Prevalence of the scuticociliate Orchitophrya stellarum in seastars from the Pacific and Atlantic oceans.

As part of a study to investigate the use of the scuticociliate Orchitophrya stellarum as a biological control for the invasive seastar Asterias amurensis in Australia, we collected prevalence data for O. stellarum from 3 seastar species (A. amurensis, A. rubens, Pisaster ochraceus) between 1996 and 1999 from the Pacific (Australia, Japan, Korea, Canada) and Atlantic (France, Netherlands, Canada) oceans. In the Pacific Ocean, for the first time, we found O. stellarum in male A. amurensis in Korea and female A. amurensis in Japan. The parasite was not detected in the invasive A. amurensis from Australia. There was no significant difference between size of infected and uninfected male seastars, nor a correlation between biased sex ratio and parasite prevalence in populations in the Pacific or Atlantic oceans. Therefore, unlike other studies, we found size and sex ratio in seastar populations in the field are unreliable indicators of parasite impacts. Regular monitoring of infected seastar populations in the field would be useful to better understand how sex ratio varies with parasite prevalence. We recommend laboratory studies under controlled conditions to determine the effect of O. stellarum on seastar populations.

Economies of parasite body size.

Parasitism has independently evolved multiple times across the entire tree of life, and there are numerous parasitic representatives from every major eukaryote kingdom. In animals alone, parasitism has independently evolved at least 200 times. If there are any organisms that one might think would have access to limitless resources, it would be parasites. You would think that living in or on the body of their host, which serves as both a habitat and a food source, would provide parasites with bountiful resources to maximise every aspect of their existence, especially reproduction. But parasitism is not a loophole out of life history trade-offs. There is still a finite amount of resources that a parasite can obtain and allocate to its many needs. Living in a resource-rich environment has allowed many parasites to grow to sizes that are of multiple orders of magnitude larger than their free-living relatives. But that does not mean that the underlying economy of nature and its limitations are inapplicable to parasites.

Your infections are what you eat: How host ecology shapes the helminth parasite communities of lizards.

Understanding how parasite communities are assembled, and the factors that influence their richness, can improve our knowledge of parasite-host interactions and help to predict the spread of infectious diseases. Previous comparative analyses have found significant influences of host ecology and life history, but focused on a few select host taxa. Host diet and habitat use play key roles in the acquisition of parasitic helminths as many are trophically transmitted, making these attributes potentially key indicators of infection risk. Given the paucity of comparative studies with non-piscine, non-avian or non-mammalian hosts, it is critical to examine the degree to which host ecology influences parasite communities in other host taxa in order to identify common drivers. We examined helminth diversity in over 350 species of lizards in relation to their body mass, ecology (diet and habitat use) and life history (clutch size, and ovo- or viviparity) using previously published data. Overall, lizard species with herbivorous diets harboured fewer types of helminths (especially larval stages), with similar results for traits that were ultimately strongly associated with diet (host mass and habitat use). Large hosts tended to be herbivores with few helminth types, whereas species utilizing arboreal habitats typically consumed some animal matter and hosted more helminths. Understanding how host ecology and life history are related to their parasite assemblages has significant implications for the risk of acquiring novel parasites. Our results indicate an overwhelming influence of host diet such that many helminths may be relatively easily acquired by hosts in new ranges, or through dietary shifts.

Fossils of parasites: what can the fossil record tell us about the evolution of parasitism?

Parasites are common in many ecosystems, yet because of their nature, they do not fossilise readily and are very rare in the geological record. This makes it challenging to study the evolutionary transition that led to the evolution of parasitism in different taxa. Most studies on the evolution of parasites are based on phylogenies of extant species that were constructed based on morphological and molecular data, but they give us an incomplete picture and offer little information on many important details of parasite-host interactions. The lack of fossil parasites also means we know very little about the roles that parasites played in ecosystems of the past even though it is known that parasites have significant influences on many ecosystems. The goal of this review is to bring attention to known fossils of parasites and parasitism, and provide a conceptual framework for how research on fossil parasites can develop in the future. Despite their rarity, there are some fossil parasites which have been described from different geological eras. These fossils include the free-living stage of parasites, parasites which became fossilised with their hosts, parasite eggs and propagules in coprolites, and traces of pathology inflicted by parasites on the host's body. Judging from the fossil record, while there were some parasite-host relationships which no longer exist in the present day, many parasite taxa which are known from the fossil record seem to have remained relatively unchanged in their general morphology and their patterns of host association over tens or even hundreds of millions of years. It also appears that major evolutionary and ecological transitions throughout the history of life on Earth coincided with the appearance of certain parasite taxa, as the appearance of new host groups also provided new niches for potential parasites. As such, fossil parasites can provide additional data regarding the ecology of their extinct hosts, since many parasites have specific life cycles and transmission modes which reflect certain aspects of the host's ecology. The study of fossil parasites can be conducted using existing techniques in palaeontology and palaeoecology, and microscopic examination of potential material such as coprolites may uncover more fossil evidence of parasitism. However, I also urge caution when interpreting fossils as examples of parasites or parasitism-induced traces. I point out a number of cases where parasitism has been spuriously attributed to some fossil specimens which, upon re-examination, display traits which are just as (if not more) likely to be found in free-living taxa. The study of parasite fossils can provide a more complete picture of the ecosystems and evolution of life throughout Earth's history.

Nematode parasite diversity in birds: the role of host ecology, life history and migration.

Previous studies have found that migratory birds generally have a more diverse array of pathogens such as parasites, as well as higher intensities of infection. However, it is not clear whether this is driven by the metabolic and physiological demands of migration, differential selection on host life-history traits or basic ecological differences between migratory and non-migratory species. Parasitic helminths can cause significant pathology in their hosts, and many are trophically transmitted such that host diet and habitat use play key roles in the acquisition of infections. Given the concurrent changes in avian habitats and migratory behaviour, it is critical to understand the degree to which host ecology influences their parasite communities. We examined nematode parasite diversity in 153 species of Anseriformes (water birds) and Accipitriformes (predatory birds) in relation to their migratory behaviour, diet, habitat use, geographic distribution and life history using previously published data. Overall, migrators, host species with wide geographic distributions and those utilizing multiple aquatic habitats had greater nematode richness (number of species), and birds with large clutches harboured more diverse nematode fauna with respect to number of superfamilies. Separate analyses for each host order found similar results related to distribution, habitat use and migration; however, herbivorous water birds played host to a less diverse nematode community compared to those that consume some animals. Birds using multiple aquatic habitats have a more diverse nematode fauna relative to primarily terrestrial species, likely because there is greater opportunity for contact with parasite infectious stages and/or consumption of infected hosts. As such, omnivorous and carnivorous birds using aquatic habitats may be more affected by environmental changes that alter their diet and range. Even though there were no overall differences in their ecology and life history compared with non-migrators, migratory bird species still harboured a more diverse array of nematodes, suggesting that this behaviour places unique demands on these hosts and warrants further study.

Evolution: how a barnacle came to parasitise a shark.

A new study on a parasitic barnacle that lives on a deep sea shark found that its closest living relatives are rocky shore barnacles. The findings provide insight into barnacle phylogeny and raise new questions about the evolution of parasitism.

Fluorescent probes as a tool for labelling and tracking the amphibian chytrid fungus Batrachochytrium dendrobatidis.

The dissemination of the virulent pathogen Batrachochytrium dendrobatidis (Bd) has contributed to the decline and extinction of many amphibian species worldwide. Several different strains have been identified, some of which are sympatric. Interactions between co-infecting strains of a pathogen can have significant influences on disease epidemiology and evolution; therefore the dynamics of multi-strain infections is an important area of research. We stained Bd cells with 2 fluorescent BODIPY fatty acid probes to determine whether these can potentially be used to distinguish and track Bd cell lines in multi-strain experiments. Bd cells in broth culture were stained with 5 concentrations of green-fluorescent BODIPY FL and red-fluorescent BODIPY 558/568 and visualised under an epifluorescent microscope for up to 16 d post-dye. Dyed strains were also assessed for growth inhibition. The most effective concentration for both dyes was 10 pM. This concentration of dye produced strong fluorescence for 12 to 16 d in Bd cultures held at 23 degrees C (3 to 4 generations), and did not inhibit Bd growth. Cells dyed with BODIPY FL and BODIPY 558/568 can be distinguished from each other on the basis of their fluorescence characteristics. Therefore, it is likely that this technique will be useful for research into multi-strain dynamics of Bd infections.

Small worms, big appetites: ratios of different functional morphs in relation to interspecific competition in trematode parasites.

Animals living in colonies or collectives composed of highly-related individuals often produce morphs that are physically and behaviourally specialised to perform specific tasks. Because such morphs are often sterile, their production represents a fitness cost for the colony and there should be an optimal ratio of the numbers of sterile specialists and reproductive members that may be adjustable to environmental conditions. Trematode parasites undergo asexual multiplication within their snail intermediate host, resulting in large numbers of clonal stages known as rediae or sporocysts, depending on the trematode species. In areas with high prevalences of infection, the host can be infected by multiple species, which can lead to intense competition for limited resources. Here, we describe the existence of specialised 'mini-rediae' in the trematode Philophthalmus sp. that are morphologically and functionally specialised for interspecific competition. Mini-rediae were observed feeding on the sporocysts of a co-occurring trematode species -Maritrema novaezealandensis. In addition, in larger snails - which are less likely to have M. novaezealandensis infections -Philophthalmus sp. produces relatively fewer mini-rediae than expected. Our findings support results from a prior study which demonstrated the existence of morphs that perform specialised functions in antagonistic interspecific interactions in trematodes, and additionally shows that the number of these morphs in each host is associated with the likelihood of encountering other species within the same host. Trematodes may thus provide interesting models for studying morphological specialisation in colonial organisms.

Intra-host competition between co-infecting digeneans within a bivalve second intermediate host: dominance by priority-effect or taking advantage of others?

We experimentally investigated the interactions between two parasites known to manipulate their host's phenotype, the trematodes Acanthoparyphium sp. and Curtuteria australis, which infect the cockle Austrovenus stutchburyi. The larval stages of both species encyst within the tissue of the bivalve's muscular foot, with a preference for the tip of the foot. As more individuals accumulate at that site, they impair the burrowing behaviour of cockles and increase the probability of the parasites' transmission to a bird definitive host. However, individuals at the foot tip are also vulnerable to non-host predators in the form of foot-cropping fish which selectively bite off the foot tip of exposed cockles. Parasites encysted at the foot base are safe from such predators although they do not contribute to altering host behaviour, but nevertheless benefit from host manipulation as all parasites within the cockle are transmitted if it is ingested by a bird. Experimental infection revealed that Acanthoparyphium sp. and C. australis have different encystment patterns within the host, with proportionally fewer Acanthoparyphium metacercariae encysting at the foot tip than C. australis. This indicates that Acanthoparyphium may benefit indirectly from C. australis and incur a lower risk of non-host predation. However, in co-infections, not only did C. australis have higher infectivity than Acanthoparyphium, it also severely affected the latter's infection success. The asymmetrical strategies and interactions between the two species suggest that the advantages obtained from exploiting the host manipulation efforts of another parasite might be offset by traits such as reduced competitiveness in co-infections.

Accumulation of diverse parasite genotypes within the bivalve second intermediate host of the digenean Gymnophallus sp.

The complex life cycle of digenean trematodes with alternating stages of asexual multiplication and sexual reproduction can generate interesting within-host population genetic patterns. Metacercarial stages found in the second intermediate host are generally accumulated from the environment. Highly mobile second intermediate hosts can sample a broad range of cercarial genotypes and accumulate genetically diverse packets of metacercariae, but it is unclear whether the same would occur in systems where the second intermediate host is relatively immobile and cercarial dispersal is the sole mechanism that can maintain genetic homogeneity at the population level. Here, using polymorphic microsatellite markers, we addressed this issue by genotyping metacercariae of the trematode Gymnophallus sp. from the New Zealand cockle Austrovenus stutchburyi. Despite the relatively sessile nature of the second intermediate host of Gymnophallus, very high genotypic diversity of metacercariae was found within cockles, with only two cockles harbouring multiple copies of a single clonal lineage. There was no evidence of population structuring at the scale of our study, suggesting the existence of a well-mixed population. Our results indicate that (i) even relatively sessile second intermediate hosts can accumulate a high diversity of genotypes and (ii) the dispersal ability of cercariae, whether passive or not, is much greater than expected for such small and short-lived organisms. The results also support the role of the second intermediate host as an accumulator of genetic diversity in the trematode life cycle.

Size-dependent pattern of metacercariae accumulation in Macomona liliana: the threshold for infection in a dead-end host.

While bivalves can acquire trematode metacercariae over their lifetime, the rate at which this accumulation takes place is not necessarily linear. The present study found that the bivalve Macomona liliana acquires very few or no metacercariae until it reaches 30 mm in size, but thereafter the rate at which it becomes infected increases exponentially. It is likely that this ontogenetic change in infection rate is associated with the increased filtration capacity and siphon diameter of larger M. liliana. The echinostome metacercariae that infect M. liliana also infect a much more common sympatric bivalve, Austrovenus stutchburyi, in which they achieve much higher infection intensity. Due to its deeper burying depth, M. liliana most likely represents a dead-end host for the echinostomes: potential definitive hosts preferentially feed upon A. stutchburyi as they are located closer to the sediment surface than M. liliana. However, due to the low infection intensity and population density of M. liliana, its overall impact as a sink for echinostome populations in the ecosystem is probably negligible.

The use of fluorescent fatty acid analogs as labels in trematode experimental infections.

We examined the utility of fluorescent fatty acid analog dyes for labeling larval trematodes to use in experimental infections. Our goals were to identify two dyes that label larval trematodes belonging to the species Maritrema novaezealandensis and Coitocaecum parvum, determine if the dyes influence survival and infectivity of larval trematodes and/or host mortality, and if larval trematodes labeled with alternative dyes could be distinguished post-infection. The two dyes tested, BODIPY FL C(12) and BODIPY 558/568 C(12), successfully labeled all treated larval trematodes, did not influence cercariae survival or infectivity, and did not influence host mortality in either host-parasite system. All larval parasites were fluorescent and distinguishable after 5 days in amphipod intermediate hosts. In addition, larval Acanthoparyphium sp. were strongly fluorescent with both dyes after 5 weeks within cockle hosts. This method should be extremely useful for experimental studies using trematode-host systems as models for addressing a range of ecological and evolutionary questions.

Ten polymorphic microsatellite loci for the trematode Curtuteria australis (Echinostomatidae).

Ten polymorphic loci were isolated and characterised from the intertidal New Zealand trematode Curtuteria australis. This common parasite manipulates the burrowing behaviour of its abundant bivalve host Austrovenus stutchburyi, with cascading impacts on the biodiversity of intertidal communities. Observed heterozygosities of the 10 loci ranged from 0.500 to 0.905, and three to 14 alleles were detected in 24 trematode metacercariae. These loci are currently being used to investigate the molecular ecology of this species within its intermediate hosts.

Recruitment rate of gymnophallid metacercariae in the New Zealand cockle Austrovenus stutchburyi: an experimental test of the hitch-hiking hypothesis.

The rate at which host organisms accumulate parasites is affected by a number of intrinsic and extrinsic factors. The New Zealand cockle Austrovenus stutchburyi is frequently parasitised by trematodes comprising of two species of echinostomes and a species of gymnophallid that use it as a second intermediate host for trophic transmission to avian definitive hosts. The echinostomes are capable of manipulating the burrowing behaviour of the cockle to enhance their transmission success, whereas the gymnophallid is not capable of host manipulation. Previous studies have found patterns of positive associations between the echinostomes and the gymnophallid. Thus, it is possible that the latter is a "hitch-hiking" parasite that preferentially infects cockles already heavily infected by echinostome metacercariae to enhance its own transmission rate. A field experiment involving cockles forced to remain either above or below the sediment surface to simulate manipulated and non-manipulated cockles was conducted to test the hitch-hiking hypothesis. The gymnophallid was not found to display any preference for either surfaced or buried cockles; therefore, it cannot be considered as a hitch-hiking parasite. Possible alternative reasons for the pattern of positive association between the gymnophallid and the echinostomes are proposed.