Differences in straggling rates between two genera of dove lice (Insecta: Phthiraptera) reinforce population genetic and cophylogenetic patterns

Differences in dispersal abilities have been implicated for causing disparate evolutionary patterns between Columbicola and Physconelloides lice (Insecta: Phthiraptera). However, no study has documented straggling (when lice are found on atypical hosts) rates within these lineages. We used the fact that the Galapagos Hawk, Buteo galapagoensis (Gould) (Falconiformes) feeds on the Galapagos Dove Zenaida galapagoensis Gould (Columbiformes) within an ecologically simplified setting. The Galapagos Dove is the only typical host of Columbicola macrourae (Wilson) and Physconelloides galapagensis (Kellogg and Huwana) in Galapagos. We quantitatively sampled and found these lice on both bird species. A DNA barcoding approach confirmed that stragglers were derived from Galapagos doves. We also collected a Bovicola sp. louse, likely originating from a goat (Capra hircus). On hawks, C. macrourae was significantly more prevalent than P. galapagensis. On doves, the two lice were equally prevalent and abundant. Differences in prevalence on hawks was a function of differences in straggling rate between lice, and not a reflection of their relative representation within the dove population. This provides further evidence that differences in dispersal abilities may drive differences in the degree of cospeciation in Columbicola and Phyconelloides lice, which have become model systems in evolutionary biology.     

Cryptic host specificity of an avian skin mite (Epidermoptidae) vectored by louseflies (Hippoboscidae) associated with two endemic Galapagos bird species

Host specificity of vectors is an important but understudied force shaping parasite evolution and the relationship between hosts and parasites. Low vector specificity may allow a vectored parasite to invade new host species, whereas high specificity of vectors may reduce the host range of the parasite and favor specialization. The 'generalist' and widely distributed avian skin mite Myialges caulotoon Speiser (Acari: Epidermoptidae) is unusual because females require an insect vector to complete their life cycle. Myialges caulotoon was previously reported from 2 lousefly (Diptera: Hippoboscidae) species, Olfersia sordida and Icosta nigra, parasitizing flightless cormorants (Phalacrocorax harrisi) and Galápagos hawks (Buteo galapagoensis), respectively, within the Galápagos Islands. This is a surprising distribution, given that the 2 lousefly species involved are relatively host-specific. Mitochondrial DNA sequences revealed 2 reciprocally monophyletic Myialges clades that sorted out perfectly with respect to their vector species, regardless of whether they were in allopatry or sympatry. One clade was restricted to flies of hawks and the other to flies of cormorants. Females of the 2 Myialges groups were also separated consistently by the shape of the sternal surface sclerotization. Mites of hawk flies were more abundant than those of cormorant flies. Within the Myialges clade associated with hawks, genetic differentiation between 2 island populations mirrored its host's patterns of differentiation.     

Patterns of parasite abundance and distribution in island populations of Galápagos endemic birds

Parasite life-history characteristics, the environment, and host defenses determine variation in parasite population parameters across space and time. Parasite abundance and distribution have received little attention despite their pervasive effects on host populations and community dynamics. We used analyses of variance to estimate the variability of intensity, prevalence, and abundance of 4 species of lice (Insecta: Phthiraptera) infecting Galápagos doves and Galápagos hawks and 1 haemosporidian parasite (Haemosporida: Haemoproteidae) infecting the doves across island populations throughout their entire geographic ranges. Population parameters of parasites with direct life cycles varied less within than among parasite species, and intensity and abundance did not differ significantly across islands. Prevalence explained a proportion of the variance (34%), similar to infection intensity (33%) and parasite abundance (37%). We detected a strong parasite species-by-island interaction, suggesting that parasite population dynamics is independent among islands. Prevalence (up to 100%) and infection intensity (parasitemias up to 12.7%) of Haemoproteus sp. parasites varied little across island populations.     

Co-phylogeography and comparative population genetics of the threatened Galápagos hawk and three ectoparasite species: ecology shapes population histories within parasite communities

Comparative microevolutionary studies of multiple parasites occurring on a single host species can help shed light on the processes underlying parasite diversification. We compared the phylogeographical histories, population genetic structures and population divergence times of three co-distributed and phylogenetically independent ectoparasitic insect species, including an amblyceran and an ischnoceran louse (Insecta: Phthiraptera), a hippoboscid fly (Insecta: Diptera) and their endemic avian host in the Galápagos Islands. The Galápagos hawk (Aves: Falconiformes: Buteo galapagoensis) is a recently arrived endemic lineage in the Galápagos Islands and its island populations are diverging evolutionarily. Each parasite species differed in relative dispersal ability and distribution within the host populations, which allowed us to make predictions about their degree of population genetic structure and whether they tracked host gene flow and colonization history among islands. To control for DNA region in comparisons across these phylogenetically distant taxa, we sequenced ~1 kb of homologous mitochondrial DNA from samples collected from all island populations of the host. Remarkably, the host was invariant across mitochondrial regions that were comparatively variable in each of the parasite species, to degrees consistent with differences in their natural histories. Differences in these natural history traits were predictably correlated with the evolutionary trajectories of each parasite species, including rates of interisland gene flow and tracking of hosts by parasites. Congruence between the population structures of the ischnoceran louse and the host suggests that the ischnoceran may yield insight into the cryptic evolutionary history of its endangered host, potentially aiding in its conservation management.     

Influence of host ecology and morphology on the diversity of Neotropical bird lice

Host‐parasite systems can be powerful arenas in which to explore factors influencing community structure. We used a comparative approach to examine the influence of host ecology and morphology on the diversity of chewing lice (Insecta: Phthiraptera) among 52 species of Peruvian birds. For each host species we calculated two components of parasite diversity: 1) cumulative species richness, and 2) mean abundance. We tested for correlations between these parasite indices and 13 host ecological and morphological variables. Host ecological variables included geographic range size, local population density, and microhabitat use. Host morphological variables included body mass, plumage depth, and standard dimensions of bill, foot and toenail morphology, all of which could influence the efficiency of anti‐parasite grooming. Data were analysed using statistical and comparative methods that control for sampling effort and host phylogeny. None of the independent host variables correlated with louse species richness when treated as a dependent variable. When richness was treated as an independent variable, however, it was positively correlated with mean louse abundance. Host body mass was also positively correlated with mean louse abundance. When louse richness and host body mass were held constant, mean louse abundance correlated negatively with the degree to which the upper mandible of the host's bill overhangs the lower mandible. This correlation suggests that birds with longer overhangs are better at controlling lice during preening. We propose a specific functional hypothesis in which preening damages lice by exerting a shearing force between the overhang and the tip of the lower mandible. This study is the first to suggest a parasite‐control function of such a detailed component of bill morphology across species. Avian biologists have traditionally focused almost exclusively on bills as tools for feeding. We suggest that the adaptive radiation of bill morphology should be reinterpreted with both preening and feeding in mind.     

Characterization of 10 microsatellite loci in an avian louse, Degeeriella regalis (Phthiraptera: Ischnocera: Philopteridae)

We isolated and characterized 10 polymorphic microsatellite loci in an ischnoceran louse, Degeeriella regalis, which parasitizes the threatened Galápagos hawk (Buteo galapagoensis) and other falconiform birds. The loci were screened across 30 individuals from two island populations in the Galápagos Islands. The number of alleles per locus ranged from two to 28. Polymorphic information content ranged from 0.14 to 0.94 and observed heterozygosity ranged from 0 to 0.67. These markers will be valuable in comparative population genetics studies in this species, which is the focus of a long-term population and disease ecology research program.     

Biogeography explains cophylogenetic patterns in toucan chewing lice

Historically, comparisons of host and parasite phylogenies have concentrated on cospeciation. However, many of these comparisons have demonstrated that the phylogenies of hosts and parasites are seldom completely congruent, suggesting that phenomena other than cospeciation play an important role in the evolution of host-parasite assemblages. Other coevolutionary phenomena, such as host switching, parasite duplication (speciation on the host), sorting (extinction), and failure to speciate can also influence host-parasite assemblages. Using mitochondrial and nuclear protein-coding DNA sequences, I reconstructed the phylogeny of ectoparasitic toucan chewing lice in the Austrophilopterus cancellosus subspecies complex and compared this phylogeny with the phylogeny of the hosts, the Ramphastos toucans, to reconstruct the history of coevolutionary events in this host-parasite assemblage. Three salient findings emerged. First, reconstructions of host and louse phylogenies indicate that they do not branch in parallel, and their cophylogenetic history shows little or no significant cospeciation. Second, members of monophyletic Austrophilopterus toucan louse lineages are not necessarily restricted to monophyletic host lineages. Often, closely related lice are found on more distantly related but sympatric toucan hosts. Third, the geographic distribution of the hosts apparently plays a role in the speciation of these lice. These results suggest that for some louse lineages biogeography may be more important than host associations in structuring louse populations and species, particularly when host life history (e.g., hole nesting) or parasite life history (e.g., phoresis) might promote frequent host switching events between syntopic host species. These findings highlight the importance of integrating biogeographic information into cophylogenetic studies.     

Comprehensive investigation of ectoparasite community and abundance across life history stages of avian host

Fitness consequences of ectoparasitism are expressed over the lifetime of their hosts in relation to variation in composition and abundance of the entire ectoparasite community and across all host life history stages. However, most empirical studies have focused on parasite species-specific effects and only during some life history stages. We conducted a systematic, year-long survey of an ectoparasite community in a wild population of house finches Carpodacus mexicanus Müller in south-western Arizona, with a specific focus on ecological and behavioral correlates of ectoparasite prevalence and abundance. We investigated five ectoparasite species: two feather mite genera – both novel for house finches – Strelkoviacarus (Analgidae) and Dermoglyphus (Dermoglyphidae), the nest mite Pellonyssus reedi (Macronyssidae), and the lice Menacanthus alaudae (Menoponidae) and Ricinus microcephalus (Ricinidae). Mite P. reedi and louse Menacanthus alaudae abundance peaked during host breeding season, especially in older birds, whereas feather mite abundance peaked during molt. Overall, breeding birds had more P. reedi than non-breeders, molting males had greater abundance of feather mites than molting females and non-molting males, and young males had more feather mites than older males. We discuss these results in relation to natural history of ectoparasites under study and suggest that ectoparasites might synchronize their life cycles to those of their hosts. Pronounced differences in relative abundance of ectoparasite species among host's life history stages have important implications for evolution of parasite-specific host defenses.     

Independent origins of the feather lice (Insecta: Degeeriella) of raptors

Although diurnal birds of prey have historically been placed in a single order due to a number of morphological characters, recent molecular phylogenies have suggested that this is a case of convergence rather than homology, with hawks (Accipitridae) and falcons (Falconidae) forming two distantly related groups within birds. The feather lice of birds have often been used as a model for comparing host and parasite phylogenies, and in some cases there is significant congruence between the two. Thus, studying the phylogeny of the lice of diurnal raptors may be of particular interest with respect to the independent evolution of hawks vs. falcons. Using one mitochondrial gene and three nuclear genes, we inferred a phylogeny for the feather louse genus Degeeriella (which are all obligate raptor ectoparasites) and related genera. This phylogeny indicated that Degeeriella is polyphyletic, with lice from falcons vs. hawks forming two distinct clades. Falcon lice were sister to lice from African woodpeckers, whereas Capraiella, a genus of lice from rollers lice, was embedded within Degeeriella from hawks. This phylogeny showed significant geographical structure, with host geography playing a larger role than host taxonomy in explaining louse phylogeny, particularly within clades of closely related lice. However, the louse phylogeny does reflect host phylogeny at a broad scale; for example, lice from the hawk genus Accipiter form a distinct clade. © 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 114 , 837–847.     

Host densities as determinants of abundance in parasite communities

Several epidemiological models predict a positive relationship between host population density and abundance of directly transmitted macroparasites. Here, we generalize these, and test the prediction by a comparative study. We used data on communities of gastrointestinal strongylid nematodes from 19 mammalian species, representing examination of 6670 individual hosts. We studied both the average abundance of all strongylid nematodes within a host species, and the two components of abundance, prevalence and intensity. The effects of host body weight, diet, fecundity and age at maturity and parasite body size were controlled for directly, and the phylogenetically independent contrast method was used to control for confounding factors more generally. Host population density and average parasite abundance were strongly positively correlated within mammalian taxa, and across all species when the effects of host body weight were controlled for. Controlling for other variables did not change this. Even when looking at single parasite species occurring in several host species, abundance was highest in the host species with the highest population density. Prevalence and intensity showed similar patterns. These patterns provide the first macroecological evidence consistent with the prediction that transmission rates depend on host population density in natural parasite communities.     

Host behaviour drives parasite genetics at multiple geographic scales: population genetics of the chewing louse,Thomomydoecus minor

Pocket gophers and their symbiotic chewing lice form a host–parasite assemblage known for a high degree of cophylogeny, thought to be driven by life history parameters of both host and parasite that make host switching difficult. However, little work to date has focused on determining whether these life histories actually impact louse populations at the very fine scale of louse infrapopulations (individuals on a single host) at the same or at nearby host localities. We used microsatellite and mtDNA sequence data to make comparisons of chewing‐louse (Thomomydoecus minor) population subdivision over time and over geographic space where there are different potential amounts of host interaction surrounding a zone of contact between two hybridizing pocket‐gopher subspecies. We found that chewing lice had high levels of population isolation consistent with a paucity of horizontal transmission even at the very fine geographic scale of a single alfalfa field. We also found marked genetic discontinuity in louse populations corresponding with host subspecies and little, if any, admixture in the louse genetic groups even though the lice are closely related. The correlation of louse infrapopulation differentiation with host interaction at multiple scales, including across a discontinuity in pocket‐gopher habitat, suggests that host behaviour is the primary driver of parasite genetics. This observation makes sense in light of the life histories of both chewing lice and pocket gophers and provides a powerful explanation for the well‐documented pattern of parallel cladogenesis in pocket gophers and chewing lice.     

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.     

Local parasite lineage sharing in temperate grassland birds provides clues about potential origins of Galapagos avian Plasmodium

Oceanic archipelagos are vulnerable to natural introduction of parasites via migratory birds. Our aim was to characterize the geographic origins of two Plasmodium parasite lineages detected in the Galapagos Islands and in North American breeding bobolinks (Dolichonyx oryzivorus) that regularly stop in Galapagos during migration to their South American overwintering sites. We used samples from a grassland breeding bird assemblage in Nebraska, United States, and parasite DNA sequences from the Galapagos Islands, Ecuador, to compare to global data in a DNA sequence registry. Homologous DNA sequences from parasites detected in bobolinks and more sedentary birds (e.g., brown‐headed cowbirds Molothrus ater, and other co‐occurring bird species resident on the North American breeding grounds) were compared to those recovered in previous studies from global sites. One parasite lineage that matched between Galapagos birds and the migratory bobolink, Plasmodium lineage B, was the most common lineage detected in the global MalAvi database, matching 49 sequences from unique host/site combinations, 41 of which were of South American origin. We did not detect lineage B in brown‐headed cowbirds. The other Galapagos‐bobolink match, Plasmodium lineage C, was identical to two other sequences from birds sampled in California. We detected a close variant of lineage C in brown‐headed cowbirds. Taken together, this pattern suggests that bobolinks became infected with lineage B on the South American end of their migratory range, and with lineage C on the North American breeding grounds. Overall, we detected more parasite lineages in bobolinks than in cowbirds. Galapagos Plasmodium had similar host breadth compared to the non‐Galapagos haemosporidian lineages detected in bobolinks, brown‐headed cowbirds, and other grassland species. This study highlights the utility of global haemosporidian data in the context of migratory bird–parasite connectivity. It is possible that migratory bobolinks bring parasites to the Galapagos and that these parasites originate from different biogeographic regions representing both their breeding and overwintering sites.     

Genetic variation in infestation with a directly transmitted ectoparasite

Genetic variation in levels of parasitism of hosts is an underlying assumption of studies of coevolution, but few such estimates are available from the field. We studied genetic variation in the abundance of the chewing louse Hirundoecus malleus on its barn swallow host Hirundo rustica. These parasites are directly transmitted and a test of genetic variation of parasite abundance would thus provide a particularly strong test. The prevalence and the abundance of the chewing lice did not differ significantly between adult male and female hosts. The resemblance in parasite intensity of H. malleus of offspring and their parents was positive and highly significant, and an analysis of extra-pair paternity in the host allowed partitioning of this resemblance between genetic and common environment effects. There was no significant resemblance in parasite intensity between extra-pair offspring and their foster parents, although the resemblance remained for within-pair offspring. This provides evidence for the abundance of directly transmitted parasites having an additive genetic component. We found no evidence of common environment effects as parents did not resemble each other with respect to lice abundance.     

Host sympatry and body size influence parasite straggling rate in a highly connected multihost,multiparasite system

Parasite lineages commonly diverge when host lineages diverge. However, when large clades of hosts and parasites are analyzed, some cases suggest host switching as another major diversification mechanism. The first step in host switching is the appearance of a parasite on an atypical host, or “straggling.” We analyze the conditions associated with straggling events. We use five species of colonially nesting seabirds from the Galapagos Archipelago and two genera of highly specific ectoparasitic lice to examine host switching. We use both genetic and morphological identification of lice, together with measurements of spatial distribution of hosts in mixed breeding colonies, to test: (1) effects of local host community composition on straggling parasite identity; (2) effects of relative host density within a mixed colony on straggling frequency and parasite species identity; and (3) how straggling rates are influenced by the specifics of louse attachment. Finally, we determine whether there is evidence of breeding in cases where straggling adult lice were found, which may indicate a shift from straggling to the initial stages of host switching. We analyzed more than 5,000 parasite individuals and found that only ~1% of lice could be considered stragglers, with ~5% of 436 host individuals having straggling parasites. We found that the presence of the typical host and recipient host in the same locality influenced straggling. Additionally, parasites most likely to be found on alternate hosts are those that are smaller than the typical parasite of that host, implying that the ability of lice to attach to the host might limit host switching. Given that lice generally follow Harrison's rule, with larger parasites on larger hosts, parasites infecting the larger host species are less likely to successfully colonize smaller host species. Moreover, our study supports the general perception that successful colonization of a novel host is extremely rare, as we found only one nymph of a straggling species, which may indicate successful reproduction.     

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.     

Life history and virulence are linked in the ectoparasitic salmon louse Lepeophtheirus salmonis

Models of virulence evolution for horizontally transmitted parasites often assume that transmission rate (the probability that an infected host infects a susceptible host) and virulence (the increase in host mortality due to infection) are positively correlated, because higher rates of production of propagules may cause more damages to the host. However, empirical support for this assumption is scant and limited to microparasites. To fill this gap, we explored the relationships between parasite life history and virulence in the salmon louse, Lepeophtheirus salmonis, a horizontally transmitted copepod ectoparasite on Atlantic salmon Salmo salar. In the laboratory, we infected juvenile salmon hosts with equal doses of infective L. salmonis larvae and monitored parasite age at first reproduction, parasite fecundity, area of damage caused on the skin of the host, and host weight and length gain. We found that earlier onset of parasite reproduction was associated with higher parasite fecundity. Moreover, higher parasite fecundity (a proxy for transmission rate, as infection probability increases with higher numbers of parasite larvae released to the water) was associated with lower host weight gain (correlated with lower survival in juvenile salmon), supporting the presence of a virulence–transmission trade‐off. Our results are relevant in the context of increasing intensive farming, where frequent anti‐parasite drug use and increased host density may have selected for faster production of parasite transmission stages, via earlier reproduction and increased early fecundity. Our study highlights that salmon lice, therefore, are a good model for studying how human activity may affect the evolution of parasite virulence.     

INTERRELATIONSHIPS OF THE SPANISH MACKERELS (PISCES: SCOMBRIDAE: SCOMBEROMORUS) AND THEIR COPEPOD PARASITES

Abstract— A cladistic analysis of the eighteen species of Spanish mackerels (Scomberomorus) was conducted using the double-lined mackerel (Grammatorcynus) as the outgroup. Based on fifty-eight osteological and morphological characters, six species groups are recognized. Comparisons of the cladogram for the regalis species group were made with a cladogram of the nine species of parasitic copepods that infest the six members of the regalis group. Incongruences were analyzed. The parasite tree was then "forced" onto the host cladogram to determine which evolutionary events of the parasites could be explained by evolutionary events of the hosts. Hypotheses of coevolution are supported in some cases, hypotheses of dispersal are proposed in others.     

Sources of variability in aggregation and sex ratios of Crataerina melbae (Diptera: Hippoboscidae) among adult colonial alpine swifts

Aggregation of Crataerina melbae flies on breeding adult alpine swifts (Apus melba) was low when compared with other host-parasite systems and varied with sampling date, year, and sex of the flies. Generalized linear models were performed to ascertain which factors, extrinsic and/or intrinsic to the host, explained variability in the number of louse flies present on a single host, i.e., abundance. Overall abundance was unrelated to any host characteristic but varied slightly among years. Abundance of female flies varied among years, but also with date of sampling, the number of females increasing as the breeding season advanced. In contrast, abundance of males decreased as the season progressed, independently of host characteristics. Despite these different patterns, the number of flies of each sex on a given host was strongly intercorrelated. These results suggest that mate attraction may explain aggregation patterns in this louse fly species. Overall sex ratio of louse flies did not differ from unity. However, the proportion of males decreased during the breeding season, as a consequence of the opposite sex-related seasonal patterns in parasite abundance. Sex-ratio variability was not related to host characteristics or to infrapopulation sizes.     

Intra- and interspecific competition among helminth parasites: effects on Coitocaecum parvum life history strategy, size and fecundity

Larval helminths often share intermediate hosts with other individuals of the same or different species. Competition for resources and/or conflicts over transmission routes are likely to influence both the association patterns between species and the life history strategies of each individual. Parasites sharing common intermediate hosts may have evolved ways to avoid or associate with other species depending on their definitive host. If not, individual parasites could develop alternative life history strategies in response to association with particular species. Three sympatric species of helminths exploit the amphipod Paracalliope fluviatilis as an intermediate host in New Zealand: the acanthocephalan Acanthocephalus galaxii, the trematode Microphallus sp. and the progenetic trematode Coitocaecum parvum. Adult A. galaxii and C. parvum are both fish parasites whereas Microphallus sp. infects birds. We found no association, either positive or negative, among the three parasite species. The effects of intra- and interspecific interactions were also measured in the trematode C. parvum. Both intra- and interspecific competition seemed to affect both the life history strategy and the size and fecundity of C. parvum. Firstly, the proportion of progenesis was higher in metacercariae sharing their host with Microphallus sp., the bird parasite, than in any other situation. Second, the intensity of intraspecific competition apparently constrained the ability of metacercariae to adopt progenesis and limited both the growth and egg production of progenetic individuals. These results show that the life history strategy adopted by a parasite may be influenced by other parasites sharing the same host.