Host population density and body mass as determinants of species richness in parasite communities: comparative analyses of directly transmitted nematodes of mammals

Epidemiological theory predicts positive correlations between host population density or body mass and species richness among parasite communities. Here I test these predictions by a comparative study of communities of directly transmitted mammalian parasites, gastrointestinal strongylid nematodes. I use data from 45 species of mammals, representing examination of 17 200 individual hosts. The variable studied was the average number of gastrointestinal strongylid nematode species per host population, and three different methods were used to obtain estimates of parasite species richness that are unbiased by number of host individuals examined. Analyses were done using the phylogenetically independent contrast method. Host population density and parasite species richness were strongly positively correlated when the effects of host body weight had been controlled for. Controlling for other variables did not change this, and the relationship was found regardless of method used to correct for uneven sampling effort among host species. A positive relationship between parasite species richness and host body weight was also found, but the effect of host densities had to be controlled for to see this. These relationships between host traits and species richness of directly transmitted parasites are stronger than patterns found using data on indirectly transmitted mammalian parasites, and suggests that links between host traits and parasite species richness are stronger than previously suggested. The results are consistent with parasite species richness being positively linked to pathogen transmission rates and reductions in transmission rates possibly increasing extinction probabilities in parasite populations. The results also suggest that parasites may exert a cost of increases in rate of population energy usage, and thus show that pathogens may be important in generating independence between body mass and rate of population energy usage among host species.     

An ecological law and its macroecological consequences as revealed by studies of relationships between host densities and parasite prevalence

Epidemiological models predict a positive relationship between host population density and abundance of macroparasites. Here I lest these by a comparative study. I used data on communities of four groups of parasites inhabiting the gastrointestinal tract of mammals, nematodes of the orders Oxyurida. Ascarida. Enoplida and Spirurida. respectively. The data came from 44 mammalian species and represent examination of 16886 individual hosts. I studied average prevalence of all nematodes within an order in a host species, a measure of community level abundance, and considered the potential confounding effects of host body weight, fecundity, age at maturity and diet. Host population density was positively correlated with parasite prevalence within the order Oxyurida, where all species have direct life cycles. Considering the effects of other variables did not change this. This supports the assumption that parasite transmission rate generally is a positive function of host population density_: It also strengthens the hypothesis that host densities generally act as important determinants of species richness among directly transmitted parasites and suggests that negative influence of such parasites on host population growth rate increase with increasing host population density among host species. Within the other three nematode orders, where a substantial number of the species have indirect life cycles, no relationships between prevalence and host population density were seen, Again. considering the effects of other variables did not affect this conclusion. This suggests that host population density is a poor predictor of species richness of indirectly transmitted parasites and that effects of such parasites on host population dynamics do not scale with host densities among species of hosts.     

Large-scale patterns of host use by parasites of freshwater fishes

Organisms that are abundant locally in a habitat patch are commonly observed to be frequent regionally, or among patches. In parasites, species present in high numbers in host individuals are also present in many individuals in the host population. On a larger scale, however, when host species are considered as patches, we may expect the opposite pattern because of the cost of producing mechanisms to evade the immune responses of several host species. Thus parasite species exploiting many host species may achieve lower average abundance in their hosts than parasite species exploiting fewer host species. This prediction was tested with data from 188 species of metazoan parasites of freshwater fish, using a comparative approach that controlled for study effort and phylogenetic influences. A negative correlation was found between the number of host species used by parasites and their average abundance in hosts, measured as either prevalence or intensity of infection. There was no evidence that parasite species fall into distinct categories based on abundance patterns, but rather that they fall along a continuum ranging from a generally low abundance in many host species, to a generally high abundance in few host species. These results applied to both ecto- and endoparasites. The pattern observed suggests the existence of a trade-off between how many host species a parasite can exploit and how well it does on average in those hosts.     

The intra- and interspecific relationships between abundance and distribution in helminth parasites of birds

1. Positive correlations between local abundance and distribution on a larger spatial scale are commonly observed among related species.
2. Within parasite species, the same relationship may be expected between prevalence and intensity of infection across host species used. Across parasite species, a positive relationship is expected between average abundance in a host population and the number of host species that can be exploited based on the resource breadth hypothesis. Trade-offs between the ability to exploit many host species and the potential for heavy infections, however, could result in a negative relationship.
3. Intraspecifically, using data on 51 helminth species parasitic in birds, prevalence and intensity of infection among host species used are generally only weakly correlated. Only in nematodes is there an overall positive relationship between prevalence and intensity.
4. A comparative analysis was performed on data from 389 species of cestodes, trematodes and nematodes parasitic in birds to determine how host specificity covaries interspecifically with abundance, measured both as prevalence and intensity of infection.
5. After controlling for phylogenetic influences and sampling effort, the number of host species used correlated positively with prevalence in all three parasite taxa, and with intensity of infection in trematodes only.
6. These results do not support the existence of a trade-off between abundance and the use of many host species, as has been found for fish parasites. Instead, whatever makes helminth parasites of birds abundant within a host population may facilitate their successful colonization of new host species.     

High parasite infection level in non‐native invasive species: it is just a matter of time

The enemy release hypothesis is often used to explain the success of non‐native species invasions. Growing evidence indicates that parasite or pathogen species richness increases over time in invasive non‐native species; however, this increase should not directly translate into release from enemy pressure as infection intensity of parasites (number of parasites per host) has a more profound impact on host fitness. The changes in intensity of parasitic infections in invasive non‐native species have not yet been thoroughly analysed in newly colonized areas. The goal of this study was to determine whether gastrointestinal parasite (nematode and trematode) infection intensity has increased with time since the populations of American mink Neovison vison were established and how host demographic parameters affect infection intensity. We tested the enemy release hypothesis by substituting space for time, evaluating parasite abundance in American mink at six sites along a chronosequence of mink invasion history. Nematode and trematode abundance increased with time since mink introduction, from a few parasites on average per mink after 16 yr, to 200–250 parasites per mink after 34 yr. The rate of increase in parasite abundance varied among demographic groups of mink (sex and age). Both nematodes and trematodes were more abundant in males than in females, and in subadults than in adults. Higher nematode abundance negatively affected body condition of mink. Our results provide evidence that non‐native species are released from enemy pressure only in the first phase of invasion, and that infection is modulated by host demographics and season. These results contribute to the evaluation of the long‐term patterns of parasite accumulation in invasive non‐native species after their colonization of new territories.     

Comparative tests of parasite species richness in primates

Some hosts harbor diverse parasite communities, whereas others are relatively parasite free. Many factors have been proposed to account for patterns of parasite species richness, but few studies have investigated competing hypotheses among multiple parasite communities in the same host clade. We used a comparative data set of 941 host-parasite combinations, representing 101 anthropoid primate species and 231 parasite taxa, to test the relative importance of four sets of variables that have been proposed as determinants of parasite community diversity in primates: host body mass and life history, social contact and population density, diet, and habitat diversity. We defined parasites broadly to include not only parasitic helminths and arthropods but also viruses, bacteria, fungi, and protozoa, and we controlled for effects of uneven sampling effort on per-host measures of parasite diversity. In nonphylogenetic tests, body mass was correlated with total parasite diversity and the diversity of helminths and viruses. When phylogeny was taken into account, however, body mass became nonsignificant. Host population density, a key determinant of parasite spread in many epidemiological models, was associated consistently with total parasite species richness and the diversity of helminths, protozoa, and viruses tested separately. Geographic range size and day range length explained significant variation in the diversity of viruses.     

Parasite community structure within and across host populations of a marine pelagic fish: how repeatable is it?

The geographical variation in parasite community structure among populations of the same host species remains one of the least understood aspects of parasite community ecology. Why are parasite communities clearly structured in some host populations, and randomly assembled in others? Here, we address this fundamental question using data on the metazoan parasite communities of different host size-classes of four distinct populations of a small pelagic fish, the Argentine anchovy, Engraulis anchoita, from the South West Atlantic. Within each fish sample, fish length was correlated with both the total intensity of parasites and species richness among infracommunities. More importantly, average fish length correlated with mean infracommunity richness and mean total intensity across the fish samples, indicating that the characteristics of parasite assemblages in a fish population are strongly influenced by the size of its fish in relation to those in other populations. Nested subset patterns were observed in about half of the fish samples. This means that the presence or absence of parasite species among fish individuals is often not random; however, no repeatability of nestedness among component communities was observed. Average fish length did not influence directly the likelihood that a parasite assemblage was significantly nested. However, variables influenced by average fish length, namely mean infracommunity richness and mean total intensity, determine the probability that a nested hierarchy will be observed; host size may thus indirectly affect parasite community structure either itself or via its influence on host movement and feeding patterns. To some extent, this apparent link may be due to the sensitivity of nestedness analyses to the proportion of presence in a presence/absence matrix; this in itself is a biological feature of the parasite community, however, which is associated with mean host length.     

Distribution and abundance of parasite nematodes: ecological specialisation,phylogenetic constraint or simply epidemiology?

We investigate the patterns of abundance‐spatial occupancy relationships of adult parasite nematodes in mammal host populations (828 populations of nematodes from 66 different species of terrestrial mammals). A positive relationship between mean parasite abundance and host occupancy, i.e. prevalence, is found which suggests that local abundance is linked to spatial distribution across species. Moreover, the frequency distribution of the parasite prevalence is bimodal, which is consistent with a core‐satellite species distribution. In addition, a strong positive relationship between the abundance (log‐transformed) and its variance (log‐transformed) is observed, the distribution of worm abundance being lognormally distributed when abundance values have been corrected for host body size.
Hanski et al. proposed three distinct hypotheses, which might account for the positive relationship between abundance and prevalence in free and associated organisms: 1) ecological specialisation, 2) sampling artefact, and 3) metapopulation dynamics. In addition, Gaston and co‐workers listed five additional hypotheses. Four solutions were not applicable to our parasitological data due to the lack of relevant information in most host‐parasite studies. The fifth hypothesis, i.e. the confounded effects exerted by common history on observed patterns of parasite distributions, was considered using a phylogeny‐based comparison method. Testing the four possible hypotheses, we obtained the following results: 1) the variation of parasite distribution across host species is not due to phylogenetic confounding effects; 2) the positive relationship between mean abundance and prevalence of nematodes may not result from an ecological specialisation, i.e. host specificity, of these parasites; 3) both a positive abundance‐prevalence relationship and a negative coefficient of variation of abundance‐prevalence relationship are likely to occur which corroborates the sampling model developed by Hanski et al. We argue that demographic explanations may be of particular importance to explain the patterns of bimodality of prevalence when testing Monte‐Carlo simulations using epidemiological modelling frameworks, and when considering empirical findings. We conclude that both the bimodal distribution of parasite prevalence and the mean‐variance power function simply result from demographic and stochastic patterns (highlighted by the sampling model), which present compelling evidence that nematode parasite species might adjust their spatial distribution and burden in mammal hosts for simple epidemiological reasons.     

Parasites and the regional distribution of bumblebee species

Parasites and regional processes may be important to structure local species assemblages In particular, it has been hypothesized that widely distributed and abundant species should harbour more parasite species which could give them a competitive advantage in local species assemblages Empirical evidence bearing on these points are scarce and mainly restricted to vertebrate hosts or plants The aim of this study was to provide data in insect hosts and to test whether the patterns in field populations conform with those correlates expected from the parasite-host distribution hypothesis We investigated species assemblages of bumblebees at 12 different sites in a mesoscale region with their parasites over two consecutive years Parasites included dipteran and hymenopteran parasitoids. nematodes, mites, and protozoa The mean number of parasite species per host species ranged from 1 to 8 To account for sampling effort, all data were corrected for sample size effects The number of parasite species per average host individual (parasite load) ranged from 0 09 to 0 75 In cross-species comparisons, the number of parasite species per host species was positively correlated with regional distribution, i e the number of sites a host species occupied m the region, and with the average local host abundance The same relationships were found for parasite load In addition, parasite load correlated positively with average colony size of the host species, but not with body size of the individuals Bumblebee species were bimodally distributed When separated into widely-distributed and locally-occurring species, common hosts harboured more parasite species than rare ones Moreover, workers of common species individually had higher parasite loads From these results, we conclude that some of the necessary preconditions for parasites being able to affect the distribution and occurrence of their hosts are met in bumblebees The findings support a general pattern that parasite loads correlate positively with local abundance and geographical distribution of their hosts, also on mesoscales usually considered in ecological studies     

Faecal egg counts and nemabiome metabarcoding highlight the genomic complexity of equine cyathostomin communities and provide insight into their dynamics in a Scottish native pony herd

Understanding the composition of gastrointestinal nematode communities may help to mitigate or exploit parasite adaptations within their host. We have used nemabiome deep amplicon sequencing of internal transcribed spacer-2 (ITS-2) ribosomal DNA to describe the temporal and host species composition of gastrointestinal nematode communities following sampling of six Scottish ponies across 57 months. In the absence of parasite control, each horse showed seasonal trends of increases and decreases in faecal egg counts, consistent with the epidemiology of equine strongylid parasites, however, the composition of parasites within individuals changed over time. Sixteen presumptive strongylid species were identified in each of the horses, 13 of which were distributed in a complex clade together with small numbers of amplicon sequences which could not be classified beyond the Cyathostominae subfamily level. Egg shedding of seven trichostrongylid species, which had previously been identified in co-grazed Soay sheep, was identified during the early spring. Faecal egg counts and the percentage of amplicon sequences assigned to each gastrointestinal nematode species were combined to describe their relative abundance across both host and time. Significant differences in species diversity between horses and between months were observed, being greatest from March to May and least from October to December. The magnitude of the individual horse effect varied between months and, conversely, the magnitude of the seasonal effect varied between individual horses. The most abundant gastrointestinal nematode in each of the horses was Cylicostephanus longibursatus (46.6% overall), while the abundance of the other strongylid species varied between horses and relative to each other. Patent C. longibursatus infections over the winter months might represent a genetic adaptation towards longer adult worm survival, or a lower rate of developmental arrest in the autumn. This study provides insight into highly complex phylogenetic relationships between closely related cyathostomin species; and describes the dynamics of egg shedding and pasture contamination of co-infecting equine gastrointestinal nematode communities. The results could be applied to determine how climatic and management factors affect the equilibrium between hosts and their parasites, and to inform the development of sustainable gastrointestinal nematode control strategies for different host species.     

Linking community assembly and structure across scales in a wild mouse parasite community

Understanding what processes drive community structure is fundamental to ecology. Many wild animals are simultaneously infected by multiple parasite species, so host–parasite communities can be valuable tools for investigating connections between community structures at multiple scales, as each host can be considered a replicate parasite community. Like free‐living communities, within‐host–parasite communities are hierarchical; ecological interactions between hosts and parasites can occur at multiple scales (e.g., host community, host population, parasite community within the host), therefore, both extrinsic and intrinsic processes can determine parasite community structure. We combine analyses of community structure and assembly at both the host population and individual scales using extensive datasets on wild wood mice (Apodemus sylvaticus) and their parasite community. An analysis of parasite community nestedness at the host population scale provided predictions about the order of infection at the individual scale, which were then tested using parasite community assembly data from individual hosts from the same populations. Nestedness analyses revealed parasite communities were significantly more structured than random. However, observed nestedness did not differ from null models in which parasite species abundance was kept constant. We did not find consistency between observed community structure at the host population scale and within‐host order of infection. Multi‐state Markov models of parasite community assembly showed that a host's likelihood of infection with one parasite did not consistently follow previous infection by a different parasite species, suggesting there is not a deterministic order of infection among the species we investigated in wild wood mice. Our results demonstrate that patterns at one scale (i.e., host population) do not reliably predict processes at another scale (i.e., individual host), and that neutral or stochastic processes may be driving the patterns of nestedness observed in these communities. We suggest that experimental approaches that manipulate parasite communities are needed to better link processes at multiple ecological scales.     

Variation in infection parameters among populations within parasite species: intrinsic properties versus local factors

Within any parasite species, variation among populations in standard infection parameters (prevalence, intensity and abundance) is an accepted fact. The proportion of hosts infected and the mean number of parasites per host are not fixed values across the entire geographic range of any parasite species. The question is whether this inter-population variation occurs within a narrow, species-specific range and is thus driven mainly by the biological features of the parasite, or whether it is substantial and unpredictable, leaving population parameters at the mercy of local conditions. Here, the repeatability of estimates of prevalence, intensity and abundance of infection was assessed across populations of the same parasite species, for 77 metazoan parasite species of Canadian freshwater fishes. Overall, parameter values from different populations of the same parasite species were more similar to each other and more different from those of other species, than expected by chance alone. Much of the variation in parameter values in the dataset was associated with differences between parasite species, rather than with differences among populations within species. This was particularly true for intensity and abundance of infection; in contrast, prevalence values, while somewhat repeatable among populations of the same species, still showed considerable variation. Among the higher taxa investigated (monogeneans, trematodes, cestodes, nematodes, acanthocephalans, copepods), there was no evidence that species of one taxon display intrinsically greater variation in population parameters than species of other taxa. Overall, the results suggest that intensity and abundance of infection are real species characters, though somewhat variable. This conclusion supports the view that the biological features of parasite species can potentially override local environmental conditions in driving parasite population dynamics.     

Distribution patterns and microhabitat segregation in gastrointestinal helminths of Sorex shrews

We studied the distribution patterns and microhabitat use in gastrointestinal helminths of the shrews Sorex araneus and S. caecutiens in Finland. The distribution of species prevalences was bimodal, and in S. araneus the abundance (mean intensity) was positively associated with commonness (prevalence), as assumed by the core-satellite species hypothesis (Hanski 1982). However, the positive correlation between prevalence and intensity was observed only when the effects of helminth body size and taxonomic group (cestodes vs nematodes) on intensity were controlled for. The nematodes of the genus Longistriata occurred predictably as core species, whereas the identity of the core cestodes was more variable between host species and regions. Helminth body size and taxonomic group were not related to the degree of aggregation in shrew populations, but helminth body size seemed to explain the differences in the distribution patterns of helminths between shrews and voles. The core species did not show more segregation in microhabitat use than randomly selected species. In fact, the two core nematodes showed largely overlapping intestinal distributions. We conclude that linear intestinal space is not a key resource for shrew nematodes, but it may be for shrew cestodes.     

Latitudinal gradient in the taxonomic composition of parasite communities

Although latitudinal gradients in diversity have been well studied, latitudinal variation in the taxonomic composition of communities has received less attention. Here, we use a large dataset including 950 surveys of helminth endoparasite communities in 650 species of vertebrate hosts to test for latitudinal changes in the relative contributions of trematodes, cestodes, nematodes and acanthocephalans to parasite assemblages. Although the species richness of helminth communities showed no consistent latitudinal variation, their taxonomic composition varied as a function of both host type and latitude. First, trematodes and acanthocephalans accounted for a higher proportion of species in helminth communities of fish, whereas nematodes achieved a higher proportion of the species in communities of bird and especially mammal hosts. Second, the proportion of trematodes in helminth communities of birds and mammals increased toward higher latitudes. Finally, the proportion of nematodes per community increased toward lower latitudes regardless of the type of host. We present tentative explanations for these patterns, and argue that new insights in parasite community ecology can be gained by searching for latitudinal gradients not only in parasite species richness, but also in the taxonomic composition of parasite assemblages.     

Evaluating the Effects of Ivermectin Treatment on Communities of Gastrointestinal Parasites in Translocated Woylies (<Emphasis Type="Italic">Bettongia penicillata</Emphasis>)

Wildlife species are often treated with anti-parasitic drugs prior to translocation, despite the effects of this treatment being relatively unknown. Disruption of normal host–parasite relationships is inevitable during translocation, and targeted anti-parasitic drug treatment may exacerbate this phenomenon with inadvertent impacts on both target and non-target parasite species. Here, we investigate the effects of ivermectin treatment on communities of gastrointestinal parasites in translocated woylies (Bettongia penicillata). Faecal samples were collected at three time points (at the time of translocation, and 1 and 3 months post-translocation) and examined for nematode eggs and coccidian oocysts. Parasite prevalence and (for nematodes) abundance were estimated in both treated and untreated hosts. In our study, a single subcutaneous injection of ivermectin significantly reduced Strongyloides-like egg counts 1 month post-translocation. Strongyle egg counts and coccidia prevalence were not reduced by ivermectin treatment, but were strongly influenced by site. Likewise, month of sampling rather than ivermectin treatment positively influenced body condition in woylies post-translocation. Our results demonstrate the efficacy of ivermectin in temporarily reducing Strongyloides-like nematode abundance in woylies. We also highlight the possibility that translocation-induced changes to host density may influence coinfecting parasite abundance and host body condition post-translocation.     

Effects of host sociality on ectoparasite population biology

Theory predicts a positive relationship between parasite infection intensity and host density. However, this generalization is complicated in natural systems by differences in life history among parasite taxa, e.g., transmissibility. Accordingly, predictions relating host density to parasite load should be specific to each parasite taxon. To illustrate this, we studied parasites that differed greatly in life history in the context of the Galapagos hawk's (Buteo galapagoensis) variably cooperative mating system. Two louse (Phthiraptera) species were collected: Colpocephalum turbinatum (Amblycera), with 53 host species, and Degeeriella regalis (Ischnocera), with 10 host species, although B. galapagoensis was the only known Galapagos host. Sixty territorial adult male hawks from 26 groups of 1-6 males were quantitatively sampled for lice. Average abundance and intensity of C. turbinatum but not D. regalis were significantly larger in large groups of hawks than small groups. Males from the same polyandrous group harbored significantly correlated abundances of C. turbinatum but not D. regalis. Prevalence, average abundance, and intensity of C. turbinatum were significantly higher than D. regalis. These are the first results to demonstrate significant differences in a suite of population responses between these louse suborders in the context of host sociality.     

Evaluating the Effects of Ivermectin Treatment on Communities of Gastrointestinal Parasites in Translocated Woylies (Bettongia penicillata)

Wildlife species are often treated with anti-parasitic drugs prior to translocation, despite the effects of this treatment being relatively unknown. Disruption of normal host–parasite relationships is inevitable during translocation, and targeted anti-parasitic drug treatment may exacerbate this phenomenon with inadvertent impacts on both target and non-target parasite species. Here, we investigate the effects of ivermectin treatment on communities of gastrointestinal parasites in translocated woylies (Bettongia penicillata). Faecal samples were collected at three time points (at the time of translocation, and 1 and 3 months post-translocation) and examined for nematode eggs and coccidian oocysts. Parasite prevalence and (for nematodes) abundance were estimated in both treated and untreated hosts. In our study, a single subcutaneous injection of ivermectin significantly reduced Strongyloides-like egg counts 1 month post-translocation. Strongyle egg counts and coccidia prevalence were not reduced by ivermectin treatment, but were strongly influenced by site. Likewise, month of sampling rather than ivermectin treatment positively influenced body condition in woylies post-translocation. Our results demonstrate the efficacy of ivermectin in temporarily reducing Strongyloides-like nematode abundance in woylies. We also highlight the possibility that translocation-induced changes to host density may influence coinfecting parasite abundance and host body condition post-translocation.

     

The Evolution of Taxonomic Diversity in Helminth Assemblages of Mammalian Hosts

Several studies have searched for the key forces behind the diversification of parasite assemblages over evolutionary time. All of these studies have used parasite species richness as their measure of diversity, thus ignoring the relatedness among parasite species and the taxonomic structure of the assemblages. This information is essential, however, if we want to elucidate which processes have caused an assemblage of parasites to acquire new species. Here, we performed a comparative analysis across 110 species of mammalian hosts in which we evaluated the effects of four host traits (body mass, population density, geographic range, and basal metabolic rate) on the diversity of their assemblages of helminth endoparasites. As measures of diversity, we used parasite species richness, as well as the average taxonomic distinctness of the assemblage and its variance; the latter measures are based on the taxonomic distance between two parasite species, computed across all possible species pairs in an assemblage. Unlike parasite species richness, both the average taxonomic distinctness and its variance were unaffected by the number of hosts examined. These two measures of parasite diversity also proved highly repeatable among host populations of the same mammalian species; in contrast, parasite species richness was unreliable as a species character, as it varied as much within a host species than among different host species. Using phylogenetically independent contrasts, and correcting for potential confounding variables, we found that host population density correlated positively with parasite species richness. There were, however, no other relationships between any of the four host traits investigated and either of our measures of parasite diversity. The processes facilitating the taxonomic diversification of parasite assemblages thus remain unclear, but their elucidation will be necessary if we are to fully understand parasite evolution.     

Parasite assemblages of Australian species of Pseudomys (Rodentia: Muridae: Murinae)

The parasite fauna of many Australian rodents is poorly known. The ectoparasite and helminth faunas of Pseudomys delicatulus, Pseudomys desertor, Pseudomys gracilicaudatus, and Pseudomys hermannsburgensis were determined and compared. In total, 12 species of arthropods, 2 cestodes, and 13 nematodes were found. Species richness of parasites was highest in P. hermannsburgensis and lowest in P. desertor. Despite the sampling effort, the number of parasite species discovered did not reach an asymptote for any of the host species, indicating that the full parasite fauna was not examined. Helminth species richness was highest in the insectivorous P. hermannsburgensis and lower in the obligate herbivores. The structure of parasite component communities was influenced by the social structure of the host species, not surprisingly, with the most highly social species having the highest richness of parasites. Habitat preferences also provided contrast between the helminth component communities, with heligmonellid nematodes occurring in damp woodlands and dominating the parasite fauna of P. gracilicaudatus. Oxyurid nematodes dominated the component communities of the 3 other species, all of which inhabit drier habitats.     

Helminth communities of the lesser sandeel Ammodytes tobianus L. off the west coast of Ireland

The helminth parasites present in 412 lesser sandeels (Ammodytes tobianus) taken from June 1996 to May 1997 from the Aran Islands on the west coast of Ireland were examined. Ten helminth parasite species were recorded, and more than 92% of the sandeels were infected with at least 1 helminth species. Seven of the species were digeneans, including Brachyphallus crenatus, Hemiurus communis, Derogenes varicus, Lecithaster gibbosus, Opechona bacillaris, Cryptocotyle lingua, and Galactosomum lacteum; 2 nematodes, including Hysterothylacium sp. and Contracaecum sp.; and 1 cestode, Scolex pleuronectis. Three of the 7 digenean species were either larvae or immature. Only 2 species, the digeneans G. lacteum and H. communis, had prevalences greater than 50%. The dominant species was G. lacteum, accounting for 67% of all parasites present. The relationship between spawned groups, host length, and season versus the abundance, prevalence, species richness, and the total number of parasites in the infracommunities was investigated. No difference was found between the parasite communities of the 2 spawning races of the host population. Mean abundance and prevalence of the different parasite species showed seasonal variation. Numbers of parasite species and numbers of parasites increased with fish length. The role of A. tobianus as an intermediate host for helminths was assessed; it was determined that most were infectious to birds or mammals, with the majority of the parasite species being autogenic (infectious to fish). The mean number of parasites per fish was nearly a quarter of the value recorded for A. tobianus in the North Sea, where a much higher intensity of infection was recorded.