Long-term stability in the richness and structure of helminth communities in eels, Anguilla anguilla, in Lough Derg, River Shannon, Ireland

Temporal dynamics of species associations in the parasite community of European eels, Anguilla anguilla, from a coastal lagoon

The resilience of biological communities is of central importance in ecology, but is difficult to investigate in nature. Parasite communities in individual hosts provide good model systems, as they allow a level of replication usually not possible with free-living communities. Here, using temporal data (2005-2017) on the communities of endohelminth parasites in European eels, Anguilla anguilla, from brackish-water lagoons in Italy, we test the resilience of interspecific associations to changes in the abundance of some parasite species and the disappearance of others. While most parasite species displayed changes in abundance over time, three trematodes that were present in the early years, two of which at high abundance, completely disappeared from the parasite community by the end of the study period. Possibly other host species required for the completion of their life cycles have declined in abundance, perhaps due to environmental changes. However, despite these marked changes to the overall community, pairwise correlations in abundance among the three most common parasite species (all trematodes) were stable over time and remained mostly unaffected by what happened to other species. We explore possible reasons for these resilient species associations within a temporally unstable parasite community inhabiting a mostly stable host population.

Interannual and Seasonal Variation in Protozoan and Metazoan Parasite Communities of Hemibrycon surinamensis, a Characid Fish Inhabiting the Brazilian Amazon

INTRODUCTION

Few studies on the effects of rainy/dry cycle and interannual variation in parasites of Amazonian fish have been carried out. We investigated the influence of interannual variation and seasonality on the community structure and infracommunities of parasites in Hemibrycon surinamensis in a tributary of the Amazon River, in northern Brazil.

MATERIALS AND METHODS

The fish were examined for the presence of ectoparasites and endoparasites.

RESULTS

In 2011, 9 parasite species were found (2 Protozoa, 2 Monogenea, 1 Digenea, 2 Nematoda and 2 Crustacea), and in 2016, 10 parasite species (3 Protozoa, 2 Monogenea, 1 Digenea, 3 Nematoda and 1 Pentastomida). No qualitative or quantitative dissimilarity in the component communities of parasites was observed between 2011 and 2016, but Brillouin Diversity Index, species richness and evenness were higher in 2016, while Berger-Parker dominance was higher in 2011. In both years, Ichthyophthirius multifiliis and Piscinoodinium pillulare were predominated. Some few species of parasites had interannual (P. pillulare and Genarchella genarchella) and seasonal (Trichodina sp., I. multifiliis, P. pillulare and J. hexops) variation patterns. Species richness of parasites was higher in the dry season, but Brillouin diversity, evenness and Berger-Parker dominance were similar between the seasons. Such seasonal variations were due to the host's the availability of infectious stages of parasites with direct life cycle.

CONCLUSIONS

Variations due to seasonal dry/rainy cycle, which influenced the availability of infective stages of parasites, and host size are the factors causing the changes interannual.

Macroparasites and their communities of the European eel Anguilla anguilla (Linnaeus) in the Czech Republic

This paper summarises the results of parasitological examinations of the European eel Anguilla anguilla (Linnaeus) in the Czech Republic, carried out at the Institute of Parasitology, Czech Academy of Sciences (previously the Czechoslovak Academy of Sciences) within the period of 50 years (1958-2008). Even though this survey is limited to the Czech Republic, it provides extensive data probably incomparable with any other study anywhere regarding the number of eels examined and parasites found. A total of 723 eels was examined from 42 localities that belong to all of the three main river drainage systems in the country, i.e. the Elbe, Danube and Oder river basins. Of the 31 species of adult and larval macroparasites including Monogenea (4 species), Trematoda (3), Cestoda (3), Nematoda (11), Acanthocephala (5), Hirudinea (1), Bivalvia (1), Copepoda (1), Branchiura (1) and Acariformes (1), most of them (30) were recorded from the Elbe River basin. These parasites can be divided into three main groups regarding their host specificity: parasites specific for eels (26%), non-specific adult parasites occurring also in other fishes (61%) and non-specific larvae (13%). The highest number (19) of parasite species was recorded in the Mácha Lake fishpond system in northern Bohemia. The parasite communities in eels from the individual localities exhibited large differences in their species composition and diversity depending on local ecological conditions. The parasite fauna of A. anguilla in the Czech Republic is compared with that in other European countries. The nematode Cucullanus egyptae Abdel-Ghaffar, Bashtar, Abdel-Gaber, Morsy, Mehlhorn, Al Quraishy et Mohammed, 2014 is designated as a species inquirenda.

The Bass Parasites of Oneida Lake, 80 Years Later

A survey of largemouth (Micropterus salmoides) and smallmouth bass (Micropterus dolomieu) parasite communities in Oneida Lake, New York, was conducted in the summer of 2012 and compared to an earlier survey conducted by Van Cleave and Mueller during the summers of 1929 to 1931. The component helminth communities between surveys were 31% similar in composition for largemouth and 28% similar for smallmouth bass. Between species, the component helminth communities were considerably more similar in the present survey (71%) than in the survey conducted by Van Cleave and Mueller (47%). Seven species reported by Van Cleave and Mueller were present in this survey and 21 species are new records for the bass of Oneida Lake. Van Cleave and Mueller did not report prevalence values for several taxa (Monogenea, Copepoda, Myxozoa, and a Trichodina sp.) that were important for separation of parasite infracommunities in species space for both bass species. These parasites represented 28% of all species found in the current survey and may be ecologically important. Several species of parasites exhibited differences in prevalence between surveys. Two species (Rhipidocotyle papillosa and Crepidostomum cornutum) were absent from this survey but were reported as common in the 1929-1931 survey and almost certainly represent extirpations that coincide with the loss of their native bivalve hosts from Oneida Lake. Other differences in the parasite communities may also be explained by the ecological disturbances in Oneida Lake over the past 81 yr. The changes in bass parasite communities between surveys emphasize the importance of recognizing the historical nature of parasite communities, especially in ecosystems with a history of large-scale changes. Most importantly our findings suggest that, similar to trends observed in free-living freshwater biotic communities, anthropogenic ecosystem disturbances may homogenize fish parasite communities.

Can a specialist parasite species of a widespread and common host species be rare? The case of Spinitectus inermis (Nematoda: Cystidicolidae) in eels Anguilla anguilla

The claim by many authors that Spinitectus inermis (Zeder, 1800), a narrowly specific parasite of European eels Anguilla anguilla (L.), is a rare species is considered at three levels: its geographical range, its frequency of occurrence compared to other eel parasites and its relative abundance in component communities. The parasite is widely distributed in freshwater throughout the European range of the eel but its occurrence is erratic and unpredictable, being known from only 8 countries. Surveys of eel parasites in the United Kingdom and in Continental Europe show that it is present in only 13% of British and 29% of continental localities. This satisfies one of the criteria for rarity. When present, its prevalence ranges from 1.8% to 43.3%, so it can be considered rare in some localities but in a few it may be common and on occasion it may be the dominant species in the gastro-intestinal community. Populations of S. inermis are almost always characterised by high levels of overdispersion, even at low prevalence. The species also displays an ability to colonise a locality following introduction there. Overall it meets many of the criteria of a rare species including a restricted distribution and a low frequency of occurrence and so it can be considered to exhibit diffusive rarity.

Scarcity of parasite assemblages in the Adriatic-reared European sea bass (Dicentrarchus labrax) and sea bream (Sparus aurata)

The shaping forces of parasite community structure still is the main subject in the ecological parasitology whilst community predictability and repeatability showed that hardly a generally applicable role is ever going to be assessed. Defining and describing parasite communities can be very useful from the epizootiological point, in order to help in the assessment of the medical and economical impact of certain parasitosis, moreover when hosts are economically valuable species. Since parasite assemblages in reared European sea bass (Dicentrarchus labrax) and sea bream (Sparus aurata) in Adriatic cage systems can play an important role in the economic feasibility of the rearing process, we evaluated their character through assessing diversity indices, nestedness of parasite communities and their differences in respect to season and composition, as well as fish growth. We observed colonization of a new monogenean species (Furnestinia echeneis) and general impoverishment of parasites populations over time in the Adriatic-reared fish parasite assemblages. Parasite assemblages differed significantly between seasons for both fish species, while species richness, evenness, diversity indices and nestedness of parasitic communities in the sea bream showed to be significantly higher compared to those in the sea bass. Such characteristics define parasite communities of both Adriatic-reared fish as species poor although structured and ordered assemblages.

Do captive conditions favor shedding of parasites in the reared Atlantic bluefin tuna (Thunnus thynnus)?

Tuna (Thunnus spp.) has been characterized by long distance migrations, highly predatory behavior and longevity, all of which in turn, enable infections with a wide spectrum of different parasitic groups, reflecting in a remarkable diversity of tuna parasite communities. Since 2003, we have been monitoring parasite communities of Atlantic bluefin tuna (Thunnus thynnus) that are caught from the wild and transferred into cages during spring-summer months, as well as assemblages in fish that exit rearing cycle during the winter harvest period after 1.5 years. Interestingly in reared tuna, parasitic populations exhibit a significant decreasing trend at the end of the rearing cycle, rarely observed in other intensive productions that represent a suitable environment for the emergence, establishment and transmission of pathogens. In order to assess epizootiological behavior of tuna parasites assemblages at the beginning (B group) and at the end (A group) of 1.5 year rearing cycle, we examined data on parasite prevalence and abundance over 4 years. The aim was to evaluate parasite diversity indices and emerging differences between newly caught and harvested fish, as well as community compositions and their nestedness in respect to the event in the rearing cycle (capture or harvest time). In order to be able to predict classification of tuna in two categories (newly caught or heavily infected and harvested or less infected fish), based on empirical didymozoids abundances and year of sampling, we built a decision tree model. Results suggest that specificities of parasite assemblages and their dynamics in tuna before and after farming have no similar precedents in aquaculture. A trend of parasitic pauperization repeating in each rearing cycle over four-years time, in once diverse and species rich parasite communities is observed, however, structures of both B and A group rearing assemblages remain nested, with the same species being core parasites (Didymosulcus katsuwonicola and Koellikerioides intestinalis). The B group exhibited significantly higher total parasite richness and mean parasites abundance, as well as the heteroxenous species richness and abundance in comparison to A group, where monoxenous species were not recorded at all. Eleven parasite species out of 26 taxa were selected as important in discriminating between B and A groups' parasites assemblages, while significantly the most abundant in B group were D. katsuwonicola, Platocystis alalongae, K. intestinalis, Koellikerioides internogastricus, Didymocystis abdominalis and Anisakis sp. It is hard to postulate the combination of factors affecting these parasite populations, but environmental, anthropogenic or host intrinsic influence has to be taken into account for further investigation.

Metazoan parasite infracommunities of the toadfish Aphos porosus (Pisces: Batrachoidiformes) in central Chile: how variable are they over time?

The present study examines variation in parasite community characteristics of the toadfish Aphos porosus across sampling years. We analyzed and compared the species composition and numeric descriptors of the parasite infrapopulations and infracommunities in 101 fish collected during the springs of 1999, 2006, and 2007 from rocky intertidal ponds in El Tabo, central Chile. Parasites were found in 94.1% of specimens necropsied. In total 5,532 parasites were collected, representing 12 taxa, of which 7 were found in the 3 annual samples. The most prevalent and abundant parasites in the 3 sampling years were larval tetraphyllidean cestodes, followed by anisakid nematodes. The community descriptors of average richness and abundance varied significantly across the 3 yr, with the exception of parasite dominance. The parasite species composition and relative abundance showed low variability among years. A few parasite taxa (Anisakis sp., Pseudoterranova sp., tetraphyllideans, and Clestobothrium crassiceps) dominated the infracommunities of the toadfish; however, tetraphyllideans were the parasites that greatly varied over time. Consequently the parasite infracommunities of the toadfish were variable among years, with a low variation in parasite composition, but large differences in the aggregate descriptors.

The ecology of parasites of freshwater fishes: the search for patterns

Developments in the study of the ecology of helminth parasites of freshwater fishes over the last half century are reviewed. Most research has of necessity been field based and has involved the search for patterns in population and community dynamics that are repeatable in space and time. Mathematical models predict that under certain conditions host and parasite populations can attain equilibrial levels through operation of regulatory factors. Such factors have been identified in several host-parasite systems and some parasite populations have been shown to persist over long time-periods. However, there is no convincing evidence that fish parasite populations are stable and regulated since in all cases alternative explanations are equally acceptable and it appears that they are non-equilibrial systems. It has proved particularly difficult to detect replicable patterns in parasite communities. Inter-specific competition, evidenced by functional and numerical responses, has been detected in several communities but its occurrence is erratic and its significance unclear. Some studies have failed to find any nested patterns in parasite community structure and richness, whereas others have identified such patterns although they are seldom constant over space and time. Departures from randomness appear to be the exception and then only temporary. It appears that parasite communities are non-equilibrial, stochastic assemblages rather than structured and organized.

Parasite communities of eels Anguilla anguilla in freshwater and marine habitats in Iceland in comparison with other parasite communities of eels in Europe

Ninety-five eels from one marine and three freshwater localities in Iceland were examined for parasites. Twenty species were found, 12 from marine habitat, 12 from freshwater and 4 species were found in both habitats. These are: Eimeria anguillae, Chilodonella hexasticha, Trichodina fultoni, T. jadranica, Myxidium giardi, Myxobolus kotlani, two Zschokkella spp., Derogenes varicus, Deropristis inflata, Diplostonmum sp., Plagioporus angulatus, Podocotyle atomon, Anisakis simplex (larva), Eustrongylides sp. (larva), Hysterothylacium aduncum (larva), Raphidascaris acus (larval and adult stages), Bothriocephalus claviceps, Proteocephalus macrocephalus, and a pseudophyllidean larva. Thirteen of these species are new parasite records from Icelandic waters. The component community of marine eels was characterized by low diversity and a high dominance of a single species. Overall, seven species of helminths were observed, up to five different species occurring in an individual fish. The component community of the freshwater eels was species-poor with low diversity and relatively high dominance of single species. A between-sites difference in the freshwater eels was considerable; only Diplositonun sp. was found at more then one sampling site. Similar to previous studies, there is a total replacement of freshwater macroparasite species by marine ones in saline waters. But unlike research abroad in which species richness decreases with higher salinity, the marine eels in Iceland have considerably higher richness than the freshwater ones. The parasite communities of freshwater eels in Iceland are, in general species-poorer, less diverse and having higher Berger Parker (BP) dominance than other eel communities in Europe. Marine eels have on the other hand comparable species richness, are less diverse and with a high BP dominance.

Utilization of brown troutSalmo truttabyAcanthocephalus clavulain an Irish lake: is this evidence of a host shift?

The population biology of the fish acanthocephalanAcanthocephalus clavulawas described from 161 wild brown trout,Salmo truttasampled over a two-year period in Clogher Lake in the west of Ireland. Overall prevalence of the parasite was 86% and the mean abundance was 53 worms per fish. Despite the presence of large numbers of worms in the trout very few females (2%) attained full reproductive maturity. This suggests that trout is an accidental host. A sample of yellow eels,Anguilla anguillawas examined at a different time from the same lake. The prevalence ofA. clavulawas 97% and the average abundance was 8 worms per fish. In contrast to the situation in trout, the proportion of female worms attaining full reproductive maturity was 61% fulfilling the expected characteristic of a preferred definitive host. The possible explanations for the very high abundance ofA. clavulain trout are discussed and include the influence of fluctuations in host populations, host diet and the absence of a potential competitor.

Long-term investigation of the composition and richness of intestinal helminth communities in the stocked population of eel, Anguilla anguilla, in Neusiedler See, Austria

Data from a long-term study of the intestinal helminth parasite community of eels, Anguilla anguilla, stocked into the shallow eutrophic Neusiedler See, Austria, were collected over an 8 year period (1994–2001). In total, 720 eels from 2 sampling sites were examined. The parasite community showed characteristics similar to those in the natural eel populations in rivers of the UK and mainland Europe: it was species poor, with only 5 species (Acanthocephalus lucii, Acanthocephalus anguillae, Raphidascaris acus, Proteocephalus macrocephalus, Bothriocephalus claviceps) comprising the component community and a maximum infracommunity richness of 4 species. Over the period, the intestinal parasite community of the sampling site in Illmitz, which was originally dominated by A. lucii, changed. As levels of A. anguillae increased to a point at which it dominated the community, diversity increased whilst dominance of a single species decreased. By contrast the community in the southern sampling site remained rather constant with a continuous high infection level of A. anguillae and low abundance of A. lucii. Both acanthocephalan species exhibited higher infection levels in larger eels and in different seasons of the year and the infection parameters were significantly different between the years of study. The significant differences in the infection levels of the 2 acanthocephalan species at the 2 sampling sites were surprising as both acanthocephalan species use the same intermediate host, Asellus aquaticus, and the sampling sites were in close proximity and were similar in terms of water quality, host size and invertebrate abundance. Differences in the fish communities of the 2 sampling sites and eel movements rather than interspecific competition are discussed as possible explanations for the differences in the parasite communities of the 2 sampling sites.

Interspecific interactions between Acanthocephala in the intestine of brown trout: are they more frequent in Ireland?

The aim of this paper was to test the hypothesis that when the 2 species of Acanthocephalan Pomphorhynchus laevis and Acanthocephalus clavula are found concurrently within the intestine of brown trout under field conditions, they have the potential to interact negatively. Evidence has shown that Acanthocephala are more likely to exhibit negative interactions with their own and other species, under both field and experimental conditions. Furthermore, the likelihood of these interactions is increased in Ireland because of the absence of certain definitive hosts and the fact that concurrent infections by two or more species of Acanthocephala are more commonly observed in fish. Data collected from wild and stocked brown trout and from 2 lakes provided an opportunity to compare the 2 potentially interacting helminth species in their fundamental and realized niche and several pieces of convincing evidence are provided here to support the hypothesis. A significant negative association between the numbers of each species found in individual fish was reported and this was consistent for both wild and stocked trout. Furthermore, an analysis of the proportions of low, moderate and high intensity infections in single and concurrent infections revealed a significant reduction in increasing intensities in concurrent infections compared to single infections. Finally, strikingly different patterns of niche inhabitation were observed, particularly for P. laevis in the presence of A. clavula in wild trout. Results from the niche width analysis also support the observations on average position in single and concurrent infections. The niche width of P. laevis when it co-occurred with A. clavula decreased markedly in high intensity infections compared to low intensity infections.