Search

We explored the extent to which δC and δD values of freshwater bryozoan statoblasts can provide information about the isotopic composition of zooids, bryozoan food and surrounding water. Bryozoan samples were collected from 23 sites and encompassed ranges of nearly 30‰ for δC and 100‰ for δD values. δC offsets between zooids and statoblasts generally ranged from −3 to +4.5‰, with larger offsets observed in four samples. However, a laboratory study with Plumatella emarginata and Lophopus crystallinus demonstrated that, in controlled settings, zooids had only 0-1.2‰ higher δC values than statoblasts, and 1.7‰ higher values than their food. At our field sites, we observed a strong positive correlation between median δC values of zooids and median δC values of corresponding statoblasts. We also observed a positive correlation between median δD values of zooids and statoblasts for Plumatella, and a positive correlation between median δD values of statoblasts and δD values of lake water for Plumatella and when all bryozoan taxa were examined together. Our results suggest that isotope measurements on statoblasts collected from flotsam or sediment samples can provide information on the feeding ecology of bryozoans and the H isotopic composition of lake water. [ABSTRACT FROM AUTHOR]

Myxozoans are extremely morphologically degenerate bilateral animals that are endoparasitic in aquatic invertebrates and fish and present intriguing questions regarding the evolution of life cycle complexity in parasites. The life cycle of myxozoans in the Class Myxosporea entails obligate cycling between worm and fish hosts. The newly-described Malacosporea are parasites of freshwater bryozoans and also occur in fish, but whether fish are true hosts is unresolved. The timing of sexuality implies that invertebrates in both classes are definitive hosts. The recent discovery that the worm-like Buddenbrockia is a malacosporean provides evidence for retention of ancestral features in the Class Malacosporea (with the Myxosporea being a more derived group) or, less likely, an evolutionary atavism. Ultrastructural evidence suggests that the life cycle of Buddenbrockia may entail a facultative polymorphism with primitive, active, worm-like stages developing in certain bryozoan hosts and derived, passive sac-like stages in others via some form of paedomorphosis. The acquisition of secondary hosts in the Myxosporea may have arisen through increased transmission via asexual proliferation in new hosts, increased host longevity and the key innovation of producing resistant, long-lived infective spores. The apparent lack of development of mature malacosporean stages in fish raises the intriguing question of whether fish are in the process of being gained or lost as hosts for the Malacosporea. Consideration of the distribution of apparently primitive and derived traits in the two myxozoan clades provides insights into the evolution of myxozoan life cycles.

Abstract Background The enigmatic wormlike parasite Buddenbrockia plumatellae has recently been shown to belong to the Myxozoa, which are now supported as a clade within Cnidaria. Most myxozoans are morphologically extremely simplified, lacking major metazoan features such as epithelial tissue layers, gut, nervous system, body axes and gonads. This hinders comparisons to free-living cnidarians and thus an understanding of myxozoan evolution and identification of their cnidarian sister group. However, B. plumatellae is less simplified than other myxozoans and therefore is of specific significance for such evolutionary considerations. Methods We analyse and describe the development of major body plan features in Buddenbrockia worms using a combination of histology, electron microscopy and confocal microscopy. Results Early developmental stages develop a primary body axis that shows a polarity, which is manifested as a gradient of tissue development, enabling distinction between the two worm tips. This polarity is maintained in adult worms, which, in addition, often develop a pore at the distal tip. The musculature comprises tetraradially arranged longitudinal muscle blocks consisting of independent myocytes embedded in the extracellular matrix between inner and outer epithelial tissue layers. The muscle fibres are obliquely oriented and in fully grown worms consistently form an angle of 12° with respect to the longitudinal axis of the worm in each muscle block and hence confer chirality. Connecting cells form a link between each muscle block and constitute four rows of cells that run in single file along the length of the worm. These connecting cells are remnants of the inner epithelial tissue layer and are anchored to the extracellular matrix. They are likely to have a biomechanical function. Conclusions The polarised primary body axis represents an ancient feature present in the last common ancestor of Cnidaria and Bilateria. The tetraradial arrangement of musculature is consistent with a medusozoan affinity for Myxozoa. However, the chiral pattern of muscle fibre orientation is apparently novel within Cnidaria and could thus be a specific adaptation. The presence of independent myocytes instead of Cnidaria-like epitheliomuscular cells can be interpreted as further support for the presence of mesoderm in cnidarians, or it may represent convergent evolution to a bilaterian condition.

Aim: The aim of this non-inferiority randomized clinical trial was to compare the efficacy of an iodoform-based paste (Guedes-Pinto -(GP)) as a filling material in pulpectomies of primary teeth, and a standard material composed by calcium hydroxide and iodoform (CaOH/Iodof paste; Vitapex ® )., Design: A total of 104 teeth from 61 children (3-8 years old) were randomly allocated to two groups according to filling materials. Children were followed up for 24 months. The primary endpoint was the treatment success rate evaluated through clinical and radiographic examinations at follow-up, and the secondary outcome was the analysis of the canal filling quality. Differences in the proportion of treatment success was calculated based on 95% confidence intervals (95% CI) and with the Miettinen and Nurminen method in the intention-to-treat population, considering a -20% of the non-inferiority limit., Results: From 104 randomized teeth, 102 were followed up after 24 months (attrition rate of 1.9%). The success rate of teeth treated with the GP paste was 86.8% (95% CI: 69.9-94.9) and 78.4% (95% CI: 61.8-89.1) with the CaOH/Iodof paste. Consequently, a non-inferiority of the GP paste was observed when compared to the CaOH/Iodof paste (P < .001)., Conclusion: The GP paste has a non-inferior success rate than the CaOH/Iodof paste used as filling material for pulpectomy in primary teeth., (© 2021 BSPD, IAPD and John Wiley & Sons Ltd.)

Evolutionary transitions of organisms between environments have long fascinated biologists, but attention has been focused almost exclusively on free-living organisms and challenges to achieve such transitions. This bias requires addressing because parasites are a major component of biodiversity. We address this imbalance by focusing on transitions of parasitic animals between marine and freshwater environments. We highlight parasite traits and processes that may influence transition likelihood (e.g., transmission mode, life cycle, host use), and consider mechanisms and directions of transitions. Evidence for transitions in deep time and at present are described, and transitions in our changing world are considered. We propose that environmental transitions may be facilitated for endoparasites because hosts reduce exposure to physiologically challenging environments and argue that adoption of an endoparasitic lifestyle entails an equivalent transitioning process as organisms switch from living in one environment (e.g., freshwater, seawater, or air) to living symbiotically within hosts. Environmental transitions of parasites have repeatedly resulted in novel forms and diversification, contributing to the tree of life. Recognizing the potential processes underlying present-day and future environmental transitions is crucial in view of our changing world and the current biodiversity crisis., (© The Author(s) 2022. Published by Oxford University Press on behalf of the Society for Integrative and Comparative Biology.)

A bryozoan survey conducted in the Amazon Basin in the vicinities of Manaus and Santarm during the high water season (May, 2018) revealed four new species described here: Fredericella adrianoi n. sp., Plumatella divae n. sp., Plumatella hartikainenae n. sp., and Plumatella spencerjonesae n. sp. Two of these species were encountered only once, suggesting that other undescribed species are likely to occur in the area. Range extensions were determined for two additional species: Plumatella pirassununga and Timwoodiellina natans. In addition, colonies were collected for the first time for two species previously known only by their statoblasts: Plumatella siolii and Plumatella marcusi. Statoblasts of Tapajosella elongata were encountered near Manaus, but the colonies remained elusive. The discovery of new species collected during two expeditions to the Amazon Basin in different seasons and years suggests that further diversity remains undetected in this and other poorly studied regions of the world. With few exceptions, plumatellid colonies described so far from the Amazon Basin are very similar in appearance, with branches wholly attached to the substratum and body walls that are soft, colorless, and transparent.

Examination of faecal material has demonstrated how a broad range of organisms are distributed by bird movements. Such research has largely focused on dispersal of plant seeds by frugivores and of freshwater organisms by waterbirds. However, with few exceptions (e.g. avian influenza, Ebola virus), there is a dearth of evidence for transport of parasites and pathogens. High-throughput sequencing methods now provide a powerful means of addressing this knowledge gap by elucidating faecal contents in unprecedented detail. We collected faeces excreted by a range of migratory waterbirds in south-west Spain and pooled faecal DNA to create libraries reflective of feeding behavior. We created sets of libraries using high-throughput metagenomic and amplicon sequencing. For the latter we employed two sets of primers to broadly target the V4 region of the 18S rRNA gene (one set amplifying the region across all eukaryotes, the other excluding amplification of metazoans). Libraries revealed a wide diversity of eukaryotes, including parasites of the faecal producers themselves, parasites of food items, or those incidentally ingested. We also detected novel microbial eukaryotic taxa and found that parasite assemblage profiles were relatively distinct. Comparing the performance of the methods used supports their joint use for future studies of diversity and abundance. Because viable stages of many parasites are likely to be present in faeces, our results suggest significant levels of bird-mediated dispersal of parasites (both from avian and other hosts). Our methods revealed much hidden biodiversity, and allowed identification of the individuals who produced the faecal samples to species level, facilitating the study of interaction networks., (© 2021 John Wiley & Sons Ltd.)

Metazoans with worm-like morphologies across diverse and disparate groups typically demonstrate motility generated by hydrostatic skeletons involving tissue layers (muscles and epithelia). Here we present representative morphological, behavioural and molecular data for parasitic cnidarians (myxozoans) that demonstrate unprecedented variation in form and function, developing as cellular hydrostats. Motile elongate plasmodia characterise a remarkable radiation of species in the genus Ceratomyxa. The vermiform plasmodia inhabit gall bladders of a range of South American freshwater fish and exhibit undulatory motility reminiscent of nematodes but achieved at the cellular level. Collective insights from ultrastructure, confocal and light microscopy along with videos depicting movements highlight key features that we propose explain the unique motility of the plasmodia. These features include cytoskeletal elements (net forming microfilaments and microtubules), a large internal vacuole, a relatively rigid outer glycocalyx and peripherally arranged mitochondria. These constituents provide collective evidence for repurposing of the cnidarian epitheliomuscular cell to support worm-like motility at the cellular level. The apparent restriction of vermiform ceratomyxids to South American freshwaters suggests an origination via Cretaceous or Miocene marine transgressions and subsequent radiation., (© 2021 Anatomical Society.)

Cnidarians are the oldest lineage of venomous animals and use nematocysts to discharge toxins. Whether venom toxins have been recruited to support parasitic lifestyles in the Endocnidozoa (Myxozoa + Polypodium ) is, however, unknown. To examine this issue we variously employed transcriptomic, proteomic, associated molecular phylogenies, and localisation studies on representative primitive and derived myxozoans (Malacosporea and Myxosporea, respectively), Polypodium hydriforme , and the free-living staurozoan Calvadosia cruxmelitensis . Our transcriptomics and proteomics analyses provide evidence for expression and translation of venom toxin homologs in myxozoans. Phylogenetic placement of Kunitz type serine protease inhibitors and phospholipase A2 enzymes reveals modification of toxins inherited from ancestral free-living cnidarian toxins, and that venom diversity is reduced in myxozoans concordant with their reduced genome sizes. Various phylogenetic analyses of the Kunitz-type toxin family in Endocnidozoa suggested lineage-specific gene duplications, which offers a possible mechanism for enhancing toxin diversification. Toxin localisation in the malacosporean Buddenbrockia plumatellae substantiates toxin translation and thus illustrates a repurposing of toxin function for endoparasite development and interactions with hosts, rather than for prey capture or defence. Whether myxozoan venom candidates are expressed in transmission stages (e.g. in nematocysts or secretory vesicles) requires further investigation., Competing Interests: Malcolm Ward is employed by Aulesa Ltd. All other authors declare that they have no competing interests., (© 2021 Hartigan et al.)

This study undertook the first investigation of malacosporean infections in Neotropical fish. We used polymerase chain reaction detection with a primer set generally targeting known malacosporeans to assay for infection in the kidney of 146 fish in 21 species belonging to 12 families collected from two areas in the Amazon Basin. Infections were found in 13 fish variously belonging to seven species in six families and included the first identification of a malacosporean infection in cartilaginous fish (a freshwater stingray). Based on ssrDNA, all infections represented a single Buddenbrockia species (Buddenbrockia sp. E) that demonstrates an exceptionally broad range of fish species infected, and countered our expectations of high Neotropical malacosporean diversity. Infections were characterized at varying and often high prevalences in fish species but sample sizes were small. Ascertaining whether highly divergent malacosporeans have not been detected by current primers, and more comprehensive sampling may reveal whether malacosporeans are truly as species poor in the Amazon Basin as present data suggest. Our results prompt speculations about evolutionary scenarios including introduction via marine incursions and patterns of host use over time.

The myxozoan parasite, Tetracapsuloides bryosalmonae has a two-host life cycle alternating between freshwater bryozoans and salmonid fish. Infected fish can develop Proliferative Kidney Disease, characterised by a gross lymphoid-driven kidney pathology in wild and farmed salmonids. To facilitate an in-depth understanding of T. bryosalmonae-host interactions, we have used a two-host parasite transcriptome sequencing approach in generating two parasite transcriptome assemblies; the first derived from parasite spore sacs isolated from infected bryozoans and the second from infected fish kidney tissues. This approach was adopted to minimize host contamination in the absence of a complete T. bryosalmonae genome. Parasite contigs common to both infected hosts (the intersect transcriptome; 7362 contigs) were typically AT-rich (60-75% AT). 5432 contigs within the intersect were annotated. 1930 unannotated contigs encoded for unknown transcripts. We have focused on transcripts encoding proteins involved in; nutrient acquisition, host-parasite interactions, development, cell-to-cell communication and proteins of unknown function, establishing their potential importance in each host by RT-qPCR. Host-specific expression profiles were evident, particularly in transcripts encoding proteases and proteins involved in lipid metabolism, cell adhesion, and development. We confirm for the first time the presence of homeobox proteins and a frizzled homologue in myxozoan parasites. The novel insights into myxozoan biology that this study reveals will help to focus research in developing future disease control strategies.

Polypodium hydriforme is a parasitic cnidarian that develops within the eggs of acipenseriform fish in the Old and New Worlds. Currently regarded as monotypic, P. hydriforme has been studied largely in the context of caviar production in Russian sturgeon species. We report the first robust epidemiological study of P. hydriforme in North American acipenseriform fish. We sampled infection prevalences (in 2017 and 2018) and intensities (in 2017) during annual surveys of American Paddlefish, Polyodon spathula, caught during spawning migration in north-eastern Oklahoma. Egg masses were characterized for the presence and intensity of P. hydriforme infection. Prevalences were similar in 2017 and 2018 (49% and 45%, respectively). Generally, a small number of eggs were infected per egg mass, but a few were heavily infected. Longer, heavier and older fish are more likely to be infected and to harbour more severe infections. In addition, infection is linked to decreases in roe fat weight independently of fish length, weight, age or roe weight. Infection thus diminishes Paddlefish energy reserves (roe fat) which could in turn impact host fitness. Our results raise questions about the impacts of infection on caviar production and Paddlefish conservation and suggest insights on infection dynamics and parasite strategies., (© 2020 John Wiley & Sons Ltd.)

Variation in dispersal capacity may influence population genetic variation and relatedness of freshwater animals thus demonstrating how life-history traits influence patterns and processes that in turn influence biodiversity. The majority of studies have focused on the consequences of dispersal variation in taxa inhabiting riverine systems whose dendritic nature and upstream/downstream gradients facilitate characterizing populations along networks. We undertook extensive, large-scale investigations of the impacts of hydrological connectivity on population genetic variation in two freshwater bryozoan species whose dispersive propagules (statoblasts) are either attached to surfaces ( Fredericella sultana ) or are released as buoyant stages ( Cristatella mucedo ) and that live primarily in either lotic ( F. sultana ) or lentic environments ( C. mucedo ). Describing population genetic structure in multiple sites characterized by varying degrees of hydrological connectivity within each of three (or four) UK regions enabled us to test the following hypotheses: (1) genetic diversity and gene flow will be more influenced by hydrological connectivity in populations of C. mucedo (because F. sultana dispersal stages are retained); (2) populations of F. sultana will be characterized by greater genetic divergence than those of C. mucedo (reflecting their relative dispersal capacities); and (3) genetic variation will be greatest in F. sultana (reflecting a propensity for genetic divergence as a result of its low dispersal potential). We found that hydrological connectivity enhanced genetic diversity and gene flow among C. mucedo populations but not in F. sultana while higher overall measures of clonal diversity and greater genetic divergence characterized populations of F. sultana . We suggest that genetic divergence over time within F. sultana populations reflects a general constraint of releasing propagules that might eventually be swept to sea when taxa inhabit running waters. In contrast, taxa that primarily inhabit lakes and ponds may colonize across hydrologically connected regions, establishing genetically related populations. Our study contributes more nuanced views about drivers of population genetic structures in passively dispersing freshwater invertebrates as outlined by the Monopolization Hypothesis ( Acta Oecologica , 23, 2002, 121) by highlighting how a range of demographic and evolutionary processes reflect life-history attributes of benthic colonial invertebrates (bryozoans) and cyclically parthenogenetic zooplankton. In addition, growing evidence that genetic divergence may commonly characterize populations of diverse groups of riverine taxa suggests that organisms inhabiting lotic systems may be particularly challenged by environmental change. Such change may predispose riverine populations to extinction as a result of genetic divergence combined with limited dispersal and gene flow., Open Research Badges: This article has earned an Open Data Badge for making publicly available the digitally-shareable data necessary to reproduce the reported results. The data is available at https://doi.org/10.5061/dryad.1tm8705., Competing Interests: None declared., (© 2019 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd.)

Environmental stress from ultraviolet radiation, elevated temperatures or metal toxicity can lead to reactive oxygen species in cells, leading to oxidative DNA damage, premature aging, neurodegenerative diseases, and cancer. The transcription factor nuclear factor (erythroid-derived 2)-like 2 (Nrf2) activates many cytoprotective proteins within the nucleus to maintain homeostasis during oxidative stress. In vertebrates, Nrf2 levels are regulated by the Kelch-family protein Keap1 (Kelch-like ECH-associated protein 1) in the absence of stress according to a canonical redox control pathway. Little, however, is known about the redox control pathway used in early diverging metazoans. Our study examines the presence of known oxidative stress regulatory elements within non-bilaterian metazoans including free living and parasitic cnidarians, ctenophores, placozoans, and sponges. Cnidarians, with their pivotal position as the sister phylum to bilaterians, play an important role in understanding the evolutionary history of response to oxidative stress. Through comparative genomic and transcriptomic analysis our results show that Nrf homologs evolved early in metazoans, whereas Keap1 appeared later in the last common ancestor of cnidarians and bilaterians. However, key Nrf-Keap1 interacting domains are not conserved within the cnidarian lineage, suggesting this important pathway evolved with the radiation of bilaterians. Several known downstream Nrf targets are present in cnidarians suggesting that cnidarian Nrf plays an important role in oxidative stress response even in the absence of Keap1. Comparative analyses of key oxidative stress sensing and response proteins in early diverging metazoans thus provide important insights into the molecular basis of how these lineages interact with their environment and suggest a shared evolutionary history of regulatory pathways. Exploration of these pathways may prove important for the study of cancer therapeutics and broader research in oxidative stress, senescence, and the functional responses of early diverging metazoans to environmental change., (© The Author(s) 2019. Published by Oxford University Press on behalf of the Society for Integrative and Comparative Biology. All rights reserved. For permissions please email: journals.permissions@oup.com.)

For many years methodological constraints limited insights on the molecular biology of non-model organisms. However, the development of various sequencing platforms has led to an explosion of transcriptomic and genomic data on non-model systems. As a consequence the molecular drivers of organismal phenotypes are becoming clearer and the chemicals that animals use to detect and respond to their environments are increasingly being revealed-this latter area inspired our symposium theme. The papers in this volume broadly address this theme by their more specific focus in one of the following general areas: 1) sensory biology and the molecular basis of perception, 2) chemicals deployed to deal with the biotic and abiotic environment, and 3) chemical interactions along the parasite-mutualist continuum. Here we outline and synthesize the content of these papers-an exercise which demonstrates that sophisticated gene repertoires enable early diverging metazoans to encode many of the signaling, sensory, defensive, and offensive capacities typically associated with animals that have complex nervous systems. We then consider opportunities and associated challenges that may delay progress in comparative functional biochemistry, a reinvigorated field that can be expected to rapidly expand with new 'omics data. Future knowledge of chemical adaptations should afford new perspectives on the comparative evolution of chemical mediators., (© The Author(s) 2019. Published by Oxford University Press on behalf of the Society for Integrative and Comparative Biology. All rights reserved. For permissions please email: journals.permissions@oup.com.)

Venomous animals can deploy toxins for both predation and defense. These dual functions of toxins might be expected to promote the evolution of new venoms and alteration of their composition. Cnidarians are the most ancient venomous animals but our present understanding of their venom diversity is compromised by poor taxon sampling. New proteomic data were therefore generated to characterize toxins in venoms of a staurozoan, a hydrozoan, and an anthozoan. We then used a novel clustering approach to compare venom diversity in cnidarians to other venomous animals. Comparison of the presence or absence of 32 toxin protein families indicated venom composition did not vary widely among the 11 cnidarian species studied. Unsupervised clustering of toxin peptide sequences suggested that toxin composition of cnidarian venoms is just as complex as that in many venomous bilaterians, including marine snakes. The adaptive significance of maintaining a complex and relatively invariant venom remains unclear. Future study of cnidarian venom diversity, venom variation with nematocyst types and in different body regions are required to better understand venom evolution., (© The Author(s) 2019. Published by Oxford University Press on behalf of the Society for Integrative and Comparative Biology. All rights reserved. For permissions please email: journals.permissions@oup.com.)

Myxozoans are widespread and common endoparasites of fish with complex life cycles, infecting vertebrate and invertebrate hosts. There are two classes: Myxosporea and Malacosporea. To date about 2500 myxosporean species have been described. By comparison, there are only five described malacosporean species. Malacosporean development in the invertebrate hosts (freshwater bryozoans) has been relatively well studied but is poorly known in fish hosts. Our aim was to investigate the presence and development of malacosporeans infecting a diversity of fish from Brazil, Europe and the USA. We examined kidney from 256 fish belonging variously to the Salmonidae, Cyprinidae, Nemacheilidae, Esocidae, Percidae, Polyodontidae, Serrasalmidae, Cichlidae and Pimelodidae. Malacosporean infections were detected and identified by polymerase chain reaction and small subunit ribosomal DNA sequencing, and the presence of sporogonic stages was evaluated by ultrastructural examination. We found five malacosporean infections in populations of seven European fish species (brown trout, rainbow trout, white fish, dace, roach, gudgeon and stone loach). Ultrastructural analyses revealed sporogonic stages in kidney tubules of three fish species (brown trout, roach and stone loach), providing evidence that fish belonging to at least three families are true hosts. These results expand the range of fish hosts exploited by malacosporeans to complete their life cycle.

How biodiversity is changing in our time represents a major concern for all organismal biologists. Anthropogenic changes to our planet are decreasing species diversity through the negative effects of pollution, habitat destruction, direct extirpation of species, and climate change. But major biotic changes-including those that have both increased and decreased species diversity-have happened before in Earth's history. Biodiversity dynamics in past eras provide important context to understand ecological responses to current environmental change. The work of assessing biodiversity is woven into ecology, environmental science, conservation, paleontology, phylogenetics, evolutionary and developmental biology, and many other disciplines; yet, the absolute foundation of how we measure species diversity depends on taxonomy and systematics. The aspiration of this symposium, and complementary contributed talks, was to promote better understanding of our common goals and encourage future interdisciplinary discussion of biodiversity dynamics. The contributions in this collection of papers bring together a diverse group of speakers to confront several important themes. How can biologists best respond to the urgent need to identify and conserve diversity? How can we better communicate the nature of species across scientific disciplines? Where are the major gaps in knowledge about the diversity of living animal and plant groups, and what are the implications for understanding potential diversity loss? How can we effectively use the fossil record of past diversity and extinction to understand current biodiversity loss?

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.

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.