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Artificial light at night (ALAN) negatively impacts organisms in many ways, from their feeding behaviors to their response and ability to deal with disease. Our knowledge of ALAN is focused on hosts, but we must also consider their parasites, which constitute half of all described animal species. Here, we assessed the impact of light exposure on a model host-parasite system (Poecilia reticulata and the ectoparasitic monogenean Gyrodactylus turnbulli). First, parasite-free fish were exposed to 12:12 h light:dark (control) or 24:0 h light:dark (ALAN) for 21 days followed by experimental infection. Second, naturally acquired G. turnbulli infections were monitored for 28 days during exposure of their hosts to a specified light regime (6:18 h, 12:12 h, or 24:0 h light:dark). Experimentally infected fish exposed to constant light had, on average, a greater maximum parasite burden than controls, but no other measured parasite metrics were impacted. Host feeding behavior was also significantly affected: fish under ALAN fed faster and took more bites than controls, whilst fish exposed to reduced light fed slower. Thus, ALAN can impact parasite burdens, even in the short term, and altering light conditions will impact fish feeding behavior. Such responses could initiate disease outbreaks or perturb food-webs with wider ecological impacts., (© 2024 The Author(s). Journal of Fish Biology published by John Wiley & Sons Ltd on behalf of Fisheries Society of the British Isles.)

Microplastics have been found in all surveyed ecosystems and in the diet of multiple species. Detrimental health impacts of microplastic consumption include reduced growth and fecundity, metabolic stress and immune alterations for both invertebrates and vertebrates. Limited information exists, however, on how disease resistance may be affected by microplastic exposure and consumption. Here, the impact of microplastic (0.01 and 0.05 mg l-1 of polypropylene) on fish host susceptibility to disease and mortality was assessed using the guppy Poecilia reticulata-gyrodactylid Gyrodactylus turnbulli system. Fish exposed to and/or consuming microplastic at both concentrations demonstrated significantly higher pathogen burdens over time compared with fish fed a plastic-free diet. Furthermore, microplastic (at both tested concentrations) was associated with increased mortality events for fish within all treatments, regardless of host infection status. This study adds to the growing body of evidence showing that microplastic pollution can be detrimental to fish welfare by reducing disease resistance.

During their different life stages, parasites undergo remarkable morphological, physiological, and behavioral "metamorphoses" to meet the needs of their changing habitats. This is even true for ectoparasites, such as the monogeneans, which typically have a free-swimming larval stage (oncomiracidium) that seeks out and attaches to the external surfaces of fish where they mature. Before any obvious changes occur, there are ultrastructural differences in the oncomiracidium's outer surface that prepare it for a parasitic existence. The present findings suggest a distinct variation in timing of the switch from oncomiracidia epidermis to the syncytial structure of the adult tegument and so, to date, there are three such categories within the Monogenea: (1) Nuclei of both ciliated cells and interciliary cytoplasm are shed from the surface layer and the epidermis becomes a syncytial layer during the later stages of embryogenesis; (2) nuclei of both ciliated cells and interciliary syncytium remain distinct and the switch occurs later after the oncomiracidia hatch (as in the present study); and (3) the nuclei remain distinct in the ciliated epidermis but those of the interciliary epidermis are lost during embryonic development. Here we describe how the epidermis of the oncomiracidium of Discocotyle sagittata is differentiated into two regions, a ciliated cell layer and an interciliary, syncytial cytoplasm, both of which are nucleated. The interciliary syncytium extends in-between and underneath the ciliated cells and sometimes covers part of their apical surfaces, possibly the start of their shedding process. The presence of membranous whorls and pyknotic nuclei over the surface are indicative of membrane turnover suggesting that the switch in epidermis morphology is already initiated at this stage. The body tegument and associated putative sensory receptors of subadult and adult D. sagittata are similar to those in other monogeneans.

The notifiable freshwater pathogen Gyrodactylus salaris Malmberg, 1957 tends to be a generalist in contrast to other monogeneans. Whilst it causes most damage to its primary host, the Atlantic salmon (Salmo salar Linnaeus), transport and reservoir hosts likely play a key role in maintaining the parasite in the environment. Here, we tested the ability of G. salaris (strain River Lierelva, southern Norway) to infect and reproduce on a population of wild caught alpine bullhead (Cottus poecilopus Heckel). Exposure of alpine bullhead yearlings (0+) to G. salaris for 24 h at low (6.5 °C) or high temperature (11.5 °C) resulted in the establishment of 1 to 104 parasites per fish. Eight to nine days post-infection at high temperature, the infection of G. salaris was eliminated, indicative of innate host immunity. In contrast, at low temperature G. salaris infections persisted for 47-48 days. The relative lengthy infection of alpine bullhead with G. salaris compared to other non-salmonids tested may be due to low temperature and high initial infection load in combination with an epibiont infection. The present results suggest that this non-salmonid may function as a temperature-dependent transport or reservoir host for G. salaris.

While group formation provides antipredatory defences, increases foraging efficiency and mating opportunities, it can be counterintuitive by promoting disease transmission amongst social hosts. Upon introduction of a pathogen, uninfected individuals often modify their social preferences to reduce infection risk. Infected hosts also exhibit behavioural changes, for example, removing themselves from a group to prevent an epidemic. Conversely, here we show how Trinidadian guppies infected with a directly transmitted ectoparasite, Gyrodactylus turnbulli, significantly increase their contact rates with uninfected conspecifics. As uninfected fish never perform this behaviour, this is suggestive of a parasite-mediated behavioural response of infected hosts, presumably to offload their parasites. In the early stages of infection, however, such behavioural modifications are ineffective in alleviating parasite burdens. Additionally, we show that fish exposed to G. turnbulli infections for a second time, spent less time associating than those exposed to parasites for the first time. We speculate that individuals build and retain an infection cue repertoire, enabling them to rapidly recognize and avoid infectious conspecifics. This study highlights the importance of considering host behavioural modifications when investigating disease transmission dynamics.

Infectious disease dynamics depend on the speed, number and fitness of parasites transmitting from infected hosts ('donors') to parasite-naive 'recipients'. Donor heterogeneity likely affects these three parameters, and may arise from variation between donors in traits including: (i) infection load, (ii) resistance, (iii) stage of infection, and (iv) previous experience of transmission. We used the Trinidadian guppy, Poecilia reticulata , and a directly transmitted monogenean ectoparasite, Gyrodactylus turnbulli , to experimentally explore how these sources of donor heterogeneity affect the three transmission parameters. We exposed parasite-naive recipients to donors (infected with a single parasite strain) differing in their infection traits, and found that donor infection traits had diverse and sometimes interactive effects on transmission. First, although transmission speed increased with donor infection load, the relationship was nonlinear. Second, while the number of parasites transmitted generally increased with donor infection load, more resistant donors transmitted more parasites, as did those with previous transmission experience. Finally, parasites transmitting from experienced donors exhibited lower population growth rates on recipients than those from inexperienced donors. Stage of infection had little effect on transmission parameters. These results suggest that a more holistic consideration of within-host processes will improve our understanding of between-host transmission and hence disease dynamics.This article is part of the themed issue 'Opening the black box: re-examining the ecology and evolution of parasite transmission'., (© 2017 The Author(s).)

Inbreeding can threaten population persistence by reducing disease resistance through the accelerated loss of gene diversity (i.e. heterozygosity). Such inbreeding depression can affect many different fitness-related traits, including survival, reproductive success, and parasite susceptibility. Empirically quantifying the effects of inbreeding on parasite resistance is therefore important for ex-situ conservation of vertebrates. The present study evaluates the disease susceptibility of individuals bred under three different breeding regimes (inbred, crossed with full siblings; control, randomly crossed mating; and fully outbred). Specifically, we examined the relationship between inbreeding coefficient (F-coefficient) and susceptibility to Gyrodactylus turnbulli infection in a live bearing vertebrate, the guppy Poecilia reticulata. Host-breeding regime significantly affected the trajectories of parasite population growth on individual fish. Inbred fish showed significantly higher mean parasite intensity than fish from the control and outbred breeding regimes, and in addition, inbred fish were slower in purging their gyrodactylid infections. We discuss the role of inbreeding on the various arms of the immune system, and argue that the increased disease susceptibility of inbred individuals could contribute to the extinction vortex. This is one of the first studies to quantify the effects of inbreeding and breeding regime on disease susceptibility in a captive bred vertebrate of wild origin, and it highlights the risks faced by small (captive-bred) populations when exposed to their native parasites., (Copyright © 2016 The Authors. Published by Elsevier Inc. All rights reserved.)

Scanning and transmission electron microscopy (TEM) were used to study the histopathological effects of the monogenean Macrogyrodactylus clarii Gussev, 1961 on the gills of the catfish Clarias gariepinus (Burchell). Suction generated during attachment created 'footprints' on host surfaces in which the host tissues were elevated above the general gill surface. 'Footprints' were bordered by four clefts caused by the muscular flaps on the anterior, lateral and posterior margins of the haptor. The hamuli points penetrate the gill tissue but no evidence was found for the insertion of the marginal hooklets. At the site of attachment, host cells adjacent to the lateral flaps often appeared compressed and widely spaced with large intercellular spaces. Desquamation of these surface epithelia was also apparent and some of the widely spaced epithelial cells had pseudopodium-like processes. Cells within the upper surface epithelial layer of the host were vacuolated and necrotic. Ruptured blood capillaries (blood spaces) in the secondary gill lamellae contained atypical compressed erythrocytes, agranular and granular leucocytes and evidence of haemorrhaging. Cells with fibrotic cytoplasm, putative phagocytes and host mucous cells were evidence of a host response at the site of parasite attachment. The possible role of these cells is discussed in relation to host resistance against infection.

The recent identification of Pseudamphistomum truncatum, (Rudolphi, 1819) (Trematoda: Opisthorchiidae) and Metorchis bilis (Braun, 1790) Odening, 1962 (synonymous with Metorchis albidus (Braun, 1893) Loos, 1899 and Metorchis crassiusculus (Rudolphi, 1809) Looss, 1899 (Trematoda: Opisthorchiidae)) in otters from Britain caused concern because of associated biliary damage, coupled with speculation over their alien status. Here, we investigate the presence, intensity and phylogeny of these trematodes in mustelids (principally otters) across Europe (Czech Republic, Denmark, France, Germany, Norway, Poland and Sweden and Britain). The trematodes were identified to species using the internal transcribed spacer II (ITS2) locus. Both parasites were found across Europe but at unequal frequency. In the German state of Saxony, eight out of eleven (73%) otters examined were infected with P. truncatum whilst this parasite was not found in either mink from Scotland (n=40) or otters from Norway (n=21). Differences in the phylogenies between the two species suggest divergent demographic histories possibly reflecting contrasting host diet or competitive exclusion, with M. bilis exhibiting greater mitochondrial diversity than P. truncatum. Shared haplotypes within the ranges of both parasite species probably reflect relatively unrestricted movements (both natural and anthropogenic) of intermediate and definitive hosts across Europe., (Copyright © 2015 The Authors. Published by Elsevier Ireland Ltd.. All rights reserved.)

The demand for ornamental fish has led to a steep rise in aquaculture for the hobbyist trade, promoting the emergence, persistence and spread of various infectious diseases. Complete control of disease outbreaks with antibiotics and chemical-based medicines is rare, but plant compounds may herald potential alternatives effective against a range of pathogens. Melafix® and Pimafix® are formulated with the essential oils cajuput (Melaleuca cajuputi) and West Indian bay (Pimenta racemosa) and are marketed against bacterial and fungal infections, respectively. Previous experiments showed high efficacy of emulsified cajuput oil against gyrodactylids; the current study tested Melafix® and Pimafix® and their individual compounds against Gyrodactylus turnbulli infecting the guppies Poecilia reticulata. In particular, a combination treatment of Melafix® and Pimafix® was highly effective at reducing in vitro survival of parasites from 15 to 2 h and eradicating 95% of gyrodactylids in vivo. The unexpected high efficacy of this combination treatment is likely explained by the high content of terpenes and phenol propanoids in the cajuput and West Indian bay oils, as well as the anti-helminthic properties of the emulsifier Crovol PK 70. Hence, Melafix® and Pimafix® effectively reduce gyrodactylid burdens on fish, increasing the chances of efficient disease control in ornamental fish.

Predation pressure can alter the morphology, physiology, life history, and behavior of prey; each of these in turn can change how surviving prey interact with parasites. These trait-mediated indirect effects may change in direction or intensity during growth or, in sexually dimorphic species, between the sexes. The Trinidadian guppy, Poecilia reticulata presents a unique opportunity to examine these interactions; its behavioral ecology has been intensively studied in wild populations with well-characterized predator faunas. Predation pressure is known to have driven the evolution of many guppy traits; for example, in high-predation sites, females (but not males) tend to shoal, and this anti-predator behavior facilitates parasite transmission. To test for evidence of predator-driven differences in infection in natural populations, we collected 4715 guppies from 62 sites across Trinidad between 2003 and 2009 and screened them for ectosymbionts, including Gyrodactylus. A novel model-averaging analysis revealed that females were more likely to be infected with Gyrodactylus parasites than males, but only in populations with both high predation pressure and high infection prevalence. We propose that the difference in shoaling tendency between the sexes could explain the observed difference in infection prevalence between males and females in high-predation sites. The infection rate of juveniles did not vary with predation regime, probably because juveniles face constant predation pressure from conspecific adults and therefore tend to shoal in both high- and low-predation sites. This represents the first evidence for age- and sex-specific trait-mediated indirect effects of predators on the probability of infection in their prey.

Parasites are typically aggregated within their host populations. The most heavily infected hosts are frequently cited as targets for optimal disease control. Yet a heavily infected individual is not necessarily highly infective and does not automatically contribute a higher proportion of infective parasitic stages than a host with fewer parasites. Here, Pseudamphistomum truncatum (Opisthorchiida) parasitic infection within the definitive otter host (Lutra lutra) is used as a model system. The hypothesis tested is that variation in parasite abundance, aggregation and egg production (fecundity, as a proxy of host infectivity) can be explained by abiotic (season and region) or biotic (host age, sex and body condition) factors. Parasite abundance was affected most strongly by the biotic factors of age and body condition, such that adults and otters with a higher condition index had heavier infections than sub-adults or those with a lower condition index, whilst there were no significant differences in parasite abundance among the seasons, regions (ecological regions defined by river catchment boundaries) or host sexes. Conversely, parasite aggregation was affected most strongly by the abiotic factors of season and region, which were supported by four different measures of parasite aggregation (the corrected moment estimate k, Taylor's Power Law, the Index of Discrepancy D, and Boulinier's J). Pseudamphistomum truncatum was highly aggregated within otters, with aggregation stronger in the Midlands (England) and Wales than in the southwestern region of the United Kingdom. Overall, more parasites were found in fewer hosts during the summer, which coincides with the summer peak in parasite fecundity. Combined, these data suggest that (i) few otters carry the majority of P. truncatum parasites and that there are more infective stages (eggs) produced during summer; and (ii) abiotic factors are most influential when describing parasite aggregation whilst biotic factors have a greater role in defining parasite abundance. Together, parasite abundance, aggregation and fecundity can help predict which hosts make the largest contribution to the spread of infectious diseases., (Copyright © 2014 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Traditional compounds used to treat fish diseases in aquaculture and the ornamental fish industry (such as formalin and malachite green) can be more toxic to the hosts than their parasites. With the reviviscence in the use of herbal products, various botanicals have been heralded as cures for particular pathogens, but the efficacy of these compounds for parasitic worms is questionable. Here, we tested a range of garlic (Allium sativum) products against a major aquarium pathogen, Gyrodactylus turnbulli, infecting the guppy (Poecilia reticulata). All garlic products significantly reduced parasite mean survival time in vitro, from 13 h to <1 h. In fully randomised trials, the number of parasites was also significantly reduced on infected fish exposed to garlic from different sources. Two garlic treatments (minced and granule forms) reduced worm burdens by 66% and 75% after three doses, whereas Chinese freeze-dried garlic and allyl disulphide were 95% effective after a single application. In fact, Chinese freeze dried garlic was equally effective as Levamisole, a licensed livestock dewormer that is highly effective against G. turnbulli but not routinely prescribed for use in fish; hence, garlic may be a potential alternative treatment for gyrodactylosis., (Crown Copyright © 2013. Published by Elsevier B.V. All rights reserved.)

Parasites with complex life cycles are expected to be disproportionately affected by climate change. Knowledge of current associations with weather and host-parasite interactions is therefore essential for the inference of future distributions. The Eurasian otter, Lutra lutra, is exposed to a range of parasites due to its large home range and use of terrestrial, freshwater and marine habitats. As such, it can act as a sentinel species for generalist parasites. Here we consider two biliary parasites recently reported in the United Kingdom, Pseudamphistomum truncatum and Metorchis albidus (Trematoda, Opisthorchiidae), and ask whether there are associations between abiotic factors (season, temperature, rainfall and the North Atlantic Oscillation) and the prevalence and intensities of these parasites in otters (n = 586). To control for biotic interactions we first examined whether particular sub-groups of the otter population (grouped by sex, age-class and condition) are more prone to infection and whether any damage is associated with the presence of these parasites. Even though mean intensities of the smaller trematode, P. truncatum (28.3 worms/host), were much higher than M. albidus (4.1), both parasite species had similar impacts on the otter. The distributions of parasites on host sexes were similar, but males suffered greater damage and regardless of sex, parasite intensity increased in older hosts. The probability of infection with either parasite was negatively associated with ground frost, minimum temperatures and rainfall, but was positively associated with warm long-term average temperatures. Although it is widely accepted that multiple variables influence parasite distributions, to our knowledge this is one of only a few studies to examine the combined impact of biotic and abiotic variables on parasites with complex life cycles within their wild definitive host. Identifying such associations can give greater accuracy to predictions concerning the distribution and spread of trematodes with future climate change., (Copyright © 2013 Australian Society for Parasitology Inc. Published by Elsevier Ltd. All rights reserved.)

In a previous study we found that female guppies shoaled more than males and that there was greater transmission of the ectoparasite Gyrodactylus turnbulli between females. Here, to test for a possible sex bias in parasite transmission, we conducted a similar experiment on single sex shoals of male and female guppies, observing host behaviour before and after the introduction of an infected shoal mate. The initial parasite burden was considerably lower in the present experiment (30 worms versus >100 worms previously) and we used a different stock of ornamental guppies (Green Cobra variety versus a Tuxedo hybrid previously). Contrary to our previous finding, males aggregated significantly more than females. Males performed 'sigmoid' displays towards each other, a courtship behaviour that is more generally directed towards females. Due to the high rate of male-male interactions, parasite transmission was 10 times higher between males than between females. Furthermore, shoaling intensity was highest for the most parasitised fish indicating that these infected fish were not avoided by non-parasitised conspecifics. These studies show that certain social behaviours including shoaling and courtship displays, appear to facilitate the transmission of gyrodactylid parasites., (Copyright © 2012 Elsevier Inc. All rights reserved.)

Despite routine screening requirements for the notifiable fish pathogen Gyrodactylus salaris, no standard operating procedure exists for its rapid identification and discrimination from other species of Gyrodactylus. This study assessed screening and identification efficiencies under real-world conditions for the most commonly employed identification methodologies: visual, morphometric and molecular analyses. Obtained data were used to design a best-practice processing and decision-making protocol allowing rapid specimen throughput and maximal classification accuracy. True specimen identities were established using a consensus from all three identification methods, coupled with the use of host and location information. The most experienced salmonid gyrodactylid expert correctly identified 95.1% of G. salaris specimens. Statistical methods of classification identified 66.7% of the G. salaris, demonstrating the need for much wider training. Molecular techniques (internal transcribed spacer region-restriction fragment length polymorphism (ITS-RFLP)/cytochrome c oxidase I (COI) sequencing) conducted in the diagnostic laboratory most experienced in the analysis of gyrodactylid material, identified 100% of the true G. salaris specimens. Taking into account causes of potential specimen loss, the probabilities of a specimen being accurately identified were 95%, 87% and 92% for visual, morphometric and molecular techniques, respectively, and the probabilities of correctly identifying a specimen of G. salaris by each method were 81%, 58% and 92%. Inter-analyst agreement for 189 gyrodactylids assessed by all three methods using Fleiss' Kappa suggested substantial agreement in identification between the methods. During routine surveillance periods when low numbers of specimens are analysed, we recommend that specimens be analysed using the ITS-RFLP approach followed by sequencing of specimens with a "G. salaris-like" (i.e. G. salaris, Gyrodactylus thymalli) banding pattern. During periods of suspected outbreaks, where a high volume of specimens is expected, we recommended that specimens be identified using visual identification, as the fastest processing method, to select "G. salaris-like" specimens, which are subsequently identified by molecular-based techniques., (Copyright © 2010. Published by Elsevier Ltd.)

Since Norway experienced the devastating Gyrodactylus salaris (Monogenea) epidemics in Atlantic salmon Salmo salar, there has been heightened interest in how to treat gyrodactylosis in fish. Here we summarize chemical treatments previously used against gyrodactylids and discuss the main problems associated with these control measures including efficacy, host toxicity, human health concerns and application of treatments. Unfortunately, for these reasons and because of the different methodologies and different parasite and host species used in previous studies, it is difficult to recommend effective chemotherapeutic treatments. However, we suggest a method for manual removal of gyrodactylids from the host suitable for use in small-scale research facilities.

Gall bladders from 273 otter carcasses, collected throughout England and Wales, were screened to assess the status of gall bladder parasites in the Eurasian otter, Lutra lutra. The digenean Pseudamphistomum truncatum had previously been found in UK otters collected between 2000 and 2007. The parasite was established in Somerset and Dorset but its distribution elsewhere in the UK was largely unknown. In the current study, P. truncatum was also found to be abundant in south Wales, with occasional cases elsewhere, but appears to be absent from the north of England. Overall, 11.7% of otters were infected with 1-238 P. truncatum. A second digenean, Metorchis albidus, previously unreported in British otters, was found in the biliary system of 6.6% of otters. M. albidus appears well established in Suffolk, Norfolk and north Essex but was recorded elsewhere rarely. Both parasites are associated with pathological damage to the otter gall bladder. The recent discovery of these two non-native parasites provides a unique opportunity to assess their impact on native British fauna.

Gyrodactylus orecchiae sp. n. (Monogenea, Gyrodactylidae) is described from the skin, fins, eyes and gills of juvenile Sparus aurata L. (gilthead seabream) following two outbreaks of gyrodactylosis amongst stocks held in inshore floating cages on the Adriatic coast of Albania and Croatia. Fish were heavily infected (1000+ gyrodactylids/fish) with G. orecchiae which reportedly resulted in approximately 2-10% mortality amongst the infected stock. Morphologically, the haptoral hooks of G. orecchiae most closely resemble those of Gyrodactylus arcuatus Bychowsky, 1933 in the approximate shape of the ventral bar with its pronounced ventral bar processes and marginal hook sickles which possess a square line to the inner edge of the sickle blade and large rounded heels. The marginal hooks are also morphologically similar to those of Gyrodactylus quadratidigitus Longshaw, Pursglove et Shinn, 2003 and Gyrodactylus colemanensis Mizelle et Kritsky, 1967, but G. orecchiae can be readily discriminated from all three species by the characteristic infolding of the hamuli roots and the shape of the marginal hook sickle. Molecular sequencing of the ITS1, 5.8S, ITS2 regions (513+157+404 bp, respectively) of G. orecchiae and alignment with other gyrodactylids for which these same genomic regions have been determined, suggests that this is a new species. No similarities were found when the ITS1 region of G. orecchiae was compared with 84 species of Gyrodactylus available on GenBank.

Previous studies on long term gyrodactylid infections on isolated fish have shown that for the majority of Gyrodactylus-fish interactions, infections peak at a maximum level and then decline as the host mounts an effective immune response. In the current study, juvenile chub collected from the wild still harboured Gyrodactylus lomi infections after 6 - 10 months in the laboratory despite being individually maintained at 12 C.

Gyrodactylids are ubiquitous fish parasites and yet, with the notable exception of Gyrodactylus salaris, few studies have reported the effect of these parasites on host survival in natural populations. Here, we assess the impact of the parasite load of gyrodactylids (G. turnbulli and G. bullatarudis) on the survival and migration of guppies (Poecilia reticulata) in their natural habitat of the Aripo River in Trinidad. The recapture rate of males declined by 19% with every additional parasite, a remarkably high figure given that the parasite load in this study ranged from zero to 20 worms. In addition, with an increased number of parasites, males were more prone to be recovered downstream. In contrast, no effect of parasitism was observed in females. The mean parasite load sharply declined after a series of flushing events during heavy seasonal downpours. The parasite load varied significantly between fish depending on their location in the river, and the size of the fish explained variation in parasite load between individuals. The present study indicates that tropical gyrodactylid parasites can play an important role in the ecology of natural fish populations, causing intense bouts of natural selection in guppies during heavy rains in the wet season.

This article reviews the history of gyrodactylid research focussing on the unique anatomy, behaviour, ecology and evolution of the viviparous forms while identifying gaps in our knowledge and directions for future research. We provide the first summary of research on the oviparous gyrodactylids from South American catfish, and highlight the plesiomorphic characters shared by gyrodactylids and other primitive monogeneans. Of these, the most important are the crawling, unciliated larva and the spike sensilla of the cephalic lobes. These characters allow gyrodactylids to transfer between hosts at any stage of the life cycle, without a specific transmission stage. We emphasise the importance of progenesis in shaping the evolution of the viviparous genera and discuss the relative extent of progenesis in the different genera. The validity of the familial classification is discussed and we conclude that the most significant division within the family is between the oviparous and the viviparous genera. The older divisions into Isancistrinae and Polyclithrinae should be allowed to lapse. We discuss approaches to the taxonomy of gyrodactylids, and we emphasise the importance of adequate morphological and molecular data in new descriptions. Host specificity patterns in gyrodactylids are discussed extensively and we note the importance of host shifts, revealed by molecular data, in the evolution of gyrodactylids. To date, the most closely related gyrodactylids have not been found on closely related hosts, demonstrating the importance of host shifts in their evolution. The most closely related species pair is that of G. salaris and G. thymalli, and we provide an account of the patterns of evolution taking place in different mitochondrial clades of this species complex. The host specificity of these clades is reviewed, demonstrating that, although each clade has its preferred host, there is a range of specificity to different salmonids, providing opportunities for complex patterns of survival and interbreeding in Scandinavia. At the same time, we identify trends in systematics and phylogeny relevant to the G. salaris epidemics on Atlantic salmon in Norway, which can be applied more generally to parasite epidemiology and evolution. Although much of gyrodactylid research in the last 30 years has been directed towards salmonid parasites, there is great potential in using other experimental systems, such as the gyrodactylids of poeciliids and sticklebacks. We also highlight the role of glacial lakes and modified river systems during the ice ages in gyrodactylid speciation, and suggest that salmon infecting clades of G. salaris first arose from G. thymalli in such lakes, but failed to spread fully across Scandinavia before further dispersal was ended by rising sea levels. This dispersal has been continued by human activity, leading to the appearance of G. salaris as a pathogen in Norway. We review the history and current status of the epidemic, and current strategies for elimination of the parasite from Norway. Finally, we consider opportunities for further spread of the parasite within and beyond Europe.

Macroparasites of vertebrates usually occur in multi-species communities, producing infections whose outcome in individual hosts or host populations may depend on the dynamics of interactions amongst the different component species. Within a single co-infection, competition can occur between conspecific and heterospecific parasite individuals, either directly or via the host's physiological and immune responses. We studied a natural single-host, multi-parasite model infection system (polystomes in the anuran Xenopus laevis victorianus) in which the parasite species show total interspecific competitive exclusion as adults in host individuals. Multi-species infection experiments indicated that competitive outcomes were dependent on infection species composition and strongly influenced by the intraspecific genetic identity of the interacting organisms. Our results also demonstrate the special importance of temporal heterogeneity (the sequence of infection by different species) in competition and co-existence between parasite species and predict that developmental plasticity in inferior competitors, and the induction of species-specific host resistance, will partition the within-host-individual habitat over time. We emphasise that such local (within-host) context-dependent processes are likely to be a fundamental determinant of population dynamics in multi-species parasite assemblages.

The viviparous monogenean Gyrodactylus salaris continues to devastate Norwegian Atlantic salmon populations despite the extreme measures taken to control this pathogen. Increased understanding of parasite biology is needed to develop alternative control and management strategies of wild Atlantic salmon. We have examined temperature-dependent survival of G. salaris, both on and off the host. At 18 degrees C, survival off the host was 1 day, but at 3 degrees C parasites survived for 4 days. However, in contrast to assumptions made by earlier authors, many parasites remained with their host following its death. Ultrastructural evidence indicated that G. salaris individuals can feed on a dead host, and laboratory tests demonstrated that worms on their hosts more than double their life-span compared with individuals maintained off the host. Experimental infections also demonstrated that establishment and subsequent population growth of parasites previously maintained on dead hosts for 3 days, was similar to that of parasites transferred directly between living hosts. Hence, for G. salaris, dead infected hosts may increase the chances of successful transmission and be a potential important infection source in rivers and hatcheries.

Reproductive success of Gvrodactylus salaris from River Lierelva was compared experimentally on 3 stocks of salmon (12.5 degrees C +/- 0.2). Isolated fish from 2 susceptible Norwegian stocks (Rivers Lier and Alta) and 1 resistant Baltic stock of salmon (River Neva) were infected with a single gravid worm in order to record the temporal sequence of births and age at death of individual parasites. Establishment success (proportion of worms surviving to give birth) was generally low and mortality high, but significantly fewer worms survived on Neva (45% with mean survival of 3.5 days) compared to Alta and Lier fish (60 % mean survival 7.9 and 5.2 days, respectively). There was a dramatic difference in parasite fecundity between the host stocks: only 2 births occurred on Neva fish compared to third and fourth births on both Alta and Lier hosts. The timing of the first birth was more variable on Neva hosts and was significantly extended (mean 2.3 days) relative to that on Alta and Neva fish (1.8 days). However, timing of the second birth did not vary on any of the 3 salmon stocks. Age-specific mortality and fecundity data are consistent with exponential population growth of G. salaris on Alta and Lier fish but eventual extinction on Neva hosts. This is the first demonstration that gyrodactylids maintained on different host stocks exhibit variations in fecundity, development and mortality, which may in turn account for the variable virulence noted when G. salaris infects different salmonid hosts.

Two new species of Gyrodactylus are described from the South American live-bearing fish Poecilia caucana from Venezuela. Gyrodactylus poeciliae n. sp. is characterised by conspicuous ventral bar processes and by marginal hooks in which the tips are barely curved. Morphologically, the species resembles G. bullatarudis and G. costaricensis, and its relationship to the former was confirmed by sequencing the Internal Transcribed Spacers (ITS-1 and ITS-2) and the 5.8S ribosomal gene of the ribosomal DNA. Gyrodactylus milleri n. sp. was characterised by large, sharply angled marginal hook sickles in which the tips overhang the toes. This species resembled most closely G. turnbulli from guppies. These descriptions confirm the occurrence of G. turnbulli and G. bullatarudis-like gyrodactylids on a range of South American poeciliid fishes.