Infection of bryozoans by Tetracapsuloides bryosalmonae at sites endemic for salmonid proliferative kidney disease

The unplumatellid Plumatella fruticosa found its home: Hirosella gen. nov. morphological arguments for the systematic placement of a freshwater bryozoan

Bryozoans are colonial, suspension-feeding lophotrochozoans. The phylum consists of the large group of chiefly marine Myolaemata and the exclusively limnic Phylactolaemata. Each colony consists of individual zooids that comprise the protective cystid and the retractable polypide. Phylactolaemates are a small group of approximately 90 species in 6 families. They feature a body wall, that can either be gelatinous, as in the families Stephanellidae, Lophopodidae, Cristatellidae and Pectinatellidae, or encrusted, as in Plumatellidae and Fredericellidae. Morphological investigations of the most specious plumatellids are rare and focus on few species. Plumatella fruticosa is of particular interest in this regard, as it shows a mosaic of plumatellid and fredericellids characters. The most recent phylogeny clusters P. fruticosa with cristatellids and pectinatellids as sister groups to fredericellids. Hence, there is considerable doubt, whether P. fruticosa is truly a plumatellid. Therefore, this study aims to reinvestigate the morphology of P. fruticosa with confocal microscopy and section-based three-dimensional reconstruction. The new data show that P. fruticosa has numerous conspicuous stumps from fragmented proliferation buds, which are otherwise only known from fredericellids. Like fredericellids, P. fruticosa grows erect, but in contrast, has a horseshoe-shaped lophophore and floatoblasts. Besides the proportions of the lophophore, the tentacle sheath and digestive tract resemble a fredericellid-like situation. Myoanatomical details like the pronounced longitudinal muscles of the vestibular wall and tentacle sheath differ from plumatellids and favour the recently proposed scenario, which places P. fruticosa next to Pectinatellidae and Cristatellidae. In addition, the intertentacular membrane of P. fruticosa shows structural similarity to cristatellids as it is attached to the tentacles via lamellae. Taking all aspects into account, we erect a new family: Hirosellidae fam. nov. including the new genus Hirosella gen. nov.

O anus, where art thou? An investigation of ctenostome bryozoans

Ctenostome bryozoans are a small group of approximately 350 currently described species that remain inadequately investigated anatomically. Recently, the importance of soft body morphology of zooids including the digestive tract has become more evident for addressing various biological aspects such as systematic, functional, or phylogenetic analyses. Particularly, the position of the anus shows considerable variation in ctenostomes and in its extreme form can either be at the lophophoral base or at the vestibular wall. However, it has never been analysed in a broader systematic, phylogenetic, or functional context. Hence, the purpose of this study is to assess the distribution of anus position among ctenostomes, analyse whether zooidal or colonial morphology affects anus position, and draw first conclusions on its functional effects. The survey shows that a vestibular anus is ubiquitously present in alcyonidioideans and several, probably closely related, walkerioideans. In other groups such as boring forms, it appears more patchily distributed, or in some currently unassignable genera, such as Monobryozoon, supports a closer relationship to alcyonidioideans. Other zooidal or colonial characters such as tentacle number or zooidal density in the colony do not show a distinct correlation to the position of the anus. It appears that the shift of the anus into a vestibular area occurred once or twice among ctenostomes; the reasons and functional effects remain unknown. Future important aspects of defecation research in bryozoans are discussed.

Effects of parasite concentrations on infection dynamics and proliferative kidney disease pathogenesis in brown trout (Salmo trutta)

Proliferative kidney disease (PKD) is an emerging disease of salmonids, which is exacerbating with increasing water temperature. Its causative agent, the myxozoan parasite Tetracapsuloides bryosalmonae, exploits freshwater bryozoans as primary hosts and salmonids as intermediate hosts. Our experiments showed that the manipulation of exposure concentrations of infective malacospores had relatively minor impacts for the disease outcomes in the fish host. In this study, brown trout (Salmo trutta) were exposed to three different exposure concentrations of T. bryosalmonae malacospores: (a) a single low parasite concentration (LC), (b) a single high parasite concentration (HC) and (c) three times a low concentration (repeat exposure, RE). Parasite dynamics in the fish host and release of fish malacospores were quantified and fish kidney histopathology was evaluated to determine PKD pathogenesis. Infection prevalence was always lower in the LC group than in the other groups over the course of the study. While the parasite proliferation phase was slower in the LC group, the maximum parasite burden did not differ significantly amongst treatments. The onset of fish malacospore release (day 45 post-exposure), indicated by detection of T. bryosalmonae DNA in the tank water, occurred at the same time point for all groups. Reduced intensity of kidney pathological development was observed in the LC treatment indicating lower disease severity. While the LC treatment resulted in reduced outcomes across several infection parameters (infection prevalence, parasite proliferation, total fish malacospores released), the overall differences were small. The RE and HC treatment outcomes were for most parameters comparable. Our results suggest that repeated exposure, as is likely to occur in the wild during the summer months, might play a more important role in the dynamics of PKD as an emerging infectious disease than the actual concentration of spores.

Proliferative Kidney Disease and Proliferative Darkening Syndrome are Linked with Brown Trout (Salmo trutta fario) Mortalities in the Pre-Alpine Isar River

For many years, brown trout (Salmo trutta fario) mortalities within the pre-alpine Isar River in Germany were reported by the Bavarian Fisheries Association (Landesfischereiverband Bayern e.V.) and local recreational anglers during August and September. Moribund fish seemed to be affected by proliferative darkening syndrome (PDS). In addition, proliferative kidney disease (PKD) caused by Tetracapsuloides bryosalmonae was discussed. To investigate this phenomenon, the present field study monitored brown trout mortalities by daily river inspection in 2017 and 2018. Moribund brown trout (n = 31) were collected and examined using histology, immunohistochemistry, qPCR, and quantitative stereology. Our investigations identified 29 (93.5%) brown trout affected by PKD. Four brown trout (12.9%) displayed combined hepatic and splenic lesions fitting the pathology of PDS. The piscine orthoreovirus 3, suspected as causative agent of PDS, was not detectable in any of the samples. Quantitative stereological analysis of the kidneys revealed a significant increase of the renal tissue volumes with interstitial inflammation and hematopoietic hyperplasia in PKD-affected fish as compared to healthy brown trout. The identified T. bryosalmonae strain was classified as part of the North American clade by phylogenetical analysis. This study highlights PKD and PDS as contributing factors to recurrent autumnal brown trout mortalities.

Estimating species distribution and abundance in river networks using environmental DNA

All organisms leave traces of DNA in their environment. This environmental DNA (eDNA) is often used to track occurrence patterns of target species. Applications are especially promising in rivers, where eDNA can integrate information about populations upstream. The dispersion of eDNA in rivers is modulated by complex processes of transport and decay through the dendritic river network, and we currently lack a method to extract quantitative information about the location and density of populations contributing to the eDNA signal. Here, we present a general framework to reconstruct the upstream distribution and abundance of a target species across a river network, based on observed eDNA concentrations and hydro-geomorphological features of the network. The model captures well the catchment-wide spatial biomass distribution of two target species: a sessile invertebrate (the bryozoan Fredericella sultana) and its parasite (the myxozoan Tetracapsuloides bryosalmonae). Our method is designed to easily integrate general biological and hydrological data and to enable spatially explicit estimates of the distribution of sessile and mobile species in fluvial ecosystems based on eDNA sampling.

Persistence, impacts and environmental drivers of covert infections in invertebrate hosts

Background

Persistent covert infections of the myxozoan, Tetracapsuloides bryosalmonae, in primary invertebrate hosts (the freshwater bryozoan, Fredericella sultana) have been proposed to represent a reservoir for proliferative kidney disease in secondary fish hosts. However, we have limited understanding of how covert infections persist and vary in bryozoan populations over time and space and how they may impact these populations. In addition, previous studies have likely underestimated covert infection prevalence. To improve our understanding of the dynamics, impacts and implications of covert infections we employed a highly sensitive polymerase chain reaction (PCR) assay and undertook the first investigation of covert infections in the field over an annual period by sampling bryozoans every 45 days from three populations within each of three rivers.

Results

Covert infections persisted throughout the year and prevalence varied within and between rivers, but were often > 50%. Variation in temperature and water chemistry were linked with changes in prevalence in a manner consistent with the maintenance of covert infections during periods of low productivity and thus poor growth conditions for both bryozoans and T. bryosalmonae. The presence and increased severity of covert infections reduced host growth but only when bryozoans were also investing in the production of overwintering propagules (statoblasts). However, because statoblast production is transitory, this effect is unlikely to greatly impact the capacity of bryozoan populations to act as persistent sources of infections and hence potential disease outbreaks in farmed and wild fish populations.

Conclusions

We demonstrate that covert infections are widespread and persist over space and time in bryozoan populations. To our knowledge, this is the first long-term study of covert infections in a field setting. Review of the results of this and previous studies enables us to identify key questions related to the ecology and evolution of covert infection strategies and associated host-parasite interactions.

Electronic supplementary material

The online version of this article (10.1186/s13071-017-2495-8) contains supplementary material, which is available to authorized users.

Tetracapsuloides bryosalmonae abundance in river water

Tetracapsuloides bryosalmonae is a myxozoan parasite of freshwater bryozoans and salmonids, causing proliferative kidney disease in the latter. To date, detection of the parasite has required collection of hosts and subsequent molecular or histological examination. The release of infectious spores from both hosts offers an opportunity to detect the parasite in water samples. We developed a novel SYBR® Green quantitative real-time PCR (qPCR) assay for T. bryosalmonae in water samples which provides an estimation of bryozoan malacospore numbers and tested the assay in 3 rivers in southern England (UK) over a period of 5 wk. The assay proved to be both highly sensitive and specific to the parasite, detecting low levels of spores throughout the study period. Larger-volume samples afforded greater detection likelihood, but did not increase the number of spores detected, possibly as a result of low and patchy spore distributions and lack of within-site replication of large-volume samples. Based on point-measurements, temperature was positively associated with the likelihood of detecting spores, possibly reflecting the temperature dependence of spore shedding from bryozoan hosts. The presence of T. bryosalmonae in water samples was predominantly influenced by spatial (sites within rivers, amongst rivers) and temporal (sampling dates) factors, while the latter also influenced quantification cycle (Cq) values and spore abundance. Environmental monitoring for infectious stages can complement traditional methods, providing faster and easier detection and avoiding potentially prolonged searching, collecting and destructive sampling of invertebrate and vertebrate hosts.

First Proliferative Kidney Disease outbreak in Austria, linking to the aetiology of Black Trout Syndrome threatening autochthonous trout populations

Proliferative Kidney Disease (PKD) was diagnosed in juvenile autochthonous brown trout Salmo trutta for the first time in Austria during summer 2014. Cytology showed Tetracapsuloides bryosalmonae sporoblasts, and histology revealed sporogonic (coelozoic) and extrasporogonic (histozoic) stages. Analysis of malacosporean ribosomal small subunit revealed that this strain is closely related to European isolates, although its source is unknown. Infection and high pathogenicity were reproduced upon a pre-restocking test with specific pathogen free (SPF) juvenile trout, resulting in 100% mortality between 28 and 46 d post exposure (dpe), with high ectoparasitosis. Fish showed grade 2 of the Kidney Swelling Index and grade 3 of the PKD histological assessment. T. bryosalmonae enzootic waters were demonstrated in further locations along the River Kamp, with infected bryozoans retrieved up to 6 km upstream of the farm with the PKD outbreak. Fredericella sultana colonies collected from these locations were cultivated in laboratory conditions. Released malacospores successfully induced PKD, and contextually Black Trout Syndrome (BTS), in SPF brown trout. In the absence of co-infections mortality occurred between 59 and 98 dpe, with kidneys enlarged up to 6.74% of total body weight (normal 1.23%). This study confirms the first isolation of a pathogenic myxozoan from an Austrian river tributary of the Danube, where its 2-host life cycle is fully occurring. Its immunosuppressant action could link PKD as a key factor in the multifactorial aetiology of BTS. This T. bryosalmonae isolation provides an impetus to undertake further multi-disciplinary research, aiming to assess the impact of PKD and BTS spreading to central European regions.

Vertical transmission ofTetracapsuloides bryosalmonae(Myxozoa), the causative agent of salmonid proliferative kidney disease

The freshwater bryozoan,Fredericella sultana, is the main primary host of the myxozoan endoparasite,Tetracapsuloides bryosalmonaewhich causes proliferative kidney disease (PKD) of salmonid fish. Because spores that develop in bryozoan colonies are infectious to fish, bryozoans represent the ultimate source of PKD. Bryozoans produce numerous seed-like dormant stages called statoblasts that enable persistence during unfavourable conditions and achieve long-distance dispersal. The possibility thatT. bryosalmonaemay undergo vertical transmission via infection of statoblasts has been the subject of much speculation since this is observed in close relatives. This study provides the first evidence that such vertical transmission ofT. bryosalmonaeis extensive by examining the proportions of infected statoblasts in populations ofF. sultanaon two different rivers systems and confirms its effectiveness by demonstrating transmission from material derived from infected statoblasts to fish hosts. Vertical transmission in statoblasts is likely to play an important role in the infection dynamics of both bryozoan and fish hosts and may substantially contribute to the widespread distribution of PKD.

Establishment of medium for laboratory cultivation and maintenance of Fredericella sultana for in vivo experiments with Tetracapsuloides bryosalmonae (Myxozoa)

The freshwater bryozoan Fredericella sultana (Blumenbach) is the most common invertebrate host of the myxozoan parasite Tetracapsuloides bryosalmonae, the causative agent of proliferative kidney disease in salmonid fish. Culture media play an important role in hatching of statoblasts and maintaining clean bryozoan colonies for Malacosporea research. We developed a novel culture medium, Bryozoan Medium C (BMC), for the cultivation and maintenance of F. sultana under laboratory conditions. Statoblasts of F. sultana were successfully hatched to produce transparent-walled, specific pathogen-free (SPF) colonies that were maintained >12 months in BMC at pH 6.65. Tetracapsuloides bryosalmonae was successfully transmitted from infected brown trout, Salmo trutta L., to newly hatched F. sultana colonies in BMC, then from the infected bryozoan to SPF brown trout. This study demonstrated the utility of BMC (pH 6.65) for hatching statoblasts, long-term cultivation of clean and transparent bryozoan colonies and maintenance of the Tetracapsuloides bryosalmonae life cycle in the laboratory for molecular genetic research and other studies such as host-parasiteinteraction.

The effects of infection by Tetracapsuloides bryosalmonae (Myxozoa) and temperature on Fredericella sultana (Bryozoa)

The myxozoan, Tetracapsuloides bryosalmonae, exploits freshwater bryozoans as definitive hosts, occurring as cryptic stages in bryozoan colonies during covert infections and as spore-forming sacs during overt infections. Spores released from sacs are infective to salmonid fish, causing the devastating Proliferative Kidney Disease (PKD). We undertook laboratory studies using mesocosm systems running at 10, 14 and 20 degrees C to determine how infection by T. bryosalmonae and water temperature influence fitness of one of its most important bryozoan hosts, Fredericella sultana, over a period of 4 weeks. The effects of infection were context-dependent and often undetectable. Covert infections appear to pose very low energetic costs. Thus, we found that growth of covertly infected F. sultana colonies was similar to that of uninfected colonies regardless of temperature, as was the propensity to produce dormant resting stages (statoblasts). Production of statoblasts, however, was associated with decreased growth. Overt infections imposed greater effects on correlates of host fitness by: (i) reducing growth rates at the two higher temperatures; (ii) increasing mortality rates at the highest temperature; (iii) inhibiting statoblast production. Our results indicate that parasitism should have a relatively small effect on host fitness in the field as the negative effects of infection were mainly expressed in environmentally extreme conditions (20 degrees C for 4 weeks). The generally low virulence of T. bryosalmonae is similar to that recently demonstrated for another myxozoan endoparasite of freshwater bryozoans. The unique opportunity for extensive vertical transmission in these colonial invertebrate hosts couples the reproductive interests of host and parasite and may well give rise to the low virulence that characterises these systems. Our study implies that climate change can be expected to exacerbate PKD outbreaks and increase the geographic range of PKD as a result of the combined responses of T. bryosalmonae and its bryozoan hosts to higher temperatures.

Endoparasitism in colonial hosts: patterns and processes

This study begins to redress our lack of knowledge of the interactions between colonial hosts and their parasites by focusing on a novel host-parasite system. Investigations of freshwater bryozoan populations revealed that infection by myxozoan parasites is widespread. Covert infections were detected in all 5 populations studied and were often at high prevalence while overt infections were observed in only 1. Infections were persistent in populations subject to temporal sampling. Negative effects of infection were identified but virulence was low. Infection did not induce mortality in the environmental conditions studied. However, the production of statoblasts (dormant propagules) was greatly reduced in bryozoans with overt infections in comparison to uninfected bryozoans. Overtly-infected bryozoans also grew more slowly and had low fission rates relative to colonies lacking overt infection. Bryozoans with covert infections were smaller than uninfected bryozoans. High levels of vertical transmission were achieved through colony fission and the infection of statoblasts. Increased fission rates may be a strategy for hosts to escape from parasites but the parasite can also exploit the fragmentation of colonial hosts to gain vertical transmission and dispersal. Our study provides evidence that opportunities and constraints for host-parasite co-evolution can be highly dependent on organismal body plans and that low virulence may be associated with exploitation of colonial hosts by endoparasites.

PROLIFERATIVE, PRESACCULAR STAGES OF TETRACAPSULOIDES BRYOSALMONAE (MYXOZOA: MALACOSPOREA) WITHIN THE INVERTEBRATE HOST FREDERICELLA SULTANA (BRYOZOA: PHYLACTOLAEMATA)

Proliferative kidney disease (PKD), caused by the malacosporean parasite, Tetracapsuloides bryosalmonae, is a major disease of salmonid culture both in western Europe and North America. The fish are infected from spores that develop within freshwater bryozoans and are released into the water column. Although sporogenesis has been studied in the bryozoan host and occurs within sacs, the formation of these sacs from presaccular stages has only been hypothesized. Examination of infected bryozoans by using a range of techniques identified proliferating, presaccular amoeboid stages of T. bryosalmonae on the body wall of the bryozoan Fredericella sultana. These stages possessed unique electron-dense bodies and were observed as aggregating within the bryozoan metacoel, differentiating to form spore sacs. Spore sac growth was associated with the assimilation of the presaccular parasites rather than through cryptomitosis of sac mural cells. This sac formation through aggregation and assimilation suggests an intriguing mechanism by which T. bryosalmonae can cross-fertilize.

Transmission of Tetracapsuloides bryosalmonae (Myxozoa: Malacosporea), the causative organism of salmonid proliferative kidney disease, to the freshwater bryozoan Fredericella sultana

Proliferative kidney disease (PKD), caused by the malacosporean parasite Tetracapsuloides bryosalmonae, causes significant losses among salmonids in Western Europe and North America. The role of salmonid fish in the life-cycle of this parasite has been conjectured upon for over a quarter of a century. To examine whether fish can transmit the infection to bryozoans, the known invertebrate host, water containing parasitized brown trout Salmo trutta was pumped into tanks containing colonies of Fredericella sultana collected from the wild. The specific parasite-free status of these colonies being first assessed, by PCR and prolonged laboratory culture. After 6 weeks exposure to the brown trout aquarium effluent, portions of these colonies displayed overt infections with T. bryosalmonae. This was in contrast to control bryozoans, derived from the experimental colonies prior to exposure, which remained T. bryosalmonae negative. In addition, spores obtained from the experimentally infected colonies were exposed to naïve rainbow trout, resulting in clinical PKD, thus completing a cycle of transmission. During the experiments, the infection was noted to inhibit statoblast formation within bryozoans and appeared to be pathogenic, finally killing the bryozoan host. These findings indicate that fish can transmit the parasite to bryozoans and are an integral part of this parasite's life-cycle.

Development of Tetracapsuloides bryosalmonae (Myxozoa: Malacosporea) in bryozoan hosts (as examined by light microscopy) and quantitation of infective dose to rainbow trout (Oncorhynchus mykiss)

The myxozoan parasite Tetracapsuloides bryosalmonae is the causative agent of proliferative kidney disease (PKD), a highly damaging disease of cultured salmonid fish. Within this study, phylactolaemate bryozoans were collected from a river known to be endemic for PKD and subsequently cultured in the laboratory. Sequential developmental stages of T. bryosalmonae were studied by light microscopy within the living bryozoan colonies, allowing the identification of stages attached to host peritoneum, consistent with previous molecular evidence of cryptic stages. Infection resulted in the production of large numbers of spores, which were released from the bryozoans. Experimental exposure of rainbow trout (Oncorhynchus mykiss) to medium in which infected bryozoans were cultured resulted in clinical PKD. Rainbow trout were exposed to known numbers of T. bryosalmonae spores collected by micromanipulation, which had been released from mature spore sacs within colonies of the bryozoan Fredericella sultana. Exposure to one spore was sufficient to lead to development of PKD. These findings indicate that small numbers of bryozoans are capable of releasing sufficient spores to infect large numbers of fish, having implications for future control methods for PKD in salmonid farming.

The phylogeography of salmonid proliferative kidney disease in Europe and North America

Salmonid proliferative kidney disease (PKD) is caused by the myxozoan Tetracapsuloides bryosalmonae. Given the serious and apparently growing impact of PKD on farmed and wild salmonids, we undertook a phylogeographic study to gain insights into the history of genealogical lineages of T. bryosalmonae in Europe and North America, and to determine if the global expansion of rainbow trout farming has spread the disease. Phylogenetic analyses of internal transcribed spacer 1 sequences revealed a clade composed of all North American sequences plus a subset of Italian and French sequences. High genetic diversity in North America and the absence of genotypes diagnostic of the North American clade in the rest of Europe imply that southern Europe was colonized by immigration from North America; however, sequence divergence suggests that this colonization substantially pre-dated fisheries activities. Furthermore, the lack of southern European lineages in the rest of Europe, despite widespread rainbow trout farming, indicates that T. bryosalmonae is not transported through fisheries activities. This result strikingly contrasts with the commonness of fisheries-related introductions of other pathogens and parasites and indicates that fishes may be dead-end hosts. Our results also demonstrate that European strains of T. bryosalmonae infect and induce PKD in rainbow trout introduced to Europe.