New insights into avian malaria infections in New Zealand seabirds.

The past few years have been marked by a drastic increase in pathogen spillover events. However, the extent and taxonomic range at which these events take place remain as crucial unanswered questions in many host-pathogen systems. Here, we take advantage of opportunistically sampled bird carcasses from the South Island of New Zealand, with the aim of identifying Plasmodium spp. infections in native and endemic New Zealand seabird species. In total, six samples from five bird species were positive for avian malaria, including four of which were successfully sequenced and identified as Plasmodium matutinum LINN1 lineage. These results provide new Plasmodium infection records in seabirds, including the first documented case in Procellariiformes in New Zealand, highlighting the potential disease risk to these species.

The energetic costs of sub-lethal helminth parasites in mammals: a meta-analysis.

Parasites, by definition, have a negative effect on their host. However, in wild mammal health and conservation research, sub-lethal infections are commonly assumed to have negligible health effects unless parasites are present in overwhelming numbers. Here, we propose a definition for host health in mammals that includes sub-lethal effects of parasites on the host's capacity to adapt to the environment and maintain homeostasis. We synthesized the growing number of studies on helminth parasites in mammals to assess evidence for the relative magnitude of sub-lethal effects of infection across mammal taxa based on this expanded definition. Specifically, we develop and apply a framework for organizing disparate metrics of parasite effects on host health and body condition according to their impact on an animal's energetic condition, defined as the energetic burden of pathogens on host physiological and behavioural functions that relate directly to fitness. Applying this framework within a global meta-analysis of helminth parasites in wild, laboratory and domestic mammal hosts produced 142 peer-reviewed studies documenting 599 infection-condition effects. Analysing these data within a multiple working hypotheses framework allowed us to evaluate the relative weighted contribution of methodological (study design, sampling protocol, parasite quantification methods) and biological (phylogenetic relationships and host/parasite life history) moderators to variation in the magnitude of health effects. We found consistently strong negative effects of infection on host energetic condition across taxonomic groups, with unusually low heterogeneity in effect sizes when compared with other ecological meta-analyses. Observed effect size was significantly lower within cross-sectional studies (i.e. observational studies that investigated a sub-set of a population at a single point in time), the most prevalent methodology. Furthermore, opportunistic sampling led to a weaker negative effect compared to proactive sampling. In the model of host taxonomic group, the effect of infection on energetic condition in carnivores was not significant. However, when sampling method was included, it explained substantial inter-study variance; proactive sampling showing a strongly significant negative effect while opportunistic sampling detected only a weak, non-significant effect. This may partly underlie previous assumptions that sub-lethal parasites do not have significant effects on host health. We recommend future studies adopt energetic condition as the framework for assessing parasite effects on wildlife health and provide guidelines for the selection of research protocols, health proxies, and relating infection to fitness.

Corynosoma strumosum (Acanthocephala) infection in marine foraging mink (Neogale vison) and river otter (Lontra canadensis) and associated peritonitis in a juvenile mink.

Corynosoma strumosum (Acanthocephala), a widespread parasite of pinnipeds, is reported in marine foraging North American mink (Neogale vison) and river otter (Lontra canadensis) on Vancouver Island, British Columbia. This is the first confirmed case of infection by C. strumosum in river otters on the west coast of North America and may be the first confirmed case of infection in wild North American mink; C. strumosum has previously been reported in river otters in Europe (Lutra lutra) and in farmed mink fed with marine fish. We also detected a case of acanthocephalan associated peritonitis in a juvenile mink. Furthermore, though infections with Corynosoma spp. are often assumed to be accidental in mustelids, some C. strumosum individuals found in mink showed signs of reproductive activity. These findings indicate that mink may be a competent definitive host and represent a reservoir in coastal habitats although further research is needed to confirm this. Investigating whether river otters may be competent hosts and determine the prevalence of infection in coastal populations would determine the potential implications of C. strumosum for coastal otters and minks. Our report indicates that mink and possibly river otter living in coastal areas are vulnerable to this previously unreported parasitic infection with mortality risk, at least in juvenile individuals.

Parasites shape community structure and dynamics in freshwater crustaceans.

Parasites directly and indirectly influence the important interactions among hosts such as competition and predation through modifications of behaviour, reproduction and survival. Such impacts can affect local biodiversity, relative abundance of host species and structuring of communities and ecosystems. Despite having a firm theoretical basis for the potential effects of parasites on ecosystems, there is a scarcity of experimental data to validate these hypotheses, making our inferences about this topic more circumstantial. To quantitatively test parasites' role in structuring host communities, we set up a controlled, multigenerational mesocosm experiment involving four sympatric freshwater crustacean species that share up to four parasite species. Mesocosms were assigned to either of two different treatments, low or high parasite exposure. We found that the trematode Maritrema poulini differentially influenced the population dynamics of these hosts. For example, survival and recruitment of the amphipod Paracalliope fluviatilis were dramatically reduced compared to other host species, suggesting that parasites may affect their long-term persistence in the community. Relative abundances of crustacean species were influenced by parasites, demonstrating their role in host community structure. As parasites are ubiquitous across all communities and ecosystems, we suggest that the asymmetrical effects we observed are likely widespread structuring forces.

Acanthocephalan infection patterns in amphipods: a reappraisal in the light of recently discovered host cryptic diversity.

Amphipods are model species in studies of pervasive biological patterns such as sexual selection, size assortative pairing and parasite infection patterns. Cryptic diversity (i.e. morphologically identical but genetically divergent lineages) has recently been detected in several species. Potential effects of such hidden diversity on biological patterns remain unclear, but potentially significant, and beg the question of whether we have missed part of the picture by involuntarily overlooking the occurrence and effects of cryptic diversity on biological patterns documented by previous studies. Here we tested for potential effects of cryptic diversity on parasite infection patterns in amphipod populations and discuss the implications of our results in the context of previously documented host-parasite infection patterns, especially amphipod-acanthocephalan associations. We assessed infection levels (prevalence and abundance) of 3 acanthocephalan species (Pomphorhynchus laevis, P. tereticollis and Polymorphus minutus) among cryptic lineages of the Gammarus pulex/G. fossarum species complex and G. roeseli from sampling sites where they occur in sympatry. We also evaluated potential differences in parasite-induced mortality among host molecular operational taxonomic units (MOTUs)-parasite species combinations. Acanthocephalan prevalence, abundance and parasite-induced mortality varied widely among cryptic MOTUs and parasite species; infection patterns were more variable among MOTUs than sampling sites. Overall, cryptic diversity in amphipods strongly influenced apparent infection levels and parasite-induced mortality. Future research on species with cryptic diversity should account for potential effects on documented biological patterns. Results from previous studies may also need to be reassessed in light of cryptic diversity and its pervasive effects.

Differential patterns of definitive host use by two fish acanthocephalans occurring in sympatry: Pomphorhynchus laevis and Pomphorhynchus tereticollis.

Parasites with complex life-cycles and trophic transmission are expected to show low specificity towards final hosts. However, testing this hypothesis may be hampered by low taxonomic resolution, particularly in helminths. We investigated this issue using two intestinal fish parasites with similar life-cycles and occurring in sympatry, Pomphorhynchus laevis and Pomphorhynchus tereticollis (Acanthocephala). We used species-specific ITS1 length polymorphism to discriminate parasite species from 910 adult acanthocephalans collected in 174 individual hosts from 12 fish species. Both P. laevis and P. tereticollis exhibited restricted host range within the community of available fish host species, and transmission bias compared to their relative abundance in intermediate hosts. The two parasites also exhibited low niche overlap, primarily due to their contrasting use of bentho-pelagic (P. laevis) and benthic (P. tereticollis) fish. Furthermore, parasite prevalence in intermediate hosts appeared to increase with taxonomic specificity in definitive host use. Comparison of P. laevis and P. tereticollis adult size in the two main definitive hosts, barbel and chub, suggested lower compatibility towards the fish species with the lowest parasite abundance, in particular in P. laevis. The determinants of low niche overlap between these two sympatric acanthocephalan species, and the contribution of definitive host range diversity to parasite transmission success, are discussed.

Progenesis and facultative life cycle abbreviation in trematode parasites: Are there more constraints than costs?

Complex life cycles provide advantages to parasites (longer life span, higher fecundity, etc.), but also represent a series of unlikely events for which many adaptations have evolved (asexual multiplication, host finding mechanisms, etc.). Some parasites use a radical strategy where the definitive host is dropped; life cycle abbreviation is most often achieved through progenesis (i.e. early maturation) and reproduction in the second intermediate host. In many progenetic species, both the typical and abbreviated life cycles are maintained. However, conditions that trigger the adoption of one or the other strategy, and the pros and cons of each parasite life history strategy, are often complex and poorly understood. We used experimental infections with the trematode Coitocaecum parvum in its fish definitive host to test for potential costs of progenesis in terms of lifespan and fecundity. We show that individuals that adopt progenesis in the intermediate host are still able to establish in the definitive host and achieve higher survival and fecundity than conspecifics adopting the typical three-host life cycle. Our results and that of previous studies show that there seems to be few short-term costs associated with progenesis in C. parvum. Potential costs of self-fertilization and inbreeding are often suggested to select for the maintenance of both life-history strategies in species capable of facultative progenesis. We suggest that, at least for our focal species, there are more constraints than costs limiting its adoption. Progenesis and the abbreviated cycle may become the typical life-history strategy while reproduction in the vertebrate definitive host is now a secondary alternative when progenesis is impossible (e.g. limited host resources, etc.). Whether this pattern can be generalized to other progenetic trematodes is unknown and would require further studies.

Evolution, phylogenetic distribution and functional ecology of division of labour in trematodes.

Division of labour has evolved in many social animals where colonies consist of clones or close kin. It involves the performance of different tasks by morphologically distinct castes, leading to increased colony fitness. Recently, a form of division of labour has been discovered in trematodes: clonal rediae inside the snail intermediate host belong either to a large-bodied reproductive caste, or to a much smaller and morphologically distinct 'soldier' caste which defends the colony against co-infecting trematodes. We review recent research on this phenomenon, focusing on its phylogenetic distribution, its possible evolutionary origins, and how division of labour functions to allow trematode colonies within their snail host to adjust to threats and changing conditions. To date, division of labour has been documented in 15 species from three families: Himasthlidae, Philophthalmidae and Heterophyidae. Although this list of species is certainly incomplete, the evidence suggests that division of labour has arisen independently more than once in the evolutionary history of trematodes. We propose a simple scenario for the gradual evolution of division of labour in trematodes facing a high risk of competition in a long-lived snail host. Starting with initial conditions prior to the origin of castes (size variation among rediae within a colony, size-dependent production of cercariae by rediae, and a trade-off between cercarial production and other functions, such as defence), maximising colony fitness (R0) can lead to caste formation or the age-structured division of labour observed in some trematodes. Finally, we summarise recent research showing that caste ratios, i.e. relative numbers of reproductive and soldier rediae per colony, become more soldier-biased in colonies exposed to competition from another trematode species sharing the same snail, and also respond to other stressors threatening the host's survival or the colony itself. In addition, there is evidence of asymmetrical phenotypic plasticity among individual caste members: reproductives can assume defensive functions against competitors in the absence of soldiers, whereas soldiers are incapable of growing into reproductives if the latter's numbers are reduced. We conclude by highlighting future research directions, and the advantages of trematodes as model systems to study social evolution.

Non-host organisms impact transmission at two different life stages in a marine parasite.

The potential for local biodiversity to 'dilute' infection risk has been shown to be particularly important in aquatic trematodes, where non-host organisms can feed on free-living infective stages (cercariae) and reduce transmission rates to target hosts. Non-host organisms could also impact transmission during other stages of the trematode life cycle. In Philophthalmus spp., cercariae encyst as metacercariae on external surfaces, where they remain exposed to the adverse effects of non-host organisms. In laboratory experiments, we tested the potential for a range of non-host organisms to (i) prey on cercariae, (ii) induce early (i.e., faster) encystment and (iii) prey on or destroy metacercariae. Our results show that intertidal anemones, and to a lesser extent clams, can consume substantial numbers of cercariae. However, we found no strong evidence that the presence of these predators causes cercariae to encyst faster as a way to escape from predation. We also found that grazing snails can reduce numbers of encysted metacercariae, either by eating or crushing them. Our findings add to the growing evidence that trematode transmission success can be strongly affected by the local diversity of non-host organisms. They also reinforce the notion that parasites are potentially important food items for many organisms, thus playing roles other than consumers in many food webs.

Caste ratio adjustments in response to perceived and realised competition in parasites with division of labour.

Colonial organisms with division of labour are assumed to achieve increased colony-level efficiency in task performance through functional specialisation of individuals into distinct castes. In social insects, ratios of individuals in different castes can adjust adaptively in response to external threats. However, whether flexibility in caste ratio also occurs in other social organisms with division of labour remains unclear. Some parasitic trematodes, in which clonal colonies within the snail intermediate host comprise a reproductive caste and a soldier caste, offer good systems to test the general nature of adaptive caste ratio adjustments. Using the trematode Philophthalmus sp. as model, we test whether trematode colonies shift their composition towards more soldiers when exposed to a sustained risk of invasion by a competitor parasite species, and/or when experiencing sustained, active competition. We also quantify the colony-level fitness impact of caste ratio adjustments, measured as the colony's output of larval infective stages. We conducted two long-term laboratory experiments on within-snail trematode colonies. First, snails harbouring Philophthalmus colonies were exposed to different levels of invasion risk by another trematode species, Maritrema novaezealandense. Second, the structure of Philophthalmus colonies was quantified after a year-long period of within-snail competition with the other trematode species. When facing the risk of invasion by a competitor, independently of the level of risk, Philophthalmus colonies showed a significant shift towards producing more soldiers, resulting in altered caste ratio. Similarly, when experiencing actual competition by another trematode established in the same snail, Philophthalmus colonies also adjusted by producing significantly more soldiers. Greater investments in defense via more soldiers had negative impacts on the establishment and size of the competitor's colonies. Nevertheless, the presence of the competitor reduced the fitness (output of infective stages) of Philophthalmus colonies, although the production of more soldiers mitigated that effect. Our findings demonstrate that trematode colonies with division of labour are capable of adaptive caste ratio adjustments in response to both the perceived threat of competition and actual competition, with trade-offs against reproductive success only apparent when soldier numbers are very high. Combined with results on social insects, our study suggests parallel adaptations of colonial organisms in phylogenetically disparate organisms.

The missing link in parasite manipulation of host behaviour.

The observation that certain species of parasite my adaptively manipulate its host behaviour is a fascinating phenomenon. As a result, the recently established field of 'host manipulation' has seen rapid expansion over the past few decades with public and scientific interest steadily increasing. However, progress appears to falter when researchers ask how parasites manipulate behaviour, rather than why. A vast majority of the published literature investigating the mechanistic basis underlying behavioural manipulation fails to connect the establishment of the parasite with the reported physiological changes in its host. This has left researchers unable to empirically distinguish/identify adaptive physiological changes enforced by the parasites from pathological side effects of infection, resulting in scientists relying on narratives to explain results, rather than empirical evidence. By contrasting correlative mechanistic evidence for host manipulation against rare cases of causative evidence and drawing from the advanced understanding of physiological systems from other disciplines it is clear we are often skipping over a crucial step in host-manipulation: the production, potential storage, and release of molecules (manipulation factors) that must create the observed physiological changes in hosts if they are adaptive. Identifying these manipulation factors, via associating gene expression shifts in the parasite with behavioural changes in the host and following their effects will provide researchers with a bottom-up approach to unraveling the mechanisms of behavioural manipulation and by extension behaviour itself.

Save your host, save yourself? Caste-ratio adjustment in a parasite with division of labor and snail host survival following shell damage.

Shell damage and parasitic infections are frequent in gastropods, influencing key snail host life-history traits such as survival, growth, and reproduction. However, their interactions and potential effects on hosts and parasites have never been tested. Host-parasite interactions are particularly interesting in the context of the recently discovered division of labor in trematodes infecting marine snails. Some species have colonies consisting of two different castes present at varying ratios; reproductive members and nonreproductive soldiers specialized in defending the colony. We assessed snail host survival, growth, and shell regeneration in interaction with infections by two trematode species, Philophthalmus sp. and Maritrema novaezealandense, following damage to the shell in the New Zealand mud snail Zeacumantus subcarinatus. We concomitantly assessed caste-ratio adjustment between nonreproductive soldiers and reproductive members in colonies of the trematode Philophthalmus sp. in response to interspecific competition and shell damage to its snail host. Shell damage, but not parasitic infection, significantly increased snail mortality, likely due to secondary infections by pathogens. However, trematode infection and shell damage did not negatively affect shell regeneration or growth in Z. subcarinatus; infected snails actually produced more new shell than their uninfected counterparts. Both interspecific competition and shell damage to the snail host induced caste-ratio adjustment in Philophthalmus sp. colonies. The proportion of nonreproductive soldiers increased in response to interspecific competition and host shell damage, likely to defend the parasite colony and potentially the snail host against increasing threats. These results indicate that secondary infections by pathogens following shell damage to snails both significantly increased snail mortality and induced caste-ratio adjustments in parasites. This is the first evidence that parasites with a division of labor may be able to produce nonreproductive soldiers according to environmental factors other than interspecific competition with other parasites.

Parasite-mediated microhabitat segregation between congeneric hosts.

Parasite-mediated competition can shape community structure and host distribution. If two species compete for resources, parasites may indirectly change the outcome of competition. We tested the role of a trematode parasite in mediating microhabitat use by congeneric isopods Austridotea annectens and Austridotea lacustris Although both isopods share resources, they rarely co-occur in the same discrete microhabitats. We set up mesocosms with and without competition and/or parasites to examine the role of parasites in host distribution and habitat segregation. Austridotea annectens showed a clear preference for one microhabitat type regardless of competition or parasitic infection. By contrast, A. lacustris showed little habitat selection in the absence of competition, but favoured sandy habitats in the presence of uninfected A. annectens and rocky habitats when competing with infected A. annectens Our results suggest that parasites in one species affect the distribution of another species, and mediate competition between these species. We demonstrated the impacts of a parasite on the microhabitat use of its host's competitor. This also represents an example of a super-extended phenotype, where a parasite affects the phenotype of a non-host.

The invasive cestode parasite Ligula from salmonids and bullies on the South Island, New Zealand.

Freshwater ecosystems are often impacted by biological invasions, including the introduction of exotic parasites capable of infecting native species. Here, we report the occurrence of the introduced tapeworm Ligula sp. from common bully, Gobiomorphus cotidianus, and quinnat salmon, Oncorhynchus tshawytscha, in Lake Hawea, South Island, New Zealand. This parasite has a complex life cycle, reaching its adult stage in fish-eating birds. Worms recovered from the body cavity of fish hosts reached huge sizes (60-300 mm long); however, their low prevalence in fish populations suggests that infections are rare or localised. Molecular analysis (internal transcribed spacer (ITS)1 and ITS2 sequences) confirms that these specimens belong to the genus Ligula and suggests tentative routes of invasion into New Zealand. Monitoring the spread of this parasite is important, as it can impact fish populations and also, when infection levels are high, those of piscivorous birds.

Differential impacts of shared parasites on fitness components among competing hosts.

Effects of parasites on individual hosts can eventually translate to impacts on host communities. In particular, parasitism can differentially affect host fitness among sympatric and interacting host species. We examined whether the impact of shared parasites varied among host species within the same community. Specifically, we looked at the impacts of the acanthocephalan Acanthocephalus galaxii, the trematodes Coitocaecum parvum and Maritrema poulini, and the nematode Hedruris spinigera, on three host species: the amphipods, Paracalliope fluviatilis and Paracorophium excavatum, and the isopod, Austridotea annectens. We assessed parasite infection levels in the three host species and tested for effects on host survival, behavior, probability of pairing, and fecundity. Maritrema poulini and C. parvum were most abundant in P. excavatum but had no effect on its survival, whereas they negatively affected the survival of P. fluviatilis, the other amphipod. Female amphipods carrying young had higher M. poulini and C. parvum abundance than those without, yet the number of young carried was not linked to parasite abundance. Behavior of the isopod A. annectens was affected by M. poulini infection; more heavily infected individuals were more active. Paracorophium excavatum moved longer distances when abundance of C. parvum was lower, yet no relationship existed with respect to infection by both M. poulini and C. parvum. The differential effects of parasites on amphipods and isopods may lead to community-wide effects. Understanding the consequences of parasitic infection and differences among host species is key to gaining greater insight into the role of parasite mediation in ecosystem dynamics.

Host taxonomy constrains the properties of trophic transmission routes for parasites in lake food webs.

Some parasites move from one host to another via trophic transmission, the consumption of the parasite (inside its current host) by its future host. Feeding links among free-living species can thus be understood as potential transmission routes for parasites. As these links have different dynamic and structural properties, they may also vary in their effectiveness as trophic transmission routes. That is, some links may be better than others in allowing parasites to complete their complex life cycles. However, not all links are accessible to parasites as most are restricted to a small number of host taxa. This restriction means that differences between links involving host and non-host taxa must be considered when assessing whether transmission routes for parasites have different food web properties than other links. Here we use four New Zealand lake food webs to test whether link properties (contribution of a link to the predator's diet, prey abundance, prey biomass, amount of biomass transferred, centrality, and asymmetry) affect trophic transmission of parasites. Critically, we do this using both models that neglect the taxonomy of free-living species and models that explicitly include information about which free-living species are members of suitable host taxa. Although the best-fit model excluding taxonomic information suggested that transmission routes have different properties than other feeding links, when including taxonomy, the best-fit model included only an intercept. This means that the taxonomy of free-living species is a key determinant of parasite transmission routes and that food-web properties of transmission routes are constrained by the properties of host taxa. In particular, many intermediate hosts (prey) attain high biomasses and are involved in highly central links while links connecting intermediate to definitive (predator) hosts tend to be dynamically weak.

Behavioural modification of personality traits: testing the effect of a trematode on nymphs of the red damselfly Xanthocnemis zealandica.

Research on animal personality is increasingly demonstrating that individuals in a population are characterised by distinct sets of behavioural traits that show consistency over time and across different situations. Parasites are known to alter the behaviour of their hosts, although their role in shaping host personality remains little studied. Here, we test the effect of trematode infection on two traits of their host's personality, activity and boldness, in nymphs of the red damselfly Xanthocnemis zealandica. Genetic analyses indicate that the undescribed trematode species falls within the superfamily Microphalloidea. Results of laboratory behavioural tests indicate that the two behavioural traits are related to each other: bolder individuals also show higher levels of spontaneous activity than shy ones. However, parasite infection had no effect on either of these behaviours or on their repeatability over three separate testing sessions. Although our findings suggest that this trematode does not influence personality traits of the damselfly host, it remains possible that other standard personality traits not tested here (exploratory tendency, aggressiveness) are affected by infection.

Variations in infection levels and parasite-induced mortality among sympatric cryptic lineages of native amphipods and a congeneric invasive species: Are native hosts always losing?

Shared parasites can strongly influence the outcome of competition between congeneric, sympatric hosts, and thus host population dynamics. Parasite-mediated competition is commonly hypothesized as an important factor in biological invasion success; invasive species often experience lower infection levels and/or parasite-induced mortality than native congeneric hosts. However, variation in infection levels among sympatric hosts can be due to contrasting abilities to avoid infection or different parasite-induced mortality rates following infection. Low parasite infection levels in a specific host can be due to either factor but have drastically different implications in interaction outcomes between sympatric hosts. We assessed acanthocephalan infection levels (prevalence and abundance) among cryptic molecular taxonomic units (MOTU) of the native G. pulex/G. fossarum species complex from multiple populations where they occur in sympatry. We concomitantly estimated the same parameters in the invasive Gammarus roeseli commonly found in sympatry with G. pulex/G. fossarum MOTUs. We then tested for potential differences in parasite-induced mortality among these alternative hosts. As expected, the invasive G. roeseli showed relatively low infection level and was not subject to parasite-induced mortality. We also found that both acanthocephalan infection levels and parasite-induced mortality varied greatly among cryptic MOTUs of the native amphipods. Contrary to expectations, some native MOTUs displayed levels of resistance to their local parasites similar to those observed in the invasive G. roeseli. Overall, cryptic diversity in native amphipods coupled with high levels of variability in infection levels and parasite-induced mortality documented here may strongly influence inter-MOTU interactions and native population dynamics as well as invasion success and population dynamics of the congeneric invasive G. roeseli.

Impacts of crustacean invasions on parasite dynamics in aquatic ecosystems: A plea for parasite-focused studies.

While there is considerable interest in, and good evidence for, the role that parasites play in biological invasions, the potential parallel effects of species introduction on parasite dynamics have clearly received less attention. Indeed, much effort has been focused on how parasites can facilitate or limit invasions, and positively or negatively impact native host species and recipient communities. Contrastingly, the potential consequences of biological invasions for the diversity and dynamics of both native and introduced parasites have been and are still mainly overlooked, although successful invasion by non-native host species may have large, contrasting and unpredictable effects on parasites. This review looks at the links between biological invasions and pathogens, and particularly at crustacean invasions in aquatic ecosystems and their potential effects on native and invasive parasites, and discusses what often remains unknown even from well-documented systems. Aquatic crustaceans are hosts to many parasites and are often invasive. Published studies show that crustacean invasion can have highly contrasting effects on parasite dynamics, even when invasive host and parasite species are phylogenetically close to their native counterparts. These effects seem to be dependent on multiple factors such as host suitability, parasite life-cycle or host-specific resistance to parasitic manipulation. Furthermore, introduced hosts can have drastically contrasting effects on parasite standing crop and transmission, two parameters that should be independently assessed before drawing any conclusion on the potential effects of novel hosts on parasites and the key processes influencing disease dynamics following biological invasions. I conclude by calling for greater recognition of biological invasions' effects on parasite dynamics, more parasite-focused studies and suggest some potential ways to assess these effects.

Linking parasite populations in hosts to parasite populations in space through Taylor's law and the negative binomial distribution.

The spatial distribution of individuals of any species is a basic concern of ecology. The spatial distribution of parasites matters to control and conservation of parasites that affect human and nonhuman populations. This paper develops a quantitative theory to predict the spatial distribution of parasites based on the distribution of parasites in hosts and the spatial distribution of hosts. Four models are tested against observations of metazoan hosts and their parasites in littoral zones of four lakes in Otago, New Zealand. These models differ in two dichotomous assumptions, constituting a 2 × 2 theoretical design. One assumption specifies whether the variance function of the number of parasites per host individual is described by Taylor's law (TL) or the negative binomial distribution (NBD). The other assumption specifies whether the numbers of parasite individuals within each host in a square meter of habitat are independent or perfectly correlated among host individuals. We find empirically that the variance-mean relationship of the numbers of parasites per square meter is very well described by TL but is not well described by NBD. Two models that posit perfect correlation of the parasite loads of hosts in a square meter of habitat approximate observations much better than two models that posit independence of parasite loads of hosts in a square meter, regardless of whether the variance-mean relationship of parasites per host individual obeys TL or NBD. We infer that high local interhost correlations in parasite load strongly influence the spatial distribution of parasites. Local hotspots could influence control and conservation of parasites.