Parasitology: parasite survives predation on its host

How Japanese eels escape from the stomach of a predatory fish

Predation shapes diversity in the defensive tactics of prey. One specialized defensive tactic is to escape the digestive system of the predator after capture1,2,3,4,5,6,7,8. While most of these defensive tactics involve passive ejection alive from predators' mouths and vents1,2,3,4,5, active escape from the digestive tracts of predators has recently been observed in certain invertebrate species6,7 and fish8. However, no study has yet uncovered the behavioral patterns and escape routes of the prey within a predator's digestive tract. Here, we report the sequential escape processes of the Japanese eel Anguilla japonica from capture to escape via the gills of predatory fish Odontobutis obscura using an X-ray video system. All captured eels had at least one portion of their bodies swallowed into the stomach of the predator. Surprisingly, after being swallowed, most individuals attempted to escape by going back up the digestive tract towards the esophagus and gill, and some of them succeeded in escaping via the predator's gill. Some eels, whose entire bodies were completely inside the stomach, exhibited circling behavior along the stomach, seemingly searching for possible escape routes. An electro-anesthetization experiment revealed that eels utilize various escape routes through gill clefts, rather than just one.

Manipulative neuroparasites: uncovering the intricacies of neurological host control

Manipulative neuroparasites are a fascinating group of organisms that possess the ability to hijack the nervous systems of their hosts, manipulating their behavior in order to enhance their own survival and reproductive success. This review provides an overview of the different strategies employed by manipulative neuroparasites, ranging from viruses to parasitic worms and fungi. By examining specific examples, such as Toxoplasma gondii, Leucochloridium paradoxum, and Ophiocordyceps unilateralis, we highlight the complex mechanisms employed by these parasites to manipulate their hosts' behavior. We explore the mechanisms through which these parasites alter the neural processes and behavior of their hosts, including the modulation of neurotransmitters, hormonal pathways, and neural circuits. This review focuses less on the diseases that neuroparasites induce and more on the process of their neurological manipulation. We also investigate the fundamental mechanisms of host manipulation in the developing field of neuroparasitology, which blends neuroscience and parasitology. Finally, understanding the complex interaction between manipulative neuroparasites and their hosts may help us to better understand the fundamentals of behavior, neurology, and host-parasite relationships.

Rampant loss of universal metazoan genes revealed by a chromosome-level genome assembly of the parasitic Nematomorpha

Parasites may manipulate host behavior to increase the odds of transmission or to reach the proper environment to complete their life cycle.1,2 Members of the phylum Nematomorpha (known as horsehair worms, hairworms, or Gordian worms) are large endoparasites that affect the behavior of their arthropod hosts. In terrestrial hosts, they cause erratic movements toward bodies of water,3,4,5,6 where the adult worm emerges from the host to find mates for reproduction. We present a chromosome-level genome assembly for the freshwater Acutogordius australiensis and a draft assembly for one of the few known marine species, Nectonema munidae. The assemblies span 201 Mbp and 213 Mbp in length (N50: 38 Mbp and 716 Kbp), respectively, and reveal four chromosomes in Acutogordius, which are largely rearranged compared to the inferred ancestral condition in animals. Both nematomorph genomes have a relatively low number of genes (11,114 and 8,717, respectively) and lack a high proportion (∼30%) of universal single-copy metazoan orthologs (BUSCO genes7). We demonstrate that missing genes are not an artifact of the assembly process, with the majority of missing orthologs being shared by the two independent assemblies. Missing BUSCOs are enriched for Gene Ontology (GO) terms associated with the organization of cilia and cell projections in other animals. We show that most cilium-related genes conserved across eukaryotes have been lost in Nematomorpha, providing a molecular basis for the suspected absence of ciliary structures in these animals.

Search performance and octopamine neuronal signaling mediate parasitoid induced changes in Drosophila oviposition behavior

Making the appropriate responses to predation risk is essential for the survival of an organism; however, the underlying mechanisms are still largely unknown. Here, we find that Drosophila has evolved an adaptive strategy to manage the threat from its parasitoid wasp by manipulating the oviposition behavior. Through perception of the differences in host search performance of wasps, Drosophila is able to recognize younger wasps as a higher level of threat and consequently depress the oviposition. We further show that this antiparasitoid behavior is mediated by the regulation of the expression of Tdc2 and Tβh in the ventral nerve cord via LC4 visual projection neurons, which in turn leads to the dramatic reduction in octopamine and the resulting dysfunction of mature follicle trimming and rupture. Our study uncovers a detailed mechanism underlying the defensive behavior in insects that may advance our understanding of predator avoidance in animals.

Vertebrates as uninfected disseminators of helminth eggs and larvae

The passive dispersal of non-mobile organisms by vertebrates (zoochory) is a common mechanism used to explain their often widespread distribution. Transport occurs either internally via the vertebrate digestive tract (endozoochory), or externally be adhering to skin, feathers or fur (ectozoochory), and its success is due to both physiological and ecological factors associated with the disseminating 'hosting' animal. Helminth eggs and larvae are generally non-mobile stages that are largely dependent on the movement of another animal, typically a host, for geographical dissemination. Studies on the zoochory of helminths by vertebrates are extensive and particularly long-standing, stretching back to the 19th century, although this literature is often overlooked when considering the biogeography of parasites. This review assesses the potential of helminths to be dispersed passively by zoochory examining evidence from both laboratory and field studies. The physiological dynamics of the vertebrate intestines and skin surface as hostile environments, as well as the characteristics of eggs and larvae which may facilitate successful transport are evaluated. The various mechanisms of helminth endo- and ectozoochory are presented and the likelihood of long-distance dispersal determined. It is concluded that zoochory is a potentially important means of disseminating parasites.

Parasite transmission between trophic levels stabilizes predator-prey interaction

Manipulative parasites that promote their transmission by altering their host’s phenotype are widespread in nature, which suggests that host manipulation allows the permanent coexistence of the host with the parasite. However, the underlying mechanism by which host manipulation affects community stability remains unelucidated. Here, using a mathematical model, we show that host manipulation can stabilise community dynamics. We consider systems wherein parasites are transmitted between different trophic levels: intermediate host prey and final host predator. Without host manipulation, the non-manipulative parasite can destabilise an otherwise globally stable prey–predator system, causing population cycles. However, host manipulation can dampen such population cycles, particularly when the manipulation is strong. This finding suggests that host manipulation is a consequence of self-organized behavior of the parasite populations that allows permanent coexistence with the hosts and plays a key role in community stability.

Successful escape of bombardier beetles from predator digestive systems

Some prey animals can escape from the digestive systems of predators after being swallowed. To clarify the ecological factors that determine the success of such an escape, we investigated how the bombardier beetle Pheropsophus jessoensis escapes from two toad species, Bufo japonicus and Btorrenticola, under laboratory conditions. Pheropsophus jessoensis ejects a hot chemical spray from the tip of the abdomen when it is attacked. Although all toads swallowed the bombardier beetles, 43% of the toads vomited the beetles 12-107 min after swallowing them. All the vomited beetles were still alive and active. Our experiment showed that Pjessoensis ejected hot chemicals inside the toads, thereby forcing the toads to vomit. Large beetles escaped more frequently than small beetles, and small toads vomited the beetles more frequently than large toads. Our results demonstrate the importance of the prey-predator size relationship in the successful escape of prey from inside a predator.

Direct consumptive interactions between mammalian herbivores and plant-dwelling invertebrates: prevalence, significance, and prospectus

Mammalian herbivores induce changes in the chemical composition, phenology, distribution, and abundance of the plants they feed on. Consequently, invertebrate herbivores (predominantly insects) that depend on those plants, and the predators and parasitoids that are associated with them, may be affected. This plant-mediated indirect interaction between mammals and invertebrates has been extensively studied, but mammalian herbivores may also directly affect plant-dwelling invertebrates (PDI) by incidentally ingesting them while feeding. The ubiquity and small size of PDI render them highly susceptible to incidental ingestion, but as common as this interaction may intuitively seem, very little is known about its prevalence and ecological consequences. Nevertheless, cases of incidental ingestion of PDI and associated adaptations for avoiding it that have been sporadically documented in several invertebrate groups and life stages allow us to carefully extrapolate and conclude that it should be common in nature. Incidental ingestion may, therefore, bear significant consequences for PDI, but it may also affect the mammalian herbivores and the shared plants. Future research on incidental ingestion of PDI would have to overcome several technical difficulties to gain better insight into this understudied ecological interaction.

The brain's Geppetto-microbes as puppeteers of neural function and behaviour?

Research on the microbiome and its interaction with various host organs, including the brain, is increasingly gaining momentum. With more evidence establishing a comprehensive microbiota-gut-brain axis, questions have been raised as to the extent to which microbes influence brain physiology and behaviour. In parallel, there is a growing literature showing active behavioural manipulation in favour of the microbe for certain parasites. However, it seems unclear where the hidden majority of microbes are localised on the parasitism-mutualism spectrum. A long evolutionary history intimately connects host and microbiota, which complicates this classification. In this conceptual minireview, we discuss current hypotheses on host-microbe interaction and argue that novel experimental approaches and theoretical concepts, such as the hologenome theory, are necessary to incorporate transgenerational epigenetic inheritance of the microbiome into evolutionary theories.

Pathways to understanding the extended phenotype of parasites in their hosts

The study of the adaptive manipulation of animal behavior by parasites is entering very exciting times. Collectively the field has moved from its important and instructional natural history phase into proximate-level studies aiming to elucidate the mechanisms by which one organism controls another. Because many cases studies involve cross-kingdom control of behaviour, the findings are sure to be exciting. In this review I examine what possible pathways we can take to understanding the controlling behavior of parasites and how host behavior has become an extended phenotype of the parasites that is often hidden from view.

First record of Neoxysomatium brevicaudatum through the non-invasive sampling of Anguis fragilis: complementary morphological and molecular detection

Relatively few studies have examined the parasite fauna of British reptiles, partly due to the cryptic nature and low population density of these hosts. Here we examined 12 populations of the slow worm Anguis fragilis which, unlike other UK lizards, occurs at locally high population densities. Morphological examination of non-invasively collected faecal samples revealed the presence of Neoxysomatium brevicaudatum and a second unidentified nematode species. Although previously unrecorded from slow worms in the UK, N. brevicaudatum was present in 38% of animals (mean intensity 70.9, range 1–686). Morphological identification was confirmed by sequencing the 18S ribosomal gene. The use of the species-specific, cytochrome oxidase I mitochondrial gene primers proved an efficient alternative to conventional, microscope screening for parasites, although the original identification of N. brevicaudatum was dependent upon morphological characters. Sequencing also identified the second, smaller nematode as belonging to the Rhabdiasidae family: this species was even more common at a prevalence of 83% (mean intensity 102.8, range 1–2000). While increasing our knowledge of the UK macroparasite fauna, this work demonstrates the benefits of a combined morphological–molecular approach.

Water-seeking behavior in worm-infected crickets and reversibility of parasitic manipulation

One of the most fascinating examples of parasite-induced host manipulation is that of hairworms, first, because they induce a spectacular "suicide" water-seeking behavior in their terrestrial insect hosts and, second, because the emergence of the parasite is not lethal per se for the host that can live several months following parasite release. The mechanisms hairworms use to increase the encounter rate between their host and water remain, however, poorly understood. Considering the selective landscape in which nematomorph manipulation has evolved as well as previously obtained proteomics data, we predicted that crickets harboring mature hairworms would display a modified behavioral response to light. Since following parasite emergence in water, the cricket host and parasitic worm do not interact physiologically anymore, we also predicted that the host would recover from the modified behaviors. We examined the effect of hairworm infection on different behavioral responses of the host when stimulated by light to record responses from uninfected, infected, and ex-infected crickets. We showed that hairworm infection fundamentally modifies cricket behavior by inducing directed responses to light, a condition from which they mostly recover once the parasite is released. This study supports the idea that host manipulation by parasites is subtle, complex, and multidimensional.

Escape of parasitic water mites from dragonfly predators attacking their damselfly hosts

Many parasites are transmitted trophically, whereas others can either succumb to, or escape from, the predators of their hosts. We examined the extent to which larval arrenurid water mites ( Arrenurus planus Marshall, 1908 and Arrenurus pollictus Marshall, 1910) parasitizing lestid damselflies ( Lestes forcipatus Rambur, 1842 and Lestes disjunctus Sélys, 1862) escape from predatory libellulid dragonflies that are consuming their hosts. We hypothesized that the brightly coloured mites would be avoided by feeding dragonflies. However, all partially engorged A. pollictus mites were eaten while their host was being consumed in staged predation trials. In contrast, half of the fully engorged mites detached and therefore escaped consumption. Trials with A. planus mites showed that they detached more readily than their congenerics, which may be due to selection on those temporary pond mites to survive desiccation stress following detachment. The effect of dragonfly predation on transitioning of mites from parasitic larvae to their free-living aquatic stages therefore depends on the degree of engorgement and the mite species.

Heterokairy as an anti-predator strategy for parasitic species

Heterokairy refers to plasticity in the timing of onset of developmental events at the level of an individual. When two developmental stages do not share the same ecological niche, referred to as 'ontogenetic niches', the control of the niche shift through a change in developmental timing can be advantageous for the individual (e.g., when mortality risk is different in the two niches). Heterokairy can arise either from plasticity in developmental rate (ontogenetic shift) or by a purely behavioral decision (behavioral shift). Parasitic species living inside of their hosts often inherit the predators of their hosts. To cope with the predation risk on their hosts, parasites and parasitoids show either host-manipulation abilities or either host-leaving strategies. Nevertheless, leaving the host should be associated with developmental costs, since the parasitic individuals are usually unable to parasitize another host. This process is thus related to the classical tradeoff between size and developmental time. Recent studies provided examples of behavioral heterokairy in invertebrates. The goal of this publication is to review and discuss recent results on developmental plasticity in parasitic species in an evolutionary perspective.

Induced niche shift as an anti-predator response for an endoparasitoid

When two developmental stages do not share the same ecological niche, the control of the niche shift through a change in developmental timing, referred to as 'heterokairy', can provide an adaptive advantage for the individual (e.g. if mortality risk is higher in the first niche). For endoparasitic species that develop inside another (host) species, mortality of the host may directly induce mortality risk for the parasite. Thus, endoparasitoid larvae should be selected for response to host predation. In this study, aphids previously parasitized by the endoparasitoid Endaphis fugitiva, Gagné and Muratori (Diptera: Cecidomyiidae), were experimentally exposed to increased mortality risks. Both simulated attack and actual predator attacks against aphid hosts induced early emergence of the parasitoid larvae. Parasitoid emergence from the aphids occurred several minutes before the predator finished feeding on the aphid, allowing enough time for the parasitoid larvae to avoid direct predation. Predator-induced emergence produced significantly smaller parasitoid larvae than controls, but, interestingly, no effect on Endaphis adult size was found. To our knowledge, this is the first evidence of induced emergence in an insect parasitoid, but we suggest that this mechanism might be at work in many other species where plasticity in development time allows the individual to perform an adaptive niche shift.

Sea lice escape predation on their host

Parasites seldom have predators but often fall victim to those of their hosts. How parasites respond to host predation can have important consequences for both hosts and parasites, though empirical investigations are rare. The exposure of wild juvenile salmon to sea lice (Lepeophtheirus salmonis) from salmon farms allowed us to study a novel ecological interaction: the response of sea lice to predation on their juvenile pink and chum salmon hosts by two salmonid predators-coho smolts and cut-throat trout. In approximately 70% of trials in which a predator consumed a parasitized prey, lice escaped predation by swimming or moving directly onto the predator. This trophic transmission is strongly male biased, probably because behaviour and morphology constrain female movement and transmission. These findings highlight the potential for sea lice to be transmitted up marine food webs in areas of intensive salmon aquaculture, with implications for louse population dynamics and predatory salmonid health.

Parasitoid increases survival of its pupae by inducing hosts to fight predators

Many true parasites and parasitoids modify the behaviour of their host, and these changes are thought to be to the benefit of the parasites. However, field tests of this hypothesis are scarce, and it is often unclear whether the host or the parasite profits from the behavioural changes, or even if parasitism is a cause or consequence of the behaviour. We show that braconid parasitoids (Glyptapanteles sp.) induce their caterpillar host (Thyrinteina leucocerae) to behave as a bodyguard of the parasitoid pupae. After parasitoid larvae exit from the host to pupate, the host stops feeding, remains close to the pupae, knocks off predators with violent head-swings, and dies before reaching adulthood. Unparasitized caterpillars do not show these behaviours. In the field, the presence of bodyguard hosts resulted in a two-fold reduction in mortality of parasitoid pupae. Hence, the behaviour appears to be parasitoid-induced and confers benefits exclusively to the parasitoid.

Parasite-mediated allochthonous input: Do hairworms enhance subsidized predation of stream salmonids on crickets?

Energy and nutrients flow in diverse pathways across heterogeneous landscapes and tightly link the discrete food webs in local habitats. However, parasitism that enhances allochthonous resource input has not been previously documented. In a well-known example of parasite manipulation of host behaviour, crickets infected by mature hairworms (Nematomorpha) seek and jump into water when the worms reach the free-living stage. We found that a large number of trout (22%–61%), an aquatic predator, preyed on camel crickets (genera Diestrammena Brunner von Wattenwyl, 1888 and Tachycines Adelung, 1902) in September in five Japanese mountain streams where this host–parasite system exists. Trout (Kirikuchi charr, Salvelinus leucomaenis japonicus (Oshima, 1961); red-spotted masu salmon, Oncorhynchus masou ishikawae Jordan and McGregor, 1925) that preyed on crickets frequently ingested hairworms, whereas trout that did not prey on crickets did not ingest hairworms. Our results strongly suggest that hairworms enhance stream salmonid predation on camel crickets. This is the first documentation of parasitism enhancing allochthonous resource input in nature. Trout ingested a greater mass of crickets than other prey species in September, and this energy influx may play an important role in food-web dynamics in headwater streams.

Hairworm anti-predator strategy: a study of causes and consequences

One of the most fascinating anti-predator responses displayed by parasites is that of hairworms (Nematomorpha). Following the ingestion of the insect host by fish or frogs, the parasitic worm is able to actively exit both its host and the gut of the predator. Using as a model the hairworm, Paragordius tricuspidatus, (parasitizing the cricket Nemobius sylvestris) and the fish predator Micropterus salmoïdes, we explored, with proteomics tools, the physiological basis of this anti-predator response. By examining the proteome of the parasitic worm, we detected a differential expression of 27 protein spots in those worms able to escape the predator. Peptide Mass Fingerprints of candidate protein spots suggest the existence of an intense muscular activity in escaping worms, which functions in parallel with their distinctive biology. In a second step, we attempted to determine whether the energy expended by worms to escape the predator is traded off against its reproductive potential. Remarkably, the number of offspring produced by worms having escaped a predator was not reduced compared with controls.