibit consistent seasonal infection patterns between years, whereas the prevalence and mean intensity of R. canadensis in dace from the downriver locality were higher in summer 1983, 1984, and 1985. The intensity of infection of each of these helminth species significantly increased with host length. The prevalences and mean intensities of P. m. minimum, Neascus sp., and R. canadensis as well as the helminth infracommunity diversity were highest in dace from the upriver locality. The major factors that influenced parasite intensity were environmental factors that occurred when and where a fish began its life, the sequence of events that occurred in each habitat the fish encountered during its life, and the length of exposure (age of fish). Dace have isolationist helminth infracommunities arising from factors including ectothermy, a simple enteric system, restricted vagility, and being gape-limited. Allogenic helminths with indirect life cycles predominate in the depauperate helminth fauna of dace.     

A new species of myxosporean (Sphaeromyxidae), a parasite of lined seahorses, Hippocampus erectus, from the Gulf of Mexico

Sphaeromyxa cannolii sp. n. is described from the bile ducts of aquaria-maintained lined seahorses (Hippocampus erectus) from the Gulf of Mexico. Spores of the new species are linear, 17-18 μm long and 5-6 μm wide, with flattened tips; polar capsules measure 4 × 3 μm. Routine necropsies of H. erectus following planned death revealed liver inflammation, bile duct obstruction, bile accumulation, and myxozoan parasites in the bile ducts of 11 of 40 animals sampled (27.5%). The presence of S. cannolii in an aquaculture setting should prompt keepers to carefully quarantine new animals and exclude annelid fauna, a potential intermediate host of myxozoans.     

Metazoan parasite communities of wild Leporinus friderici (Characiformes: Anostomidae) from Amazon River system in Brazil

This study compared the structure of metazoan parasite communities in two populations of Leporinus friderici from two rivers of the Amazon River system in Brazil. Jainus leporini, Urocleidoides paradoxus, Urocleidoides sp., Tereancistrum parvus, Tereancistrum sp., Clinostomum marginatum, Procamallanus (Spirocamallanus) inopinatus, Contracaecum sp., Octospiniferoides incognita and Ergasilus sp. were found in both hosts. There were differences in species richness of parasites, Shannon index and the evenness between both host populations, because the parasite community showed a similarity of only 33%. Parasites of both host populations had an aggregated dispersion. Host size also contributes to the differences between the populations of hosts investigated, and the feeding habit contributes to the occurrence of endoparasites in L. friderici, an intermediate host for these parasites. The existence of variation in infracommunity and community of parasites for the same host species from different localities indicates the presence of an uneven distribution in terms of species and density of parasites, which are intermediate hosts, and some of them constitute food items for L. friderici in the localities surveyed.     

The parasitic communities of the minnow (Phoxinus phoxinus L>)

Phoxinus phoxinus being a widely distributed through Palaearctic and ecologically specialised species is a good model to study parasite communities. 72 specimens of P. phoxinus from 5 localities of the Siberia and Mongolia has been studied by the author and 651 specimens--from 5 localities in Kola Peninsula by the other researches. 595 specimens were studied by B. S. Shulman (1983) year around. To describe infra- and component parasite communities the Brillouin, Shannon, Berger-Parker indexes were used. Communities within individual fish were compared within, and between localities with quantitative percentage similarity index (Esch e. a., 1988). The Brillouin, Shannon and Berger-Parker indexes can be calculated based on abundance or total number of parasites of each species for component or compound communities. Intensity cannot be used. Significant negative correlation was observed between the Brillouin and Berger-Parker dominance indexes (r = -0.984, p < 0.01, n = 66) in infracommunities. The Brillouin index was significant positive correlated with the number of parasites in infracommunity (r = 0.407, p < 0.01, n = 66) and with the number of species in infracommunity (r = 0.92, p < 0.01, n = 66). Berger-Parker dominance index was significant negative correlated with the number of parasite in infacommunity (r = -0.815, p < 0.01, n = 66) and with the number of species in infracommunity (r = -0.268, p < 0.05). Total number of parasites and the number of species in infracommunity was positive correlated significantly (r = 0.565, p < 0.01, n = 66). These correlations remain the same at the component community level for the Shannon/Berger-Parker indexes (r = -0.965, p < 0.01, n = 10) and total number of parasites/number of species (r = 0.789, p < 0.01, n = 10) only. Similarity index was not correlate with any other indexes and parameters. No significant correlation was found between the Brillouin, Berger-Parker indexes and geographical characters of localities (longitude, latitude, altitude above sea-level). Weak correlation (r = 0.551, p < 0.098, n = 10) was observed only between the Berger-Parker dominance index and the latitude. 37 metazoan parasites have been found in all localities (specialists--40%, allogenic species--22%, rare species--40%). Allogenic generalist species of the genus Diplostomum (D. phoxini in most cases) were dominant in 60% of the component communities and 36% of infracommunities on average. 20% of the component communities were dominant by autogenic specialists Gyrodactylus magnificus and only 12% of the infracommunities were dominated by this species. Trematode and nematode larvae were core species in most localities. Frequency distribution of infracommunity richness varied from locality to locality being overdispersed in combine sample. Parasite communities at least on the infracommunity level demonstrate high variability of all parameters. Some tendency to decrease the diversity seems to be present depending on remoteness from prospective centre of origin of the genus Phoxinus.     

Structure of parasite component communities of didelphid marsupials: insights from a comparative study

The parasite fauna of the gray four-eyed opossum, Philander opossum (Linnaeus, 1758), and the common opossum, Didelphis marsupialis Linnaeus, 1758, in Camp du Tigre, French Guiana, is characterized. Nine species from the gastrointestinal system were recovered from both species, which shared 80% of their parasites. The parasite fauna comprised several monoxenous species (63%) and was dominated by Aspidodera raillieti Travassos, 1914, which exhibited high levels of prevalence and abundance in both communities. Only 2 species (Moennigia sp. and Spirura guianensis) had been recorded in other species of mammals. Both species richness and taxonomic composition at the level of component communities from this locality were compared against 11 communities present in the Virginia ( Didelphis virginiana ), white-bellied (Didelphis albiventris), and common opossum from Argentina, Brazil, Mexico, and the United States. Neither host phylogeny nor taxonomy accounted for statistical differences in species richness. There was no statistical difference among species richness values among the 9 localities studied. Taxonomic similarity was analyzed by means of the Jaccard's similarity index, including all, and only common species (occurring in prevalence >10%). The results suggest that sympatric species of marsupials share more species of parasites than parasite communities occurring in conspecific marsupials from different localities. As a consequence, taxonomic composition of these parasite communities varied depending on the locality. Probably, marsupials of the monophyletic Didelphini offer the same compatibility toward their parasites, by presenting them with similar habitats. Subtle differences in lifestyles of the marsupials may determine the chance of encounter between the symbionts and prevent some parasites from completing their life cycles. Further and more rigorous tests are necessary to determine the roles of encounter and compatibility filters, as well as the role of chance, in the structuring of parasite communities in marsupials.     

Description of a new species of myxozoan from Notropis hudsonius in the Great Lakes region of Canada

Myxobolus burti n. sp. is described from striated muscle of Notropis hudsonius (Cyprinidae) collected from localities in Lake Superior, Lake Michigan, Lake Huron, Lake St. Claire, Lake Erie, Lake Ontario, and the St. Lawrence River. Myxobolus burti is intracellular, forming thin-walled, cigar-shaped plasmodia within striated muscle cells of the body. It exhibits disporoblastic, oval spores, approximately 10 μm long, 8 μm wide, and 6 μm thick, with a length-to-width ratio of 1.3 ∶ 1. They resemble most closely those of Myxobolus bellus Kudo, 1934, and Myxobolus mutabilis Kudo, 1934, but have polar capsules unequal in length (5.3 vs. 4.7 μm) and oblique filament coils. The new parasite was not encountered in routine examination of other small-bodied fishes at collection locations and thus looks to be specific to N. hudsonius. A comparative plate of similar species encountered during surveys of these fishes in the northeast is presented to contrast the uniqueness of the new species.     

Myxozoan pathogens in cultured Malaysian fishes. I. Myxozoan infections of the sutchi catfish Pangasius hypophthalmus in freshwater cage cultures

Cage-cultured sutchi catfish Pangasius hypophthalmus (Sauvage, 1878), a favourite food fish in Southeast Asia, proved to be infected by 6 myxozoan species. Three species belonged to the genus Hennegoides (H. berlandi, H. malayensis, and H. pangasii), 1 to Henneguya (H. shariffi) and 2 to Myxobolus (M. baskai, and M. pangasii). Five myxozoans infected the gills and 1 was found on the spleen. Myxozoans infecting the gills were characterised by a specific site selection. H. shariffi sp. n. and H. berlandi sp. n. formed plasmodia in the multi-layered epithelium of the gill filaments. Of the 2 vascular species H. pangasii sp. n. developed in the gill arteries, while M. baskai sp. n. infected the capillary network of the gill lamellae. Plasmodia of H. malayensis sp. n. were found inside the cartilaginous gill rays of the filaments. Large plasmodia of M. pangasii sp. n. were located in a groove of the spleen but they affected only the serosa layer covering the spleen.     

Myxobolus albi n. sp. (Myxozoa) from the Gills of the Common Goby Pomatoschistus microps Krøyer (Teleostei: Gobiidae)

ABSTRACT. A recent investigation into the myxozoan fauna of common gobies, Pomatoschistus microps , from the Forth Estuary in Scotland, revealed numerous myxosporean cysts within the gill cartilage. They were composed of polysporous plasmodia containing myxobolid spores that were morphologically different from the other known species of Myxobolus and from the myxosporeans previously recorded from this host (i.e. the ceratomyxid Ellipsomyxa gobii , infecting the gall bladder, and the kudoid Kudoa camarguensis , infecting the muscle tissues). Spores were ovoid, 9.4 × 9.1 μm with a thickness of 6.6 μm, with two pyriform polar capsules, the polar filaments of which had four to five turns. Molecular analysis of the parasite's small subunit rDNA region, based upon a contiguous sequence of 1,558 base pairs, discriminated it from other myxosporean species that have been characterized so far. A comparison of the spore morphology and the molecular sequences determined for this new isolate with other myxozoans described to date, confirmed its identity as a previously unknown myxobolid supporting the proposal that this isolate be elevated to the species level as a new species within the genus Myxobolus . A phylogenetic analysis places this new myxobolid, Myxobolus albi n. sp., in a basal position of a clade containing the majority of Henneguya spp. sequenced to date and various Myxobolus spp.     

Phylogeny, ultrastructure, histopathology and prevalence of Myxobolus oliveirai sp. nov., a parasite of Brycon hilarii (Characidae) in the Pantanal wetland, Brazil

This paper presents the morphological, histological and ultrastructural characteristics of Myxobolus oliveirai sp. nov., a parasite of the gill filaments in Brycon hilarii from the Brazilian Pantanal. Out of 216 B. hilariispecimens examined (126 wild and 90 cultivated), 38.1% of wild specimens (n = 48) were infected. The parasites form elongated plasmodia primarily in the tip of gill filaments, reaching about 3 mm in length. A thorough comparison with all the Myxobolus species described from South American hosts, as well as nearly all the Myxobolus species described so far is provided. Partial sequencing of the 18S rDNA gene revealed a total of 1,527 bp. The Myxobolus species parasite of B. hilarii did not match any of the Myxozoa available in GenBank. In the phylogenetic analysis, M. oliveirai sp. nov. composed a monophyletic group with eight other species: five species of Myxobolus parasites of mugilid fishes, two parasites of pangasiid and one of centrarchid. Infection prevalence values of the parasite revealed no significant differences between wet and dry seasons or between males and females. The importance of the infection to the farming of the host species is emphasized.     

Parasites of native and nonnative fishes of the Little Colorado River, Grand Canyon, Arizona

A 2-yr, seasonal, parasitological study of 1,435 fish, belonging to 4 species of native fishes and 7 species of nonnative fishes from the lower Little Colorado River (LCR) and tributary creeks, Grand Canyon, Arizona, yielded 17 species of parasites. These comprised 1 myxozoan (Henneguya exilis), 2 copepods (Ergasilus arthrosis and Lernaea cyprinacea), 1 acarine (Oribatida gen. sp.), 1 piscicolid leech (Myzobdella lugubris), 4 monogeneans (Gyrodactylus hoffmani, Gyrodactylus sp., Dactylogyrus extensus, and Ligictaluridus floridanus), 4 nematodes (Contracaecum sp., Eustrongylides sp., Rhabdochona sp., and Truttaedacnitis truttae), 3 cestodes (Bothriocephalus acheilognathi, Corallobothrium fimbriatum, and Megathylacoides giganteum), and 2 trematodes (Ornithodiplostomum sp. and Posthodiplostomum sp.). Rhabdochona sp. was the only adult parasite native to the LCR. Infection intensities of Ornithodiplostomum sp. and B. acheilognathi were positively correlated with length of the humpback chub Gila cypha. Adult helminths showed a high degree of host specificity, except B. acheilognathi, which was recovered from all fish species examined but was most abundant in cyprinids. Abundance of B. acheilognathi in the humpback chub was highest in the fall and lowest in the summer in both reaches of the LCR. There was no major taxonomic difference in parasite assemblages between the 2 different reaches of the river (LC1 and LC2). Parasite community diversity was very similar in humpback chub, regardless of sampling site or time. The parasite fauna of the LCR is numerically dominated by B. acheilognathi and metacercariae of Ornithodiplostomum sp. The richest and most diverse component community occurred in a nonnative species, the channel catfish Ictalurus punctatus, but infracommunity species richness was highest in a native host, humpback chub.     

Description of a new species of Myxobolus (Myxozoa: myxobolidae) based on morphological and molecular data

Myxobolus ampullicapsulatus n. sp. was isolated from the gills of Carassius auratus auratus (L., 1758) in Chongqing, China. Myxospores were pyriform, measuring 16.5-19.5 microm long x 8.5-10.0 microm wide x 7.0 microm thick. Two equal polar capsules were ampullaceous, measuring 7.0-10.0 microm long x 2.5-4.0 microm wide, containing polar filaments coiled 9-10 turns. Spore length of this species exceeds that of the majority of other Myxobolus spp., and those overlapping in this dimension can be differentially diagnosed by other characters. Furthermore, the small subunit ribosomal DNA (SSU rDNA) of M. ampullicapsulatus n. sp. is unique among myxozoans sequenced to date. Phylogenetic analyses of the SSU rDNA gene sequence placed this species in a clade composed exclusively of gill parasites, most closely related to Myxobolus longisporus, which also infects the gills of cyprinid fishes in China.     

The secondary structure of the unusually long 18S ribosomal RNA of the myxozoan Sphaerospora truttae and structural evolutionary trends in the Myxozoa

The nearly complete 18S rRNA sequence of the myxozoan parasite Sphaerospora truttae shows an extraordinary length (2,552bp) in comparison with other myxozoans and with metazoans in general (average 1,800-1,900bp). The sequence shows nucleotide insertions in most variable regions of the 18S rRNA (V2, V4, V5 and V7), with especially large expansion segments in V4 and V7. In the myxozoans, nucleotide insertions and specific secondary structures in these regions of the gene were found to be strongly related to large scale phylogenetic clustering and thus with the invertebrate host type. Whereas expansion segments were generally found to be absent in the malacasporeans and the clade of primary marine myxozoan species, they occur in all taxa of the clade containing freshwater species, where they showed a consistent secondary structure throughout. The longest expansion segments occur in S. truttae, Sphaerospora elegans and Leptotheca ranae, which represent a clade that has emerged after the malacosporeans and before the radiation of all other myxozoan genera. These three species demonstrate structural links to the malacosporeans as well as other unique features. A smaller number of nucleotide insertions in different subhelices and specific secondary structures appear to have evolved independently in two marine genera, i.e. Ceratomyxa and Parvicapsula. The secondary structural elements of V4 and V7 of the myxozoan 18S rRNAs were found to be highly informative and revealed evolutionary trends of various regions of the gene hitherto unknown, since previous analyses have been based on primary sequence data excluding these regions. Furthermore, the unique features of the V4 region in S. truttae allowed for the design of a highly specific PCR assay for this species.