Body size,trophic level,and the use of fish as transmission routes by parasites

Within food webs, trophically transmitted helminth parasites use predator–prey links for their own transfer from intermediate prey hosts, in which they occur as larval or juvenile stages, to predatory definitive hosts, in which they reach maturity. In large taxa that can be used as intermediate and/or definitive hosts, such as fish, a host species’ position within a trophic network should determine whether its parasite fauna consists mostly of adult or larval helminths, since vulnerability to predation determines an animal’s role in predator–prey links. Using a large database on the helminth parasites of 303 fish species, we tested whether the proportion of parasite species in a host that occur as larval or juvenile stages is best explained by their trophic level or by their body size. Independent of fish phylogeny or habitat, only fish body length emerged as a significant predictor of the proportion of parasites in a host that occur as larval stages from our multivariate analyses. On average, the proportion of larval helminth taxa in fish shorter than 20 cm was twice as high as that for fish over 100 cm in length. This is consistent with the prediction that small fishes, being more vulnerable to predation, make better hosts for larval parasites. However, trophic level and body length are strongly correlated among fish species, and they may have separate though confounded effects on the parasite fauna exploiting a given species. Helminths show varying levels of host specificity toward their intermediate host when the latter is the downstream host involved in trophic transmission toward an upstream definitive host. Given this broad physiological compatibility of many helminths with fish hosts, our results indicate that fish body length, as a proxy for vulnerability to predators, is a better predictor of their use by helminth larvae than their trophic level based on diet content.     

Comparing the richness of metazoan ectoparasite communities of marine fishes: controlling for host phylogeny

Parasite communities are the product of acquisitions and losses of parasite species during the evolutionary history of their host. When comparing the parasite communities of different host species to assess the role of ecological variables as determinants of parasite species richness, a correction must be made for the possible phylogenetic inheritance of parasites from ancestral hosts independent of host ecology. We performed a comparative analysis of the metazoan ectoparasite communities on the heads and gills of 111 species of marine fish. The influences of host body size, host schooling behaviour and water temperature were tested after controlling for both sampling and phylogenetic effects. Overall, water temperature correlated positively with both parasite species richness and abundance, whereas fish size only correlated with parasite abundance. The correlation across all fish species between water temperature and parasite species richness was dependent on an outlier point. The results, however, generally held when fish from different biogeographical areas (Pacific and Atlantic) were analysed separately. In all analyses, parasite species richness always correlated strongly with parasite abundance. There was no evidence that schooling fish taxa harboured richer or more abundant ectoparasite communities than their non-schooling sister taxa, possibly because of the small number of contrasts available for that test. Overall, whereas both water temperature and host size affect the number of parasite individuals that can be harboured by a fish, only temperature appears important as a determinant of ectoparasite community richness. Received: 30 May 1996 / Accepted: 23 October 1996     

Avoidance of host resistance in the oviposition-site preferences of rose bitterling

A contemporary outcome of dynamic host–parasite coevolution can be driven by the adaptation of a parasite to exploit its hosts at the population and species levels (parasite specialisation) or by local host adaptations leading to greater host resistance to sympatric parasite populations (host resistance). We tested the predominance of these two scenarios using cross-infection experiments with two geographically distant populations of the rose bitterling, Rhodeus ocellatus, a fish brood parasite of freshwater mussels, and four populations of their mussel hosts (two Anodonta woodiana and two Unio douglasiae populations) with varying degrees of geographic sympatry and local coexistence. Our data support predictions for host resistance at the species level but no effect of local coexistence between specific populations. Rhodeus ocellatus showed a preference for allopatric host populations, irrespective of host species. Host mussel response, in terms of ejection of R. ocellatus eggs, was stronger in the more widespread and abundant host species (A. woodiana) and this response tended to be higher in sympatric populations. These outcomes provide support for the importance of host resistance in bitterling oviposition-site decisions, demonstrating that host choice by R. ocellatus is adaptive by minimizing egg ejections. These findings imply that R. ocellatus, and potentially other bitterling species, may benefit from exploiting novel hosts, which may not possess appropriate adaptive responses to parasitism.     

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.     

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

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Determinants of host-specificity in parasites of freshwater fishes.

Factors responsible for interspecific variability in host-specificity were investigated within 15 genera (including 176 species) of metazoan parasites found in Canadian freshwater fish. For each species in a genus, the parasite's number of known hosts was determined from published host-parasite records. The effects of the total number and mean size of potential hosts (i.e. all fish species belonging to the family or families that include a parasite's known hosts) on number of hosts of congeneric species were evaluated using multiple regressions. Since parasite species that have been recorded often tend to have greater numbers of known hosts than do seldom-recorded parasites, it was necessary to control for the confounding effect of study intensity. In all parasite genera, whether from highly specific taxa such as monogeneans or from less host-specific ones, there was a positive relationship between the number of potential hosts and the number of known hosts. However, no consistent relationships were observed between the mean size of potential hosts and number of known hosts. These results suggest that the availability of suitable host species may have been a key factor limiting the colonization of new hosts by fish parasites.     

Experimental infections with the tropical monogenean,Gyrodactylus bullatarudis: Potential invader or experimental fluke?

Introduced exotic species have the potential to spread their associated parasites to native species which can be catastrophic if these hosts are immunologically naïve to the novel parasite. The guppy (Poecilia reticulata) has been disseminated worldwide outside of its native habitat and therefore could be an important source of infection to native fish species. Its parasite fauna is dominated by the ectoparasitic monogeneans, Gyrodactylus turnbulli and Gyrodactylus bullatarudis. The current study tested the host specificity of G. bullatarudis by experimentally infecting a range of isolated fish hosts, including temperate species. Surprisingly, the parasite was capable of establishing and reproducing, for several days, on the three-spined stickleback when transferred directly to this host. We also established that G. bullatarudis could be transmitted under aquarium conditions at both 25 °C and 15 °C. At the higher temperature, the parasite was even capable of reproducing on this atypical host. The implications of these findings are discussed in terms of host specificity, host switching and climate change.     

Experimental infections of the monogenean Gyrodactylus turnbulli indicate that it is not a strict specialist

Parasites represent a threat to endangered fish species, particularly when the parasite can host switch and the new host is vulnerable. If the parasite is highly host specific then successful host switching should be a rare occurrence; however, the host range of many parasites which are assumed to be specialists has never been tested. This includes the monogenean Gyrodactylus turnbulli, a well-studied ectoparasite found caudally on its known host, the guppy, Poecilia reticulata. In this study, we monitored parasite establishment and reproduction on a range of poeciliids and more distantly related fish. Individually maintained fish were experimentally infected with a single parasite and monitored daily to establish whether G. turnbulli could survive and reproduce on other fish species. Gyrodactylus turnbulli can infect a wider range of hosts than previously considered, highlighting the fact that host specificity can never be assumed unless experimentally tested. Our findings also have significant implications for parasite transmission to novel hosts and provide further insight into the evolutionary origins of this ubiquitous group of fish pathogens. Previous molecular evidence indicates that host switching is the key mechanism for speciation within the genus Gyrodactylus. Until recently, most Gyrodactylus spp. were assumed to be narrowly host specific. However, our findings suggest that even so-called specialist species, such as G. turnbulli, may represent a threat to vulnerable fish stocks. In view of the potential importance of host switching under artificial conditions, we propose to describe this as 'artificial ecological transfer' as opposed to 'natural ecological transfer', host switching under natural conditions.     

Predicting the similarity of parasite communities in freshwater fishes using the phylogeny,ecology and proximity of hosts

Parasite communities tend to be dissimilar in hosts that are geographically, phylogenetically, ecologically and developmentally distant from one another. The decay of community similarity is a powerful and increasingly common method of studying parasite beta diversity, but most studies have examined only a single type of distance. Here, we evaluate distances based on the phylogeny, ecology, spatial proximity and size of hosts, as predictors of the similarity of parasite communities in individual hosts, host populations and host species. We surveyed parasites in six species of fish collected simultaneously from six localities in the St. Lawrence River, Canada, and species in a common group of larval parasites were discriminated using DNA sequences from barcode region of cytochrome c oxidase I. Distances based on the habitat use patterns of host species were good predictors of short‐term, ecological similarity of parasite communities, such as that operating at the scale of the individual host. The genetic distance between host species was associated with almost all types of similarity at all scales, particularly qualitative and phylogenetic similarity of parasite communities at the level of populations and meta‐populations of hosts. The trophic level, diet, spatial proximity and size of hosts were poor predictors of parasite community similarity. The increased taxonomic resolution provided by molecular data increased the explanatory power of regression models, and different factors were implicated when parasite species were distinguished with DNA barcodes than when larval parasites were lumped into morphospecies, as is commonly practiced.     

Circulation of parasites among fishes from lakes in the Caribou Mountains,Alberta, Canada

Circulation of 24 macroparasite species among 12 species of fish was evaluated within samples of hosts collected from 9 lakes on an isolated plateau in northern Alberta, Canada. Twenty-seven parasite taxa (24 species plus the larval stages of Triaenophorus crassus, T. stizostedionis, and Raphidascaris acus) had the potential to be circulated among hosts. Sixteen parasite taxa were recovered from a single host species within a lake. Of the 11 remaining nonspecialist taxa, 4 were larval stages that matured in fish or birds and 7 were adults. Eight of the 11 cases of circulation among hosts involved lake whitefish, and this host was involved in the transmission of 5 species to piscivorous fishes. Despite evidence for the circulation of 7 taxa among the 4 species of sympatric Salmonidae, 60-99% of all worms were recovered from just 1 species of host. These results indicated that approximately 60% of the parasite taxa that infected fish in these lakes were absolute host specialists. The remaining 40% of parasite taxa had restricted host ranges, with most examples of parasite circulation limited to the 2 species of sympatric coregonid.     

Do Parasitic Trematode Cercariae Demonstrate a Preference for Susceptible Host Species?

Many parasites are motile and exhibit behavioural preferences for certain host species. Because hosts can vary in their susceptibility to infections, parasites might benefit from preferentially detecting and infecting the most susceptible host, but this mechanistic hypothesis for host-choice has rarely been tested. We evaluated whether cercariae (larval trematode parasites) prefer the most susceptible host species by simultaneously presenting cercariae with four species of tadpole hosts. Cercariae consistently preferred hosts in the following order: Anaxyrus ( = Bufo) terrestris (southern toad), Hyla squirella (squirrel tree frog), Lithobates ( = Rana) sphenocephala (southern leopard frog), and Osteopilus septentrionalis (Cuban tree frog). These host species varied in susceptibility to cercariae in an order similar to their attractiveness with a correlation that approached significance. Host attractiveness to parasites also varied consistently and significantly among individuals within a host species. If heritable, this individual-level host variation would represent the raw material upon which selection could act, which could promote a Red Queen “arms race” between host cues and parasite detection of those cues. If, in general, motile parasites prefer to infect the most susceptible host species, this phenomenon could explain aggregated distributions of parasites among hosts and contribute to parasite transmission rates and the evolution of virulence. Parasite preferences for hosts belie the common assumption of disease models that parasites seek and infect hosts at random.     

Demersal fish parasite fauna around the South Shetland Islands: high species richness and low host specificity in deep Antarctic waters

A total of nine Antarctic fish species belonging to five families were examined for their endohelminth parasite fauna. The fishes Parachaenichthys charcoti (Bathydraconidae), Chaenocephalus aceratus (Channichtyidae), Paradiplospinus gracilis (Gempylidae), Muraenolepis microps (Muraenolepididae), Gobionotothen gibberifrons, Lepidonotothen larseni, L. nudifrons, L. squamifrons, and Trematomus eulepidopus (Nototheniidae) were caught between 80 and 608 m trawling depth off the Antarctic Peninsula (Elephant Island, King George Island) in 1996. Nineteen different parasites species comprising five Digenea, two Cestoda, four Nematoda, and eight Acanthocephala were found. Pseudophyllidean cestodes, the nematodes Contracaecum radiatum and C. osculatum as well as the acanthocephalan Corynosoma bullosum were the most common, infesting eight of the fish species studied with prevalences reaching 100%. Pseudoterranova decipiens s.l. was the only parasite that was isolated from all studied fish species; however, at a lower intensity. The observed parasite host specificity was low, and the species richness in a single fish ranged from one to eleven in a C. aceratus. This icefish and the moray cod M. microps were the most heavily infested fish, harbouring many adult and larval parasitic stages. The benthodemersal P. gracilis had only two larval parasite species, while the nototheniids had very similar parasite communities, harbouring a total of 8–14 species. Larval mammalian parasites were found to utilize fish, especially the nototheniids and channichthyids, as a common transmission route into their final hosts. The fish parasites parallel explored different benthic host systems to reach the most suitable host. In contrast to the coast and continental shelf, the meso/bathypelagiac zone appears to be species poor and is inhabited by few larval forms. The fish parasite fauna off the South Shetland Islands can be characterized by generalistic parasites that distribute within Antarctic waters according to the feeding ecology and depth range of their teleost hosts, not only horizontally but also extending vertically into the deep sea.     

Parasitic community of Fransciscodoras marmoratus (Reinhardt, 1874) (Pisces: Siluriformes, Doradidae) from the upper S?o Francisco River, Brazil.

One hundred and thirteen specimens of Franciscodoras marmoratus (Reinhardt, 1874) were collected in the upper S?o Francisco River (18 degrees 12' 32" S, 45 degrees 15' 41" W, state of Minas Gerais) between September, 1999 and January, 2004 to investigate their parasite fauna. From this total, 45 (39.8%) were afflicted by at least one parasite species. The parasitic richness consisted of six species represented by Hirudinea (n = 20), Monogenoidea (n = 25), Eucestoda (n = 55), Nematoda (n = 1, n = 2) and Acanthocephala (n = 41) found in the dry and wet periods making a total of 144 specimens. Proteocephalus renaudi Chambrier & Vaucher, 1994 was the only species with prevalence higher than 10% and a typical aggregate distribution pattern. The prevalence, intensity and abundance of P. renaudi were not influenced by the total length or sex of the hosts or by the collection periods. The relative condition factor indicated that the health of the P. renaudi hosts was not significantly affected in relation to fish not infected by parasites. The fish stocked in tanks before necropsy were opportunistically infested by Lernaea cyprinacea Yashuv, 1959. The various parasites found indicate that F. marmoratus is omnivorous and a potential definitive host. The parasite species, except for Acanthocephala, have expanded their known geographic distribution to the S?o Francisco River Basin. The parasite community was considered isolationist because of the low endoparasite diversity, infrapopulations with low intensity, lack of evidence of parasite interactions and sparse signs of parasite aggression against their hosts.     

Native and invasive hosts play different roles in host–parasite networks

Parasites are often key players in biological invasions since they can mediate the impact of host invasions or can themselves become invasive species. However, the nature and extent of parasite-mediated invasions are often difficult to delineate. Here, we used individual-based, weighted bipartite networks to study the roles (degrees of interactions of individuals in a modular network according to their within- and among-module connections) played by native and invasive host individuals to their parasite communities. We studied two phylogenetically and ecologically close fish species, Mugil cephalus s.l. and Planiliza haematocheilus (Teleostei: Mugilidae). Planiliza haematocheilus is native to the Sea of Japan and invasive in the Sea of Azov whereas, M. cephalus s.l. is native to both seas. Based on the common evolutionary history that drives native host–parasite networks, we hypothesised that 1) native networks have higher modularity than invaded ones; and 2) invasive hosts in the invaded area play a peripheral role to structure parasite communities. We analysed the whole parasite community and subsets based on transmission strategy and host specificity of the parasite species to establish whether modularity and host roles are related to these features in the native and invaded areas. All networks were found to be modular. However, modularity tended to be higher in networks of the native area rather than those of the invaded area. Host individuals of both fish species played similar roles in the native area, whereas invasive hosts played a peripheral role in the networks of the invaded area. We propose that long-term monitoring of the roles of invasive hosts in parasite communities can be a useful proxy for estimating the maturity of the establishment of the invasive hosts in an ecosystem.     

Host manipulation by Ligula intestinalis: a cause or consequence of parasite aggregation?

Previous investigations suggest that the infection of the cyprinid roach, Rutilus rutilus, with the larval plerocercoid forms of the cestode, Ligula intestinalis, creates behavioural and morphological changes in the fish host, potentially of adaptive significance to the parasite in promoting transmission to definitive avian hosts. Here we consider whether these behavioural changes are important in shaping the distribution of parasite individuals across the fish population. An examination of field data illustrates that fish infected with a single parasite were more scarce than expected under the negative binomial distribution, and in many months were more scarce than burdens of two, three or more, leading to a bimodal distribution of worm counts (peaks at 0 and >1). This scarcity of single-larval worm infections could be accounted for a priori by a predominance of multiple infection. However, experimental infections of roach gave no evidence for the establishment of multiple worms, even when the host was challenged with multiple intermediate crustacean hosts, each multiply infected. A second hypothesis assumes that host manipulation following an initial single infection leads to an increased probability of subsequent infection (thus creating a contagious distribution). If manipulated fish are more likely to encounter infected first-intermediate hosts (through microhabitat change, increased ingestion, or both), then host manipulation could act as a powerful cause of aggregation. A number of scenarios based on contagious distribution models of aggregation are explored, contrasted with alternative compound Poisson models, and compared with the empirical data on L. intestinalis aggregation in their roach intermediate hosts. Our results indicate that parasite-induced host manipulation in this system can function simultaneously as both a consequence and a cause of parasite aggregation. This mutual interaction between host manipulation and parasite aggregation points to a set of ecological interactions that are easily missed in most experimental studies of either phenomenon.     

Ecological determinants of parasite acquisition by exotic fish species

Disease‐mediated threats posed by exotic species to native counterparts are not limited to introduced parasites alone, since exotic hosts frequently acquire native parasites with possible consequences for infection patterns in native hosts. Several biological and geographical factors are thought to explain both the richness of parasites in native hosts, and the invasion success of free‐living exotic species. However, the determinants of native parasite acquisition by exotic hosts remain unknown. Here, we investigated native parasite communities of exotic freshwater fish to determine which traits influence acquisition of native parasites by exotic hosts. Model selection suggested that five factors (total body length, time since introduction, phylogenetic relatedness to the native fish fauna, trophic level and native fish species richness) may be linked to native parasite acquisition by exotic fish, but 95% confidence intervals of coefficient estimates indicated these explained little of the variance in parasite richness. Based on R²‐values, weak positive relationships may exist only between the number of parasites acquired and either host size or time since introduction. Whilst our results suggest that factors influencing parasite richness in native host communities may be less important for exotic species, it seems that analyses of general ecological factors currently fail to adequately incorporate the physiological and immunological complexity of whether a given animal species will become a host for a new parasite.     

Ecology of the acanthocephalan Sclerocollum rubrimaris Schmidt and Paperna, 1978 (Rhadinorhynchidae: Gorgorhynchinae) from wild populations of rabbitfish (genus Siganus) in the northern Red Sea

The abundance, host relationships and microhabitat of Sclerocollum rubrimaris Schmidt & Paperna. 1978 (Rhadinorhynchidae: Gorgorhynchinae) were investigated in wild populations of the herbivorous rabbitfish ( Siganus spp.) from the Gulf of Eilat, northern Red Sea. Siganus rivulatus (Forsskål, 1775) and S. argenteus (Quoy & Gaimard, 1825), judging by the high infection prevalences, are the primary definite hosts, whereas S. luridus (Ruppell, 1828) was only rarely infected and thus regarded as an accessory host. Overdispersion of S. rubrimaris suprapopu-lations was greatest in S. rivulatus. Juvenile worms occurred year-round, but enhanced recruitment was evident in both host populations between February and July. A high parasite abundance observed during spring (March-May) was correlated with an extensive period of pre-spawning algal grazing by the host fish. In S. rivulatus , parasite abundance increased with increasing host size, whereas in S. argenteus it was highest in young hosts, and decreased in larger fish. The variability of infection patterns in the three rabbitfish species probably stems not only from parasite specificity but also from the different habitats, feeding habits and diet preferences. Sclerocollum rubrimaris has a restricted host range and is not known to infect other herbivorous coral reef fish. The main region of attachment of S. rubrimaris is the anterior 5–25% of the gut in both S. rivulatus and S. argenteus , and the mean position of immature worms is significantly more posterior than that of sexually mature individuals.     

Distance decay of similarity among parasite communities of three marine invertebrate hosts

The similarity in species composition between two communities generally decays as a function of increasing distance between them. Parasite communities in vertebrate definitive hosts follow this pattern but the respective relationship in intermediate invertebrate hosts of parasites with complex life cycles is unknown. In intermediate hosts, parasite communities are affected not only by the varying vagility of their definitive hosts (dispersing infective propagules) but also by the necessary coincidence of all their hosts in environmentally suitable localities. As intermediate hosts often hardly move they do not contribute to parasite dispersal. Hence, their parasite assemblages may decrease faster in similarity with increasing distance than those in highly mobile vertebrate definitive hosts. We use published field survey data to investigate distance decay of similarity in trematode communities from three prominent coastal molluscs of the Eastern North-Atlantic: the gastropods Littorina littorea and Hydrobia ulvae, and the bivalve Cerastoderma edule. We found that the similarity of trematode communities in all three hosts decayed with distance, independently of local sampling effort, and whether or not the parasites used the mollusc as first or second intermediate host in their life cycle. In H. ulvae, the halving distance (i.e. the distance that halves the similarity from its initial similarity at 1 km distance) for the trematode species using birds as definitive hosts was approximately two to three times larger than for species using fish. The initial similarities (estimated at 1 km distance) among trematode communities were relatively higher, whereas mean halving distances were lower, compared to published values for parasite communities in vertebrate hosts. We conclude that the vagility of definitive hosts accounts for a high similarity at the local scale, while the strong decay of similarity across regions is a consequence of the low probability that all necessary hosts and suitable environmental conditions coincide on a large scale.     

指环虫属的早期辐射及其与宿主鲤科鱼类的协同进化关系

本研究利用28SrDNAC1-D2区序列分析采自鲤科鱼类中6亚科宿主和寄生在花鲈、梅花鲈上的共17种指环虫的系统发育关系。同时,通过比较宿主鲤科鱼类与指环虫的系统发育树,检验指环虫与其宿主是否存在协同进化关系。结果表明:17种指环虫形成5个进化支(Clade),其中寄生在团头鲂(亚科)和鲢、鳙(鲢亚科)上的6种指环虫聚为一支(Clade1),而它们的宿主鱼类在系统发育分析中也表现为近缘关系;寄生在鲮鱼(野鲮亚科)上的D.quanfami(Clade5)位于系统树最基部,鲫鱼和鲤鱼(鲤亚科)的寄生指环虫处在系统树的次基部位置,而鲤亚科与野鲮亚科组成的姐妹群在宿主系统树上同样处在基部位置,寄生虫和宿主在进化上较为原始的地位得到了很好地相互印证。因而,本研究首次利用分子系统学手段分析指环虫属远缘物种间的系统关系,揭示了指环虫属与宿主鱼类之间存在协同进化关系。另外,本研究首次发现,野鲮亚科鱼类也可能是指环虫类的早期宿主,这与先前认为鲤亚科鱼类为指环虫类的祖先宿主的推测有所不同。     

Nested assemblages resulting from host size variation: the case of endoparasite communities in fish hosts

Nested species subsets are a common pattern in many types of communities found in insular or fragmented habitats. Nestedness occurs in some communities of ectoparasites of fish, as does the exact opposite departure from random assembly, anti-nestedness. Here, we looked for nested and anti-nested patterns in the species composition of communities of internal parasites of 23 fish populations from two localities in Finland. We also compared various community parameters of nested and anti-nested assemblages of parasites, and determined whether nestedness may result simply from a size-related accumulation of parasite species by feeding fish hosts. Nested parasite communities were characterised by higher prevalence (proportion of infected fish) and intensities of infection (number of parasites per fish) than anti-nested communities; the two types of non-random communities did not differ with respect to parasite species richness, however. In addition, the correlation between fish size and the number of parasite species harboured by individual fish was much stronger in nested assemblages than in anti-nested ones, where it was often nil. These results were shown not to be artefacts of sampling effort or host phylogeny. They apply to both assemblages of adult and larval parasites, which were treated separately. Since species of larval parasites are extremely unlikely to interact with one another in fish hosts, the establishment of nestedness appears independent of the potential action of interspecific interactions. The species composition of these parasite communities is not determined from within the community, but rather by the extrinsic influence of host feeding rates and how they amplify differences among parasite species in probabilities of colonisation or extinction. Nested patterns occur in parasite communities whose fish hosts accumulate parasites in a predictable fashion proportional to their size, whereas anti-nested communities occur in parasite communities whose fish hosts do not, possibly because of dietary specialisation preventing them from sampling the entire pool of parasite species available locally. Thus, nestedness in parasite communities may result from processes somewhat different from those generating nested patterns in free-living communities.