Crepidula fornicata is not a first intermediate host for trematodes: who is?

Trematode larvae must generally invade a molluscan intermediate host, usually a gastropod, before they can reach reproductive maturity in another definitive host. The research literature to date has focused almost exclusively on the documented specificity between particular trematode species and particular molluscan hosts; little attention has been paid to gastropod species that do not appear to serve as hosts. We sampled Rhode Island and Massachusetts populations of the marine gastropod Crepidula fornicata to determine whether this widespread species serves as a first intermediate host for trematodes. We also sampled from the same habitat populations of Littorina littorea and Ilyanassa obsoleta, gastropods known to serve as first intermediate hosts for several trematode species. All individuals were examined by dissection for the presence of sporocysts, rediae, or developing cercariae. Although 4-28% of L. littorea (N=112) and I. obsoleta (N=84) were infected by larvae of at least one trematode species, no individuals of C. fornicata sampled from the same locations were so infected (N=136). A survey of the Biological Abstracts computer database indicates that snails in only about 10% of marine gastropod families are known to serve as first intermediate hosts for trematodes. We suggest that more attention be paid to marine gastropods that appear not to be infected by trematode miracidia. Such species may productively serve as new models for understanding trematode host specificity and gastropod resistance to infection.     

Can helminth community patterns be amplified when transferred by predation from intermediate to definitive hosts?

Helminth communities in definitive hosts are formed by the acquisition of packets of larvae arriving each time an intermediate host is consumed. It is thus possible that associations between parasite species or other aspects of community structure get transferred from intermediate to definitive hosts. Earlier computer simulations showed that associations between 2 parasite species, in particular positive associations, could be transferred up the food chain. Here, we alter some of the assumptions of previous models and generate new simulations of several ways in which source infracommunities in intermediate hosts can be transferred to target infracommunities in definitive hosts. In particular, we introduced nonrandom selection of intermediate hosts by predatory definitive hosts, to mimic the phenomenon of host manipulation by parasites; this consisted in biasing predation toward intermediate hosts harboring a certain parasite species. Overall, our results show that positive covariances between 2 parasite species can not only be transferred but can also be amplified during transmission to definitive hosts; significant covariance between parasite species can even appear in the definitive hosts when none existed in the intermediate hosts. Negative covariance was not as readily transferred to definitive hosts and amplified, in part because of properties of the presence-absence covariance index. Amplification of covariance results from intermediate host manipulation as well as from other processes taking place during transmission. These results suggest that the patterns of association between helminth species in definitive hosts cannot be taken to reflect the processes acting inside those hosts: they may simply be inherited, with amplification, from intermediate hosts.     

Do trematode parasites disrupt defence-cell signalling in their snail hosts?

More than a decade ago, it was postulated that components derived from trematode parasites block receptors on the defence cells of their snail intermediate hosts, thus preventing host-cell activation and parasite elimination. This phenomenon has still not been investigated extensively. However, recent work concerning the molecular regulation of the molluscan defence response provides a new framework for studies that focus on an extension of this original concept - subversion of host cell signalling by trematode parasites. The hypothesis is that, to facilitate survival and replication in their intermediate hosts, trematode parasites down regulate host defence responses by interfering with key signal-transduction pathways in snail defence cells.     

Biology of Microplitis kewleyi (Hymenoptera: Braconidae): A parasite of Agrotis ipsilon (Lepidoptera: Noctuidae) larvae

Microplitis kewleyi Muesebeck is a gregarious internal parasite of larvae of the black cutworm Agrotis ipsilon (Hufnagel). Studies of the biology of the parasite revealed that there was an inverse relationship between host instar and parasite preference. Duration of development from egg to pupa ranged from 18 days at 27°C to 68.7 days at 16°C. Development from egg to pupa took 13.5–21.6 days when fourth and first instar host larvae, respectively, were parasitized. A larger number of parasites emerged from hosts parasitized in the fourth instar (22.4) than the first instar (11.5). Parasite pupation occurred when the host was in the fifth/sixth instar, depending on the instar parasitized. Thirty‐nine per cent of host larvae exposed as first instars to parasites died before parasite emergence. This decreased to 0% for host larvae exposed as fourth instars. The sex ratio was 1:1.2 (M:F). Thirty‐seven per cent of hosts exposed diurnally were stung, compared to 24% exposed nocturnally. Mean daily progeny was highest (12) on the first day, decreasing to zero after 20 days. Percent host parasitism was also highest on the first day (35%) decreasing to nearly 0% after 18 days. There appear to be three parasite larval instars. Host larvae often remained alive after parasite emergence.     

Patterns of infestation by the trombiculid mite Eutrombicula alfreddugesi in four sympatric lizard species (genus Tropidurus) in northeastern Brazil

We studied the parasitism by the chigger mite Eutrombicula alfreddugesi on four sympatric lizard species of the genus Tropidurus in Morro do Chapéu, Bahia state, Brazil: T. hispidus, T. cocorobensis, T. semitaeniatus and T. erythrocephalus. For each species, we investigated the patterns of infestation and analyzed to which extent they varied among the hosts. We calculated the spatial niche breadth of the chigger mite on the body of each host species and the distribution of mites along the hosts' bodies for each Tropidurus species. All four species of Tropidurus at Morro do Chapéu were parasited by the chigger mite, with high (97-100%) prevalences. Host body size significantly explained the intensity of mite infestation for all species, except T. erythrocephalus. The body regions with highest intensity of infestation in the four lizard species were the mite pockets. The spacial niche width of the chigger varied consistently among the four lizards species studied being highest for T. erytrocephalus and lowest for T. cocorobensis. We conclude that the distribution and intensity with which lizards of the genus Tropidurus are infested by Eutrombicula alfreddugesi larvae results from the interaction between aspects of host morphology (such as body size and the occurrence and distribution of mite pockets) and ecology (especially microhabitat use).     

Prevalence, intensity, and differential development of Pseudodelphis oligocotti (Nematoda: Dracunculoidea) in sympatric fish hosts of the northeastern Pacific coast

Counter to expectations of coevolved parasite-host relationships, parasites frequently infect hosts that never contribute to their reproduction, making the identification of a parasite's true host-specificity problematic. Pseudodelphis oligocotti (Nematoda: Dracunculoidea) infects several coastal Pacific fishes, but its course of development appears highly variable, suggesting that incidence does not reflect effective host range. To determine the host range of P. oligocotti and describe its relationship to various potential hosts, 24 fish species were examined from several British Columbia localities for prevalence, intensity, and extent and tissue location of parasite development. Pseudodelphis oligocotti infects 9 species of fishes from 5 orders, of which penpoint gunnel, Apodichthys flavidus, showed the highest prevalence and intensity, up to 80% and 19 (+/- 17.1 SD) worms per host, respectively. Although subadult and adult P. oligocotti occurred in all 9 fishes, larvigerous P. oligocotti only occurred in A. flavidus and rarely in the northern clingfish, Gobiesox maeandricus. Infective first-stage larvae were recovered from gill tissue of A. flavidus. Thus, at most only 2 of the 9 host species infected by P. oligocotti actually contribute to its transmission. The occurrence of P. oligocotti in diverse hosts may be accounted for by the parasite's indiscriminant mode of transmission via ingestion of free-living intermediate copepod hosts, where highly exposed or more suitable fishes (or both) are closely related by diet and microhabitat. This study demonstrates how parasite transmission and host ecology can greatly affect observed host range and ultimately its potential for expansion.     

Tests of ecological equivalence of two species of terrestrial gastropods as second intermediate hosts of Panopistus pricei (Trematoda: Brachylaimidae)

Metacercariae of Panopistus pricei (Brachylaimidae) are common parasites of 2 species of terrestrial gastropods (Neohelix albolabris; Webbhelix multilineata) in southeastern Nebraska. Field data were collected to determine if individuals of N. albolabris and W. multilineata function as ecologically equivalent second intermediate hosts of P. pricei along a transect on the western edge of the Missouri River. Metacercariae were recovered and measured from samples of 30 snails of each species collected at each of 6 sites; whole kidneys from snails of both species were examined to quantify microhabitat use. Microhabitat use of P. pricei did not differ between host species; in both N. albolabris and W. multilineata, metacercariae were concentrated in the primary ureter and occurred throughout the kidney proper. Prevalences and mean abundances did not differ between host species at any site, nor did the relationship between parasite abundance and host size. Prevalences and mean abundances across sites were positively correlated between host species. At 2 sites, demographics of metacercariae differed between host species, suggesting short-term differences in the history of encounters with cercariae in the environment and differences in transmission to shrew definitive hosts. Overall, N. albolabris and W. multilineata appear to be equivalent and required second intermediate hosts of P. pricei in the area studied.     

When trophically‐transmitted parasites combine predation enhancement with predation suppression to optimize their transmission

Trophically‐transmitted parasites are known for their ability to enhance predation of their intermediate host but they are less known for their ability to suppress predation. We review recent literature on host manipulation explaining why and when in its life cycle a parasite benefits from preventing the predation of its host. Predation suppression occurs in intermediate hosts as long as the parasite larva has not reached the developmental conditions allowing it to successfully establish in the next host (competency). We also examine the possibility that predation suppression may occur in hosts harbouring competent larvae (post competency) since some parasites have been shown to manipulate host behaviour in a way that decreases the risk of parasite death through non‐host predation (i.e. the consumption of its intermediate host by a predator that does not risk infection). Predation suppression when the parasite is competent has to be considered with respect to non‐host predation risk and is not mutually exclusive with predation enhancement. We use the recent theoretical advances in host manipulation to investigate the conditions under which predation suppression could evolve post competency.     

Host resources govern the specificity of swiftlet lice: size matters

1. An important component of parasite diversity is the specificity for particular host taxa shown by many parasites. Specificity is often assumed to imply adaptive specialization by the parasite to its host, such that parasites are incapable of surviving and reproducing on 'foreign' hosts.
2. Specificity, however, need not be due to adaptation to particular hosts. Some parasites may be specific simply because they are incapable of dispersing among host taxa. For example, 'permanent' parasites like chewing lice spend their entire lifecycle on the body of the host and require direct contact between hosts for dispersal.
3. The role of adaptive constraints in parasite host-specificity has seldom been tested in natural populations. We conducted such a test by comparing the relative fitness of host-specific lice experimentally transferred among closely related species of cave swiftlets in northern Borneo.
4. The survival of lice in most of these transfers was significantly reduced in proportion to the mean difference in feather barb size between the donor and recipient species of hosts. Thus, adaptation to a particular resource on the body of the host does appear to govern the specificity of swiftlet lice.
5. In transfers where lice survived, microhabitat shifting on the body of the host was observed, whereby the mean barb diameter of the feathers on which the lice occurred was held 'constant'.     

The guest playing host: colonization of the introduced Mediterranean gecko, Hemidactylus turcicus, by helminth parasites in southeastern Louisiana.

Parasite surveys of exotic hosts offer the opportunity to examine parasite colonization on different scales (i.e., host individual, host population, host species, and new geographic locality). Ten helminths (Macracanthorhynchus ingens, Mesocestoides lineatus, Oochoristica javaensis, Haematoloechus varioplexus, Mesocoelium monas, Telorchis corti, Cosmocercoides variabilis, Oswaldocruzia leidyi, Skrjabinoptera sp., and a larval acuariid nematode) were recovered from the exotic Mediterranean gecko Hemidactylus turcicus, in southeastern Louisiana. Only 1 exotic parasite, O. javaensis, colonized a new geographic locality, but 7 local helminths colonized a new host species. Helminth communities of H. turcicus were similar in structure to what has been hypothesized or observed for lizards. Thus, communities were composed of generalists and were depauperate (i.e., colonization of individual geckos or host populations was rare for most of the helminths); however, there was significant variation in community structure among local habitats. Although the gecko's behavioral and physiological attributes predict colonization by monoxenous helminths, only 2, C. variabilis and O. leidyi, were recovered. Eight heteroxenous helminths, 2 of which (the acuariid and O. javaensis) were the most widely distributed and abundant, were the better colonizers. The gecko's generalist diet may have exposed it to a diverse parasite fauna and thus been important in determining the helminths that could colonize.     

Host manipulation by parasites in the world of dead-end predators: adaptation to enhance transmission?

Trophically transmitted parasites often alter their intermediate host's phenotype, thereby predisposing the hosts to increased predation. This is generally considered a parasite strategy evolved to enhance transmission to the next hosts. However, the adaptive value of host manipulation is not clear as it may be associated with costs, such as increased susceptibility to predators that are unsuitable next hosts for the parasites. We examined the ratio between the benefits and costs of host manipulation for transmission success of Acanthocephalus lucii (Acanthocephala), a parasite that alters the hiding behaviour and pigmentation of its isopod hosts. We experimentally compared the susceptibility of infected and uninfected isopods to predation by perch (Perca fluvialis; definitive host of the parasite) and dragonfly larvae (dead end). We found that the parasite predisposed the isopods to predation by both predators. However, the increased predation vulnerability of the infected isopods was higher towards perch. This suggests that, despite the costs due to non-host predation, host manipulation may still be advantageous for the parasite.     

Molluscan defence mechanisms: immunity or population biology?

In recent studies of associations between host and parasite, there has been a tendency to consider the internal defence mechanisms of the mollusc as pivotal in the relationship. However, evolution is a population-based phenomenon, and it is by considering populations rather than individuals that the success of these associations can best be appreciated. Among susceptible host species within the biocoenosis, truly resistant strains appear to be remarkably rare. Clive Shiff here examines mechanisms of defence or avoidance among molluscan hosts of tremotodes from both the internal and ecological perspective.     

Learning of host microhabitat form byExeristes roborator (F.) (Hymenoptera: Ichneumonidae)

Females of the polyphagous, ichneumonid ectoparasitoid, Exeristes roborator(F.), learned the form of artificial host microhabitats in the laboratory. Females given hosts only in a microhabitat of one form subsequently concentrated their responses on this microhabitat when given a choice between it and a microhabitat of another form, neither of which contained hosts. Control females did not display similar preferences. The parasitoid did not appear to distinguish the form of a microhabitat until after it contacted it. When hosts were present in test microhabitats, learning of form was apparently overridden, possibly because the parasitoids detected the presence of hosts before they assessed form or because their responses were shaped by learning of host-related factors. Together with learning of other microhabitat characteristics, learning of form may contribute to recognition of plant structures that are likely to contain suitable hosts by E. roborator.     

Biochemical and immunological adaptation in schistosome parasitism

The objective of this review is to clarify aspects of immunological and biochemical adaptations of schistosomes to their intermediate and final mammalian hosts. Adaptations to the mammalian hosts are traced in relation to cercarial penetration of mammalian skin, glucose transport and metabolism. The unusual ability of schistosome surface membrane to escape immune recognition and damage are reviewed. Moreover, the behavioural changes induced in the intermediate hosts by schistosomes are considered. The evolutionary adaptation to molluscan hosts aims to increase the probability of transmission of the parasite into its mammalian host. This review inspires more hope for further design of anti-schistosome drugs through disturbing aspects of biochemical and immunological adaptations in schistosome parasitism.     

颗粒体病毒、微红绒茧蜂、菜青虫相互关系的初步研究

近几年来,利用颗粒体病毒防治菜粉蝶(Pieris rapae以下称菜青虫)幼虫的工作,在我国已取得了较好的进展(梁东瑞等,1979;河北省菜青虫颗粒体病毒研究协作组,1981)。病毒杀虫剂与化学杀虫剂的显著区别之一,是前者具有选择性,不直接杀伤天敌昆虫。为了明确颗粒体病毒防治菜青虫对寄生蜂种群的影响,以便进一步了解病毒治虫在保持生态平衡上的意义,我们对颗粒体病毒、微红绒茧蜂(Apanteles rubecula)、菜青虫的相互关系作了一些实验室研究,现报道初步结果如后。     

The effects of parasite age and intensity on variability in acanthocephalan-induced behavioural manipulation

Numerous parasites with complex life cycles are able to manipulate the behaviour of their intermediate host in a way that increases their trophic transmission to the definitive host. Pomphorhynchus laevis, an acanthocephalan parasite, is known to reverse the phototactic behaviour of its amphipod intermediate host, Gammarus pulex, leading to an increased predation by fish hosts. However, levels of behavioural manipulation exhibited by naturally-infected gammarids are extremely variable, with some individuals being strongly manipulated whilst others are almost not affected by infection. To investigate parasite age and parasite intensity as potential sources of this variation, we carried out controlled experimental infections on gammarids using parasites from two different populations. We first determined that parasite intensity increased with exposure dose, but found no relationship between infection and host mortality. Repeated measures confirmed that the parasite alters host behaviour only when it reaches the cystacanth stage which is infective for the definitive host. They also revealed, we believe for the first time, that the older the cystacanth, the more it manipulates its host. The age of the parasite is therefore a major source of variation in parasite manipulation. The number of parasites within a host was also a source of variation. Manipulation was higher in hosts infected by two parasites than in singly infected ones, but above this intensity, manipulation did not increase. Since the development time of the parasite was also different according to parasite intensity (it was longer in doubly infected hosts than in singly infected ones, but did not increase more in multi-infected hosts), individual parasite fitness could depend on the compromise between development time and manipulation efficiency. Finally, the two parasite populations tested induced slightly different degrees of behavioural manipulation.     

Seasonal dynamics of Skrjabinoptera phrynosoma (Nematoda) infection in horned lizards from the Alvord Basin: temporal components of a unique life cycle

The nematode Skrjabinoptera phrynosoma is a stomach parasite of horned lizards in the genus Phrynosoma. This nematode demonstrates a distinctive life cycle wherein entire gravid females harboring infective eggs exit with lizard feces. Pogonomyrmex spp. harvester ants collect these females and feed them to their larvae, which are the only stages of the intermediate host that can become infected. We hypothesized that the seasonal dynamics of nematode abundance within lizard hosts would be correlated with the seasonal availability of suitable intermediate hosts. To describe seasonal variation of nematode population variables and elucidate the timing of critical events in the parasite life cycle, nematodes were collected from both hosts across three collection periods in the ant-and-lizard activity season of 2008 in the Alvord Basin of southeastern Oregon. Among 3 collection periods, and across the activity season, nematodes were harvested from individual Phrynosoma platyrhinos , and the distribution of developmental categories and body lengths of nematodes was analyzed to determine the seasonal change in nematode population composition. Pogonomyrmex spp. ants were collected in pit-fall traps and dissected to determine infection prevalence. The abundance of non-gravid female and juvenile nematodes collected from lizards' stomachs decreased significantly between the early and late collection period, which was likely a consequence of the sequential conversion of these developmental categories to gravid females. The presence of gravid female nematodes peaked in cloacal and fecal collections during mid-season. The body lengths of male nematodes increased as the activity season progressed, perhaps due to growth, but their abundance remained the same. Smaller juvenile nematodes were present in late-season collections from lizards, possibly indicating new acquisitions from infected ants. We propose that once a set population of male nematodes establishes in lizards' stomachs, newly acquired juvenile nematodes develop into non-gravid females that mate, become gravid females, and exit the lizard mid-season. We additionally suggest that the exit of females may be timed with the peak foraging activity of ant intermediate hosts and access to larval ants in the nests. Infection prevalence in the intermediate host was low, with only 1 of 6,000 dissected harvester ants containing a single larval nematode. The temporal dynamics of S. phrynosoma populations within P. platyrhinos at this northern locale is most likely driven by the seasonal availability of harvester ant intermediate hosts.     

Parasite-induced alteration of plastic response to predation threat: increased refuge use but lower food intake in Gammarus pulex infected with the acanothocephalan Pomphorhynchus laevis

Larvae of many trophically-transmitted parasites alter the behaviour of their intermediate host in ways that increase their probability of transmission to the next host in their life cycle. Before reaching a stage that is infective to the next host, parasite larvae may develop through several larval stages in the intermediate host that are not infective to the definitive host. Early predation at these stages results in parasite death, and it has recently been shown that non-infective larvae of some helminths decrease such risk by enhancing the anti-predator defences of the host, including decreased activity and increased sheltering. However, these behavioural changes may divert infected hosts from an optimal balance between survival and foraging (either seeking food or a mate). In this study, this hypothesis was tested using the intermediate host of the acanthocephalan parasite Pomphorhynchus laevis, the freshwater amphipod Gammarus pulex. We compared activity, refuge use, food foraging and food intake of hosts experimentally infected with the non-infective stage (acanthella), with that of uninfected gammarids. Behavioural assays were conducted in four situations varying in predation risk and in food accessibility. Acanthella-infected amphipods showed an increase in refuge use and a general reduction in activity and food intake. There was no effect of parasite intensity on these traits. Uninfected individuals showed plastic responses to water-borne cues from fish by adjusting refuge use, activity and food intake. They also foraged more when the food was placed outside the refuge. At the intra-individual level, refuge use and food intake were positively correlated in infected gammarids only. Overall, our findings suggest that uninfected gammarids exhibit risk-sensitive behaviour including increased food intake under predation risk, whereas gammarids infected with the non-infective larvae of P. laevis exhibit a lower motivation to feed, irrespective of predation risk and food accessibility.     

Egg mimicry by the Pacific koel: mimicry of one host facilitates exploitation of other hosts with similar egg types

When brood parasites exploit multiple host species, egg rejection by hosts may select for the evolution of host‐specific races, where each race mimics a particular host's egg type. However, some brood parasites that exploit multiple hosts with the ability to reject foreign eggs appear to have only a single egg type. In these cases, it is unclear how the parasite egg escapes detection by its hosts. Three possible explanations are: 1) host‐specific races are present, but differences in egg morphology are difficult for the human eye to detect; 2) the brood parasite evolves a single egg type that is intermediate in appearance between the eggs of its hosts; 3) or the parasite evolves mimicry of one of its hosts, which subsequently allows it to exploit other species with similar egg morphology. Here we test these possibilities by quantifying parameters of egg appearance of the brood‐parasitic Pacific koel Eudynamys orientalis and seven of its hosts. Koel eggs laid in the nests of different hosts did not show significant differences in colour or pattern, suggesting that koels have not evolved host‐specific races. Koel eggs were similar in colour, luminance and pattern to the majority of hosts, but were significantly more similar in colour and luminance to one of the major hosts than to two other major hosts, supporting hypothesis 3. Our findings suggest that mimicry of one host can allow a brood parasite to exploit new hosts with similar egg morphologies, which could inhibit the evolution of host defences in naïve hosts.     

Cryptic speciation and patterns of phenotypic variation of a highly variable acanthocephalan parasite

An investigation of a parasite species that is broadly host- and habitat-specific and exhibits alternative transmission strategies was undertaken to examine intraspecific variability and if it can be attributed to cryptic speciation or environmentally induced plasticity. Specimens of an acanthocephalan parasite, Leptorhynchoides thecatus, collected throughout North America were analysed phylogenetically using sequences of the cytochrome oxidase I gene and the internal transcribed spacer region. Variation in host use, habitat use, and transmission were examined in a phylogenetic context to determine if they were more likely phylogenetically based or due to environmental influences. Results indicated that most of the variation detected can be explained by the presence of cryptic species. The majority of these species have narrow host and microhabitat specificities although one species, which also may comprise a complex of species, exhibits broad host and habitat specificity. Alternate transmission pathways only occurred in two of the cryptic species and correlate with host use patterns. Taxa that mature in piscivorous piscine hosts use a paratenic fish host to bridge the trophic gap between their amphipod intermediate host and piscivorous definitive host. One potential example of environmentally induced variation was identified in three populations of these parasites, which differ on their abilities to infect different host species.