Ecology of a diplozoon parasite on the gills of the African cyprinid Barbus neumayeri

In oxygen‐deficient waters, the difficulties of oxygen uptake in gill parasites and their fish hosts may influence host and parasite densities, site selection by the parasite, and effects of the parasite on host condition. This study quantified the prevalence and intensity of the gill monogenean Neodiplozoon polycotyleus in the African cyprinid fish Barbus neumayeri from an intermittent forest stream in western Uganda. Oxygen levels were low in the stream over the 12‐month study, averaging only 2.5 mg litre^?1 (monthly range = 1.2–4.3 mg litre^?1). However, parasite prevalence was high (47.2%), suggesting high tolerance to low oxygen in N. polycotyleus. The prevalence of parasites varied with host body size, with the highest frequency of occurrence in the middle size classes. Prevalence also varied over the year; seasonal peaks of rainfall coincided with a lower frequency of N. polycotyleus. The significantly nonrandom frequency distribution of parasites among hosts suggests regulation of parasite numbers. Of the hosts infected, 37.1% harboured one N. polycotyleus parasite, and 62.9% harboured two parasites. No fish were infected with more than two diplozoons. There was evidence for strong site specificity by N. polycotyleus within hosts; 77.7% of the parasites were located on the filaments of the second gill arch, which may relate to increased oxygen availability. In addition, only one of the 178 infected fish had more than one parasite on one side of the branchial basket. Although N. polycotyleus is undoubtedly parasitic, we found no evidence of a negative parasitic effect on the condition or reproductive status of B. neumayeri.     

Introduced bullheads Cottus gobio and infection with plerocercoids of Schistocephalus cotti in the Utsjoki, an Arctic river in Finland

Bullheads Cottus gobio, first found in the River Utsjoki, an Arctic river in Finland, in 1979, were sampled at six sites in August 1991 (two of these sites were also sampled previously in June and July 1991). Mean total length (L_T) of C. gobio was 45 mm (n= 1080 fish). Overall, there were slightly more females than males, with males larger than females (mean L_T males 52·1 and females 47·6 mm). Prevalence of infection with plerocercoids of Schistocephalus cotti varied from 20·0 to 43·0% over the six sites in August. Juvenile C. gobio had a prevalence of infection of 3·5% whereas in mature fish prevalence was consistently higher in females than in males (38·8 v. 20·1%). There was one plerocercoid per infected juvenile C. gobio. In mature fish the mean number varied between 1·3 and 1·8: 49% had one worm per fish, 38% two or three worms and 13% had four to eight plerocercoids. In juvenile C. gobio, S. cotti were randomly distributed (overdispersion index 1·10), whereas occurrence was slightly aggregated in the L_T groups 40–59 and ≥60 mm (overdispersion indices both 1·90). Mean mass of infected fish was higher in all L_T groups compared to uninfected C. gobio. After removal of plerocercoids, mean mass of fish tissue remaining was lower compared with uninfected fish for the two L_T groups 40–59 and ≥ 60 mm, but higher in C. gobio <40 mm L_T. Prevalence and mean intensity of infection over the 3 months did not differ either for the two sites individually or the pooled data. Mean mass of both uninfected and infected fish increased over this period, and were both highest in August as was the mean worm burden. There was, however, no difference in Fulton’s condition factor at any time. Mean mass of plerocercoids in C. gobio of ≥40 mm L_T increased significantly comparing June and July with August. Comparison of backcalculated L_T of infected and non‐infected bullheads of different age‐groups did not show any significant differences in growth at any age. In each parasite infrapopulation as the number of worms increased so their mean mass decreased significantly but at the same time the total mass of worms in the fish host remained more or less constant in relation to length. Aggregation of parasites in the fish population was low, as the majority were infected by one S. cotti plerocercoid. Gonad development was suppressed in infected C. gobio.     

Local diversity reduces infection risk across multiple freshwater host‐parasite associations

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Habitat and host associations of the fish‐burrowing parasite Artystone minima (Cymothoidae: Isopoda) in eastern Amazonia

Cymothoid fish parasites settle on hosts in ways that may impact fish health and energetics. High abundances of Artystone minima observed in Nannostomus beckfordi from the Jeju River in eastern Amazonia were investigated to answer the following questions: (a) What factors are associated with the high prevalence at this locality?; (b) Is high abundance associated with co‐infestation of alternative hosts?; and (c) Is parasite presence associated with host species growth and/or reproduction? Fish assemblages were sampled quarterly (August 2017–May 2018) from five habitats along with environmental data. Parasitic indices were calculated, and parasite presence used to evaluate differences in growth of hosts using analysis of covariance considering host sex and sampling season (wet vs. dry). Parasites were only abundant in one of the habitats, a large, shallow backwater bay with macrophytes. Abiotic environmental factors (flow and depth) likely impact parasite transmission and are, therefore, particularly important in producing these local patterns. Two secondary hosts, Hyphessobrycon cf. rosaceus and Moenkhausia collettii, were found in the wet season. Based on host biology compared to other fish in the habitat, parasite infestation is inferred to be depth associated and long‐term infestation is apparently limited in alternative hosts. Parasite presence was significantly associated with reduced weight (standardized for length) of female Nannostomus beckfordi in the wet season. Furthermore, ovaries of non‐parasitized females from the wet season presented a range of maturation stages, while parasitized females were all immature, indicating a significant association of parasites with host reproductive capacity. Abstract in Portuguese is available with online material     

The role of the European bitterling (<Emphasis Type="Italic">Rhodeus amarus,</Emphasis> Cyprinidae) in parasite accumulation and transmission in riverine ecosystems

In aquatic ecosystems, fish play a key role in parasite accumulation and transmission to predacious animals. In the present study, realized on seven populations of a small cyprinid fish species, the European bitterling Rhodeus amarus, we investigated (1) the role of the European bitterling as a potential intermediate or paratenic host, (2) the ability of the fish to accumulate parasites with similar final host group, and (3) its significance as a potential source of parasite infection in the ecosystem in respect to habitat characteristics. A total of 36 parasite species were recorded; 31 species (90% of all parasite specimens) were classified as endoparasites. Most of the endoparasites were found in the larval life stage, using bitterling as an intermediate or paratenic host. In particular, parasite community structure showed significantly higher proportions of allogenic parasites in comparison with autogenic. The supposed co-occurrence of parasite species with identical final host groups showed only a weak association. The adjacent reservoir areas were a significant determinant of both the total and infracommunity parasite species richness and for the mean parasite abundance. No relationship between the distance of sampling site from the adjacent reservoir and parasite community characteristics was found. As a small-sized fish with a wide distribution range and high local abundances, the European bitterling can represent a natural prey for a wide range of piscivorous predators. Due to its susceptibility to the number of larval endoparasites, this fish species may therefore fulfill the role as important transmitter of parasites to their final hosts.     

Deep-water life cycle of Anisakis paggiae (Nematoda: Anisakidae) in the Irminger Sea indicates kogiid whale distribution in north Atlantic waters

The study of the beryciform Anoplogaster cornuta from the Irminger Sea (north Atlantic) revealed the presence of the anisakid nematode Anisakis paggiae inside the body cavity, representing a new host and locality record. This deep-sea fish was infected with Anisakis larvae at a prevalence of 57.1% and a mean intensity of 2.2, with no correlation between the fish standard length and the number of accumulated A. paggiae. Kogiid whales (Kogia breviceps, K. sima), the typical final hosts of this parasitic nematode, have not yet been recorded so far in the north. Because A. cornuta does not migrate outside the Irminger Sea, and by using the parasite as an indicator for the presence of the final hosts, A. paggiae must have been introduced through migratory kogiid final hosts. This would extend their range of distribution into the Irminger Sea. The depth range of the meso- and bathypelagic A. cornuta and the frequent occurrence of Anisakis inside this deep-sea fish demonstrate an oceanic deep-water life cycle for A. paggiae in the north Atlantic.     

Mancae of the parasitic cymothoid isopod, <Emphasis Type="Italic">Anilocra apogonae</Emphasis>: early life history,host-specificity,and effect on growth and survival of preferred young cardinal fishes

Juvenile parasitic cymothoid isopods (mancae) can injure or kill fishes, yet few studies have investigated their biology. While the definitive host of the adult cymothoids is usually a single host from a particular fish species, mancae may use so-called optional intermediate hosts before settling on the definitive host. Little, however, is known about these early interactions. The cymothoid isopod, Anilocra apogonae, infests the definitive host, Cheilodipterus quinquelineatus. This study examined their host preference among potential optional intermediate hosts. Their effect on the growth and mortality of the young of three apogonid fishes, including the definitive host, was investigated. The number of mancae produced per brood was positively correlated with female length. When given a choice of intermediate hosts, significantly more mancae attached to Apogon trimaculatus (Apogonidae) than to Apogon nigrofasciatus. When presented with Ap. trimaculatus and Pomacentrus amboinensis (Pomacentridae), mancae only attached to Ap. trimaculatus suggesting that mancae may show a taxonomic affiliation with preferred hosts. Mancae fed on all three apogonid species, with C. quinquelineatus being fed on earlier than Ap. trimaculatus and Ap. nigrofasciatus. Mancae feeding frequency, adjusted for fish survival, was lowest on C. quinquelineatus and highest on Ap. trimaculatus. Infested apogonids had reduced growth and increased mortality compared with uninfested fish. A. apogonae mancae can use several species of young apogonid fishes as optional intermediate hosts. Via reduced growth and increased mortality, mancae have the potential to negatively influence definitive host populations and also other young species of apogonid fishes.     

Brain‐encysting trematodes and altered monoamine activity in naturally infected killifish Fundulus parvipinnis

This paper presents novel evidence to address mechanisms by which trematode parasites effect behavioural changes in naturally infected fish hosts. California killifish Fundulus parvipinnis infected with the brain‐encysting trematode Euhaplorchis californiensis display conspicuous swimming behaviours that render them 30 times more likely to be eaten by birds, the parasite's final host. Prevalence of E. californiensis reaches nearly 100% in most F. parvipinnis populations, with parasite biomass constituting almost 2% of F. parvipinnis biomass in some locations. Despite having thousands of cysts on their brains, infected fish grow and mature at rates comparable to those of uninfected populations. The lack of general pathology combined with the specificity of the altered behaviours suggests that the behavioural changes are due to parasite manipulation. The monoamine neurotransmitters serotonin and dopamine, which control locomotion and social behaviour in fishes and other vertebrates, were examined to explore the underlying mechanisms of this behaviour modification. Whereas previous studies were similarly conducted with experimentally infected fish, in this study, brain dopaminergic and serotonergic activity were analysed in naturally infected fish to assess how E. californiensis may alter F. parvipinnis monoamines in a naturally occurring system. A parasite density‐associated decrease in serotonergic activity occurred in the hippocampus of naturally infected fish, as well as a decrease in dopaminergic activity in the raphe nuclei, suggesting that E. californiensis inhibits serotonin and dopamine signaling in naturally infected F. parvipinnis. The neurochemical profile of infected fish is consistent with the hypothesis that E. californiensis affects brain monoaminergic systems in order to induce impulse‐driven, active, and aggressive behaviour in its hosts.     

Host activity and wasp experience affect parasitoid wasp foraging behaviour and oviposition on nematode‐infected larvae of the forestry pest Hylobius abietis

1. Entomopathogenic nematodes (EPN) are currently being used as introduced biological control agents against the larvae of the native European forestry pest Hylobius abietis L. which develop under the bark of stumps and roots of newly dead conifer trees. 2. The potential for resource competition between gregarious ectoparasitoid Bracon hylobii Ratz and EPN by recording oviposition and related behaviours of B. hylobii females on EPN‐infected H. abietis larvae was investigated. Wasps did not parasitise EPN‐infected host larvae that were dead when presented, but naÏve and experienced wasps parasitised live EPN‐infected hosts. NaÏve wasps parasitised live EPN‐infected hosts significantly less frequently than healthy hosts only when the infected larvae were close to death (i.e. died during 24‐h trial). Parasitism by experienced wasps was unaffected by host infection. 3. Wasp probing and oviposition were positively associated with the amount of host movement. Preventing H. abietis larvae from chewing on bark significantly reduced parasitism by naÏve, but not experienced wasps. 4. The number of eggs per clutch was not affected by bark chewing or EPN‐infection of H. abietis larvae. 5. NaÏve and experienced B. hylobii parasitised two abnormal hosts (larvae of coleopteran Rhagium bifasciatum Fabricius and lepidopteran Galleria mellonella L.), both of which moved and chewed on bark during trials. 6. It was concluded that B. hylobii can use vibrational cues generated by host movement and feeding to locate hosts at short range and accepts unsuitable (EPN‐infected or abnormal) hosts as long as these create such cues. The implications for competition between B. hylobii and EPN and possible ways of minimising it when applying EPN are discussed.     

Parasite‐induced alterations of host behaviour in a riverine fish: the effects of glochidia on host dispersal

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A checklist of parasites in non‐native populations of rotan Perccottus glenii Dybowski, 1877 (Odontobutidae)

This review presents data on diversity, prevalence, and distribution of protistan and metazoan parasites in non‐native populations of the rotan Perccottus glenii. The information was collected using peer reviewed journals, difficult to access grey literature, and original data of the authors. As a whole, 70 literature sources from 1971 to 2012 were analyzed. The data originate from five of the 17 countries already colonized by P. glenii. A total of 97 parasite species/taxa are listed, each with the location in/on the host, geographical localities, countries, and literature sources. Where available, the number of fish examined as well as the prevalence and range of infection intensity are cited from the respective sources. Attention is given to host‐specific parasites because the rotan is a vector of their new geographical distribution. The current state and perspectives for further research are discussed.     

Host–parasite interactions and vectors in the barn swallow in relation to climate change

Recent climate change has affected the phenology of numerous species, and such differential changes may affect host–parasite interactions. Using information on vectors (louseflies, mosquitoes, blackflies) and parasites (tropical fowl mite Ornithonyssus bursa, the lousefly Ornithomyia avicularia, a chewing louse Brueelia sp., two species of feather mites Trouessartia crucifera and Trouessartia appendiculata, and two species of blood parasites Leucozytozoon whitworthi and Haemoproteus prognei) of the barn swallow Hirundo rustica collected during 1971–2008, I analyzed temporal changes in emergence and abundance, relationships with climatic conditions, and changes in the fitness impact of parasites on their hosts. Temperature and rainfall during the summer breeding season of the host increased during the study. The intensity of infestation by mites decreased, but increased for the lousefly during 1982–2008. The prevalence of two species of blood parasites increased during 1988–2008. The timing of first mass emergence of mosquitoes and blackflies advanced. These temporal changes in phenology and abundance of parasites and vectors could be linked to changes in temperature, but less so to changes in precipitation. Parasites had fitness consequences for hosts because intensity of the mite and the chewing louse was significantly associated with delayed breeding of the host, while a greater abundance of feather mites was associated with earlier breeding. Reproductive success of the host decreased with increasing abundance of the chewing louse. The temporal decrease in mite abundance was associated with advanced breeding of the host, while the increase in abundance of the lousefly was associated with earlier breeding. Virulence by the tropical fowl mite decreased with increasing temperature, independent of confounding factors. These findings suggest that climate change affects parasite species differently, hence altering the composition of the parasite community, and that climate change causes changes in the virulence of parasites. Because the changing phenology of different species of parasites had both positive and negative effects on their hosts, and because the abundance of some parasites increased, while that of other decreased, there was no consistent temporal change in host fitness during 1971–2008.     

Prevalence and intensity of Allocreadium isoporum (Digenea: Allocreadiidae) in three endemic species of cyprinids (Capoeta spp.) in Turkey,in relation to season,host size and sex

Three species of cyprinids, i.e., Capoeta antalyensis (n = 112), C. caelestis (n = 99) and C. angorae (n = 178) were collected from the Anatolian Region of Turkey to carry out this investigation. A total of 389 specimens of Capoeta were examined for Allocreadium isoporum infections throughout the four different seasons of 2012. The prevalence and mean intensity of infection of A. isoporum was investigated in relation to the host species, the seasons across the year, host size, and sex. Two‐hundred and twenty‐two specimens of a trematode identified as A. isoporum were found in these fishes. One‐hundred seventy‐eight C. angorae were caught, and 70 A. isoporum were recorded in 8 of 178 fish examined, with prevalence and mean intensity of infection of 4.49% and 8.7% parasites per infected fish, respectively. In the 112 specimens of C. antalyensis, 73 individuals of A. isoporum were found, with a prevalence of 19.6% and a mean intensity of 3.3. As for the infection in C. caelestis, a total of 79 specimens of A. isoporum were found in 17 of the 99 fish examined, with prevalence and mean intensity of infection of 17.1% and 4.6%, respectively. Prevalence and mean intensity values in relation to host size and sex, as well as the seasonality of infections was analyzed and the results were compared with other studies. The trematode A. isoporum is reported for the first time parasitising three endemic species of Capoeta spp. in Turkey, adding to the knowledge of geographical distribution and host range of this trematode species.     

Coextirpation of host–affiliate relationships in rivers: the role of climate change,water withdrawal,and host‐specificity

The role of climate‐related disturbances on complex host–affiliate relationships remains understudied, largely because affiliate species vary in host use and are often differentially susceptible to disturbance relative to their hosts. Here we report the first set of host–affiliate species–discharge relationships (SDR) in freshwater and examine how anticipated shifts in water availability (flow) will impact coextirpations. We used SDR for freshwater mussels and fish across 11 regions (over 350 rivers) in the continental United States that we coupled to future water availability (2070) to model mussel and fish coextirpations. We also used river‐specific host–affiliate matrices (presence–absence) to evaluate how host‐specificity (mean number of hosts used by an affiliate) and host‐overlap (extent to which affiliates share hosts) relate to extirpation vulnerability. We found that the strength and predictability of SDR models vary geographically and that mussels were more susceptible to flow alterations than fish. These patterns of extirpations were strongest in the southeast where: (1) flow reductions are expected to be greatest; (2) more species are lost per unit flow; (3) and more mussels are expected to be lost per unit of fish. We also found that overall mussel losses associated with reduction in habitat (water availability) were greater than those associated with loss of fish hosts which we assumed to be a function of host redundancy. These findings highlight the utility of SDR as a tool for conservation efforts but they also demonstrate the potential severity of reductions in mussel and fish richness as consequence of climate change and water use. Mussels provide key ecosystem services but face multiple pronged attacks from reductions in flow, habitat, and fish hosts. These losses in biodiversity and ecosystem functions can translate into major effects on food webs and nutrient recycling.     

Evolution of virulence under intensive farming: salmon lice increase skin lesions and reduce host growth in salmon farms

Parasites rely on resources from a host and are selected to achieve an optimal combination of transmission and virulence. Human‐induced changes in parasite ecology, such as intensive farming of hosts, might not only favour increased parasite abundances, but also alter the selection acting on parasites and lead to life‐history evolution. The trade‐off between transmission and virulence could be affected by intensive farming practices such as high host density and the use of antiparasitic drugs, which might lead to increased virulence in some host–parasite systems. To test this, we therefore infected Atlantic salmon (Salmo salar) smolts with salmon lice (Lepeophtheirus salmonis) sampled either from wild or farmed hosts in a laboratory experiment. We compared growth and skin damage (i.e. proxies for virulence) of hosts infected with either wild or farmed lice and found that, compared to lice sampled from wild hosts in unfarmed areas, those originating from farmed fish were more harmful; they inflicted more skin damage to their hosts and reduced relative host weight gain to a greater extent. We advocate that more evolutionary studies should be carried out using farmed animals as study species, given the current increase in intensive food production practices that might be compared to a global experiment in parasite evolution.     

Observations on the Occurrence of the Parasitic Ciliate Synophrya in Decapods in Coastal Waters off the Southeastern United States*

SYNOPSIS. Past reports of Synophrya, the only apostome ciliate known to harm its host, indicated that it was restricted in its distribution and in the hosts it can infect. In recent collections of decapods from the off-shore waters of North Carolina and Georgia, 30% of all specimens were infected with Synophrya. Forty-four percent of all the species collected had individuals infected with Synophrya, and 63% of all the families collected contained species that had infected individuals. There is no obvious structural or phylogenetic relationship between these families that explain why they are infected. The presence of Synophrya may be related to salinity since decapods captured in estuaries were never infected and decapods captured close to shore were only rarely infected. The salinities of these waters range from 15–35%, but the salinity of the off-shore waters where the great majority of infected specimens was found ranges only from 35–36%.     

A comparison of fatty acid,cholesterol and vitamin composition in sea bass [Dicentrarchus labrax (Linnaeus, 1758)] and sea bream [Sparus aurata (Linnaeus, 1758)] from three cage farm areas: Antalya and Muğla (Turkey) and İskele (Northern Cyprus)

In the present study the fatty acids, cholesterol and vitamin composition in farmed sea bass (8 fish per species per farm; weight range: 389.6–395.8 g, total length range: 297–316 mm) and sea bream (8 fish per species per farm; weight range: 386.8–391.7 g, total length range: 263–268 mm) from three cage farms (?skele in northern Cyprus, Antalya and Mu?la in Turkey) were compared during the harvesting period in June–July 2011. The results showed that the muscles of D. labrax and S. aurata farmed fish were rich in n‐3 fatty acids, but with important differences. For example, the muscles of sea bass farmed in ?skele were rich in docosahexaenoic acid (DHA) and n‐3 polyunsaturated fatty acids (PUFA). Palmitic acid (C16:0) was the primary saturated fatty acid, and oleic acid (C18:1 n‐9) the primary monounsaturated fatty acid in the muscle and liver samples of the cage‐farmed sea bass and sea bream. There were no significant differences in the cholesterol content in the muscles of sea bream farmed in ?skele, Antalya or Mu?la. In conclusion, the n‐3/n‐6 ratio in the muscle of farmed S. aurata and D. labrax is within the recommended limits for a healthy human diet, being very suitable for human nutrition.     

Effect of the pupal age of Calliphora erythrocephala (Diptera: Calliphoridae) on the reproductive biology of Melittobia acasta (Walker) (Hymenoptera: Chalcidoidea: Eulophidae)

A laboratory experiment was conducted to determine the effect of the pupal age of Calliphora erythrocephala (Meigen) on the reproductive biology (in terms of number, size, developmental time and longevity of progeny) of the parasitoid Melittobia acasta Walker. Melittobia acasta females of uniform size were given five C. erythrocephala pupae from one of four experimental age groups: 17–24 h, 24–48 h, 48–72 h and 72–96 h, for parasitization. The mean number of progeny produced from the experimental age groups for a 24 h period were 2, 7.6, 15.6 and 13.6, respectively. The parasitoids preferred hosts that were 48–72 h old. There were no significant differences in the mean development time (18.2 days) and size of progeny (mean head width = 0.38 ± 0.01 mm) produced from the experimental host age groups. The longevity of progeny from the four host age groups varied (range: 4–39 days), with those from the 48–72 h group living longest (mean = 25 days). The F₁ females from the 48–72 h group were reproductively more successful than those from the other groups, producing a mean F₂ progeny of 912 individuals when compared with 867, 801 and 757 individuals from the 24–48 h, 72–96 h and 17–24 h age groups, respectively. These findings make significant contributions to our knowledge of the breeding and utilization of this parasitoid for the biological control of dipteran flies in pigsties and poultry houses.     

Co‐evolutionary patterns in congeneric monogeneans: a review of Dactylogyrus species and their cyprinid hosts

Patterns associated with the evolution of parasite diversity, speciation and diversification were analysed using Dactylogyrus species (gill monogeneans) and their cyprinid hosts as a model. The aim of this study was to use this highly specific host–parasite systems to review: (1) the diversity and distribution of Dactylogyrus species, (2) the patterns of organization and structure of Dactylogyrus communities, (3) the evolution and determinants of host specificity and (4) the mode of Dactylogyrus speciation and co‐evolutionary patterns in this Dactylogyrus–cyprinid systems. Dactylogyrus are a highly diverse group of parasites, with their biogeography and distribution clearly linked to the evolutionary history of their cyprinid hosts. The coexistence of several Dactylogyrus species on one host is facilitated by increasing niche distances and the differing morphology of their reproductive organs. The positive interspecific and intraspecific interactions seem to be the most important factors determining the structure of Dactylogyrus communities. Host specificity is partially constrained by parasite phylogeny. Being a strict specialist is an ancestral character for Dactylogyrus, being the intermediate specialists or generalists are the derived characters. The evolution of attachment organ morphology is associated with both parasite phylogeny and host specificity. Considering larger and long‐lived hosts or hosts with several ecological characters as the measures of resource predictability, specialists with larger anchors occurred on larger or longer‐living fish species. Intra‐host speciation, a mode of speciation not often recorded in parasites, was observed in Dactylogyrus infecting sympatric cyprinids. Sister parasite species coexisting on the same host occupied niches that differed in at least one niche variable. Intra‐host speciation, however, was not observed in Dactylogyrus species of congeneric hosts from geographically isolated areas, which suggested association by descent and host‐switching events.     

Different thermal conditions of lakes affect host–parasite systems: A case study of Viviparus contectus (Millet, 1813) and digenean trematodes

1. Thermal disturbance of aquatic ecosystems directly and/or indirectly affects interspecific interactions, including parasitism. Both hosts and parasites respond differently to environmental changes, thus, predicting how host–parasite systems behave under the influence of disturbance remains a challenge. The aim of the study was to check how the differences in thermal conditions of lakes affect life-history traits of hosts and the level of parasitism, using a Viviparus contectus–digenean trematodes model.
2. Overall, we examined 480 individuals of V. contectus collected from a thermally polluted lake (TPL) and a natural lake (NL). Host features, including body size and fecundity, as well as the prevalence and species richness of digenean trematodes in snail populations were investigated.
3. We found that V. contectus from the TPL were significantly larger, heavier, and females were more fertile than snails collected from the NL. A total of 20.4% of the collected snails were infected with digenean larvae. The species richness of parasites was twice as high in the NL compared to the TPL (six and three species, respectively). A significant difference in the percentage of snails infected with parasites was identified between both types of lakes, with a higher prevalence of V. contectus in the NL (31.3%) compared to the TPL (7.3%).
4. These results indicate that host–parasite systems follow the environmental changes in lakes due to thermal pollution by increasing fertility and metabolism rate of viviparid hosts and by decreasing the prevalence and diversity of digenean trematodes.