Ecological determinants of parasite acquisition by exotic fish species

Evolution of parasites in the Anthropocene: new pressures, new adaptive directions

The Anthropocene is seeing the human footprint rapidly spreading to all of Earth's ecosystems. The fast-changing biotic and abiotic conditions experienced by all organisms are exerting new and strong selective pressures, and there is a growing list of examples of human-induced evolution in response to anthropogenic impacts. No organism is exempt from these novel selective pressures. Here, we synthesise current knowledge on human-induced evolution in eukaryotic parasites of animals, and present a multidisciplinary framework for its study and monitoring. Parasites generally have short generation times and huge fecundity, features that predispose them for rapid evolution. We begin by reviewing evidence that parasites often have substantial standing genetic variation, and examples of their rapid evolution both under conditions of livestock production and in serial passage experiments. We then present a two-step conceptual overview of the causal chain linking anthropogenic impacts to parasite evolution. First, we review the major anthropogenic factors impacting parasites, and identify the selective pressures they exert on parasites through increased mortality of either infective stages or adult parasites, or through changes in host density, quality or immunity. Second, we discuss what new phenotypic traits are likely to be favoured by the new selective pressures resulting from altered parasite mortality or host changes; we focus mostly on parasite virulence and basic life-history traits, as these most directly influence the transmission success of parasites and the pathology they induce. To illustrate the kinds of evolutionary changes in parasites anticipated in the Anthropocene, we present a few scenarios, either already documented or hypothetical but plausible, involving parasite taxa in livestock, aquaculture and natural systems. Finally, we offer several approaches for investigations and real-time monitoring of rapid, human-induced evolution in parasites, ranging from controlled experiments to the use of state-of-the-art genomic tools. The implications of fast-evolving parasites in the Anthropocene for disease emergence and the dynamics of infections in domestic animals and wildlife are concerning. Broader recognition that it is not only the conditions for parasite transmission that are changing, but the parasites themselves, is needed to meet better the challenges ahead.

Each coin has 2 sides: a positive role of alien Potamopyrgus antipodarum (Grey, 1843) snails in reducing the infection of native lymnaeids with trematodes

The change in the distribution of organisms in freshwater ecosystems due to natural or manmade processes raises the question of the impact of alien species on local communities. Although most studies indicate a negative effect, the positive one is more difficult to discern, especially in multispecies systems, including hosts and parasites. The purpose of the study was to check whether the presence of an alien host, Potamopyrgus antipodarum, reduces the intensity of Echinoparyphium aconiatum metacercariae in a native host, Radix spp. We additionally tested the impact of water temperature and the biomass of the alien host on the dilution effect. We experimentally studied (1) the lifespan of echinostome cercariae in different temperatures, (2) the infectivity of cercariae toward the alien host and native host, and (3) the impact of different biomass of the alien host on the intensity of metacercariae in the native host. We found that cercarial survival and infectivity were temperature dependent. However, cercarial survival decreased with increasing temperature, contrary to cercarial infectivity. Echinostome cercariae entered the renal cavity of both the native host and alien host, and successfully transformed into metacercariae. The number of metacercariae in the native host decreased with the increasing biomass of the alien host. Our results indicate that lymnaeids may benefit from the co-occurrence with P. antipodarum, as the presence of additional hosts of different origins may reduce the prevalence of parasites in native communities. However, the scale of the dilution effect depends not only on the increased spectrum of susceptible hosts but also on the other variables of the environment, including water temperature and host density.

The functional traits of host fish can act as good predictors for parasite composition in a neotropical floodplain

Parasite diversity can be influenced by the interaction of environmental factors and host traits, but understanding which traits can be decisive for the establishment of the parasite may provide subsidies for a better understanding of the host-parasite relationship. In this study, we investigated whether functional traits, diet, and host phylogeny can predict the similarity of the endoparasite composition of a fish assemblage in a Brazilian floodplain. Of the three evaluated components, the host's diet was the factor that showed the greatest influence on the composition and similarity of endoparasites, demonstrating the highest value of the explanation. The functional traits and phylogeny, despite presenting significant values (unique effect and global effect), showed low explainability in the composition of the endoparasites. When analyzing the joint effects, all components showed significant influence. Hosts that live in the same environment that are phylogenetically related and have a similar ecology have a certain degree of homogeneity in their parasite assemblages and, because they are endoparasites (which are acquired trophically along the chain), diet is the main driver of parasite richness and similarity. Overall, host traits can be one of the main determinants of parasite composition, so studies that address the functional traits of the host provide a representation of local diversity and define the possible patterns of these parasite communities.

Components influencing parasitism by Dadaytrema oxycephala (Digenea: Cladorchiidae) in Neotropical fish

The components that mold the structure of parasitic fauna are used as objects of study in an attempt to find patterns in their distribution. It is known that phylogeny (represented by specificity), host ecological traits (for example, feeding habits, position of the water column, reproductive strategies, body size, and age), and the environment affect the distribution and occurrence of parasites. In tropical regions, digeneans show high diversity, and the species Dadaytrema oxycephala is known to parasitize a wide range of host species. In this context, the objective of the present study is to analyze the components that affect the occurrence of D. oxycephala in Neotropical fish. We used data from the literature that contained the abundance of this parasite, as well as the geographic location and host species, and evaluated the influence of ecological traits, specificity, and latitude on parasite abundance, using a generalized linear mixed model (GLMM). The abundance of D. oxycephala can be explained by trophic level and position in the water column and latitude. However, coevolutionary processes are also extremely important, and the distribution of this parasite was not equal, showing high abundance for the genus Piaractus, which are the preferred hosts, even if the parasite is considered generalist. In short, host ecological traits are the important components in the distribution and occurrence of D. oxycephala, as well as the latitude.

Infection of Diplostomum spp. in invasive round gobies in the St Lawrence River, Canada

The round goby (Neogobius melanostomus) is a successful invader of the Great Lakes-St Lawrence River basin that harbours a number of local parasites. The most common are metacercariae of the genus Diplostomum. Species of Diplostomum are morphologically difficult to distinguish but can be separated using molecular techniques. While a few species have been sequenced from invasive round gobies in this study system, their relative abundance has not been documented. The purpose of this study was to determine the species composition of Diplostomum spp. and their relative abundance in round gobies in the St Lawrence River by sequencing the barcode region of cytochrome c oxidase I. In 2007-2011, Diplostomum huronense (=Diplostomum sp. 1) was the most common, followed in order by Diplostomum indistinctum (=Diplostomum sp. 4) and Diplostomum indistinctum sensu Galazzo, Dayanandan, Marcogliese & McLaughlin (2002). In 2012, the most common species infecting the round goby in the St Lawrence River was D. huronense, followed by D. indistinctum and Diplostomum gavium (=Diplostomum sp. 3). The invasion of the round goby in the St Lawrence River was followed by a decline of Diplostomum spp. in native fishes to low levels, leading to the previously published hypothesis that the presence of the round goby has led to a dilution effect. Herein, it is suggested that despite the low infection levels in the round goby, infections still may lead to spillback, helping to maintain Diplostomum spp. in native fishes, albeit at low levels.

Marine Parasites and Disease in the Era of Global Climate Change

Climate change affects ecological processes and interactions, including parasitism. Because parasites are natural components of ecological systems, as well as agents of outbreak and disease-induced mortality, it is important to summarize current knowledge of the sensitivity of parasites to climate and identify how to better predict their responses to it. This need is particularly great in marine systems, where the responses of parasites to climate variables are less well studied than those in other biomes. As examples of climate's influence on parasitism increase, they enable generalizations of expected responses as well as insight into useful study approaches, such as thermal performance curves that compare the vital rates of hosts and parasites when exposed to several temperatures across a gradient. For parasites not killed by rising temperatures, some simple physiological rules, including the tendency of temperature to increase the metabolism of ectotherms and increase oxygen stress on hosts, suggest that parasites' intensity and pathologies might increase. In addition to temperature, climate-induced changes in dissolved oxygen, ocean acidity, salinity, and host and parasite distributions also affect parasitism and disease, but these factors are much less studied. Finally, because parasites are constituents of ecological communities, we must consider indirect and secondary effects stemming from climate-induced changes in host-parasite interactions, which may not be evident if these interactions are studied in isolation.

A neglected fish stressor: mechanical disturbance during transportation impacts susceptibility to disease in a globally important ornamental fish

The transport of fish in aquaculture and the ornamental trade exposes fish to multiple stressors that can cause mass mortalities and economic loss. Previous research on fish transport has largely focussed on chemical stress related to deterioration in water quality. However, mechanical disturbance during routine fish transport is unpredictable and is a neglected potential stressor when studying fish welfare. Stress-induced immunosuppression caused by mechanical disturbance can increase the chances of contracting infections and can significantly increase infection burden. Here, using a model host-parasite system (guppy Poecilia reticulata and the monogenean ectoparasite Gyrodactylus turnbulli) and a new method of bagging fish (Breathing Bags™), which reduces mechanical disturbance during fish transport, we investigated how parasite infections contracted after simulated transport impact infection trajectories on a globally important ornamental freshwater species. Guppies exposed to mechanical transport disturbance suffered significantly higher parasite burden compared to fish that did not experience transport disturbance. Unfortunately, there was no significant reduction in parasite burden of fish transported in the Breathing Bags™ compared to standard polythene carrier bags. Thus, transport-induced mechanical disturbance, hitherto neglected as a stressor, can be detrimental to disease resistance and highlights the need for specific management procedures to reduce the impact of infectious diseases following routine fish transport.

Limited parasite acquisition by non-native Lepomis gibbosus (Actinopterygii: Centrarchidae) at two ponds in the Upper Rhine basin, Germany

Metazoan parasite communities of Lepomis gibbosus (Centrarchidae), one of the most successfully introduced fish species in Europe, were studied at two isolated ponds (Knielingen, Tropfen) along the Upper Rhine in Germany. Nine parasite taxa were observed, including North American species co-introduced to Europe (ancyrocephalid monogeneans, diplostomid trematodes), circumpolar species infecting L. gibbosus in both their native and non-native ranges (bothriocephalid cestodes) and locally acquired parasitic nematodes. Both parasite communities consisted predominantly of North American species. Acquisition of local parasites was not observed at Tropfen, where the fish community comprised just two species, with L. gibbosus dominant. Low prevalence and abundance of acquired parasites was found at Knielingen, which supported a diverse fish community. At Tropfen, a high abundance of the North American parasite Posthodiplostomum centrarchi probably contributed to the lower condition index, hepatomegaly and splenomegaly observed. Due to low local parasite competency, L. gibbosus appears to have no significant impact on parasite dynamics in affected habitats.

A minimalist macroparasite diversity in the round goby of the Upper Rhine reduced to an exotic acanthocephalan lineage

The round goby, Neogobius melanostomus, is a Ponto-Caspian fish considered as an invasive species in a wide range of aquatic ecosystems. To understand the role that parasites may play in its successful invasion across Western Europe, we investigated the parasitic diversity of the round goby along its invasion corridor, from the Danube to the Upper Rhine rivers, using data from literature and a molecular barcoding approach, respectively. Among 1666 parasites extracted from 179 gobies of the Upper Rhine, all of the 248 parasites barcoded on the c oxidase subunit I gene were identified as Pomphorhynchus laevis. This lack of macroparasite diversity was interpreted as a loss of parasites along its invasion corridor without spillback compensation. The genetic diversity of P. laevis was represented by 33 haplotypes corresponding to a haplotype diversity of 0·65 ± 0·032, but a weak nucleotide diversity of 0·0018 ± 0·00015. Eight of these haplotypes were found in 88·4% of the 248 parasites. These haplotypes belong to a single lineage so far restricted to the Danube, Vistula and Volga rivers (Eastern Europe). This result underlines the exotic status of this Ponto-Caspian lineage in the Upper Rhine, putatively disseminated by the round goby along its invasion corridor.

Enigmatic decline of a common fish parasite (Diplostomum spp.) in the St. Lawrence River: Evidence for a dilution effect induced by the invasive round goby

As they integrate into recipient food webs, invasive exotic species may influence the population dynamics of native parasites. Here we assess the potential impact of the Eurasian round goby (Neogobius melanostomus) on the abundance of eyeflukes of the genus Diplostomum, which are common parasites in fishes of the St. Lawrence River (Canada). Analyses of data collected over nearly two decades revealed that the infection levels in three native fish [spottail shiner (Notropis hudsonius), golden shiner (Notemigonus crysoleucas) yellow perch (Perca flavescens)] declined sharply throughout the St. Lawrence River after the introduction of the goby. At two sites where data were collected at regular time intervals, declines of Diplostomum spp. in spottail shiners occurred within two years of the goby's first recorded appearance, with prevalence dropping as much as 77-80% between pre-invasion and post-invasion periods. Furthermore, in localities where gobies remained scarce, infection in native species did not change significantly over time. Altogether, these observations suggest that gobies play a role in the eyefluke collapse. The decline in populations of the main definitive host (ring-billed gulls, Larus delawarensis) and changes in hydrology during periods of parasite recruitment were not strongly supported as alternate explanations for this phenomenon. Since other snail-transmitted trematodes with similar life cycles to Diplostomum spp. did not show the same decreasing pattern, we conclude that eyeflukes did not decline as a result of snail depletion due to goby predation. Rather, we suggest that gobies acted as decoys, diluting the infection. As Diplostomum spp. occurred at lower abundance in gobies than in native fish hosts, the replacement of native fish with exotic gobies in the diet of gulls might have played a part in reducing parasite transmission. In contrast to the typically negative impact of invasions, the goby-induced decline of this pathogen may have beneficial effects for native fishes.

A case of complete loss of gill parasites in the invasive cichlid Oreochromis mossambicus

This study investigates the recent evolution of a rich parasite community associated with one of the world's most invasive species, the cichlid fish Oreochromis mossambicus. Populations from the species' native range (Mozambique) are compared to a population from New Caledonia (Wester Pacific), an island where the species was introduced in 1954. The results support the complete local extinction of the gill parasite community in the course of the invasion process. Up to six gill parasite species per locality were documented in the O. mossambicus native range, and previous surveys consistently reported at least one parasite species introduced along African cichlid species established out of Africa. The absence of parasites in New Caledonia is therefore exceptional. This can be attributed to local factors, such as a strong initial population bottleneck, the likely absence of multiple host introductions, and the frequent occurrence of brackish watersheds that might enhance the probability for natural deparasitation.

Parasites of native Cichlidae populations and invasive Oreochromis niloticus (Linnaeus, 1758) in tributary of Amazonas River (Brazil)

This study provides the first investigation on acquisition of parasites in invasive O. niloticus by parasite species of native Cichlidae from the Igarapé Fortaleza basin, Northern Brazil. There were examined 576 specimens of 16 species of native cichlids and invasive O. niloticus collected in the main channel and the floodplain area of this tributary of Amazon River. The invasive O. niloticus was poorly parasitized having only Ichthyophthirius multifiliis, Trichodina centrostrigeata, Paratrichodina africana, Trichodina nobilis (Protozoa) and Cichlidogyrus tilapiae (Monogenoidea), and this host has not acquired any parasite species common to the native ichthyofauna region. In contrast, species of native cichlids showed rich fauna of parasites with predominance of Monogenoidea species, larvae and adults of Nematoda, Digenea, Cestoidea and Acanthocephala, besides four species of Protozoa and four Crustacea. However, only T. nobilis was acquired by native fish, the Aequidens tetramerus, which is a new host for this exotic Trichodinidae. In O. niloticus, well established in the region, the small number of helminth species may be associated with its rusticity, good adaptation in the new environment and also the presence of native parasites with relative specificity, but without ability to complete its life cycle in this invasive host of this ecosystem.

Trophic structure, stability, and parasite persistence threshold in food webs

Food web structure of free-living species is an important determinant of parasite species richness. Downwardly asymmetric predator-prey interactions (where there are more prey than predator species) have been shown, both theoretically and empirically, to harbour more trophically transmitted parasite species than expected due to chance. Here, we demonstrate that this could be due to the increase in the basic reproductive ratio that the addition of non-host prey species to a system creates. However, we note that the basic reproductive ratio is only increased by those prey that stabilise oscillations in a predator-prey system, and is decreased by those that do not.