Host density increases parasite recruitment but decreases host risk in a snail-trematode system.

Most species aggregate in local patches. High host density in patches increases contact rate between hosts and parasites, increasing parasite transmission success. At the same time, for environmentally transmitted parasites, high host density can decrease infection risk to individual hosts, because infective stages are divided among all hosts in a patch, leading to safety in numbers. We tested these predictions using the California horn snail, Cerithideopsis californica (=Cerithidea californica), which is the first intermediate host for at least 19 digenean trematode species in California estuaries. Snails become infected by ingesting trematode eggs or through penetration by free-swimming miracidia that hatch from trematode eggs deposited with final-host (bird or mammal) feces. This complex life cycle decouples infective-stage production from transmission, raising the possibility of an inverse relationship between host density and infection risk at local scales. In a field survey, higher snail density was associated with increased trematode (infected snail) density, but decreased trematode prevalence, consistent with either safety in numbers, parasitic castration, or both. To determine the extent to which safety in numbers drove the negative snail-density-trematode-prevalence association, we manipulated uninfected snail density in 83 cages at eight sites within Carpinteria Salt Marsh (California, USA). At each site, we quantified snail density and used data on final-host (bird and raccoon) distributions to control for between-site variation in infective-stage supply. After three months, overall trematode infections per cage increased with snail biomass density. For egg-transmitted trematodes, per-snail infection risk decreased with snail biomass density in the cage and surrounding area, whereas per-snail infection risk did not decrease for miracidium-transmitted trematodes. Furthermore, both trematode recruitment and infection risk increased with infective-stage input, but this was significant only for miracidium-transmitted species. A model parameterized with our experimental results and snail densities from 524 field transects estimated that safety in numbers, when combined with patchy host density, halved per capita infection risk in this snail population. We conclude that, depending on transmission mode, host density can enhance parasite recruitment and reduce per capita infection risk.

A synthetic workflow for coordinated direct observation and genetic tagging applied to a complex host-parasite interaction.

An important aspect influencing host specificity is a parasite's compatibility, or ability, to infect a potential host. Here, we examine the compatibility between different trematode genotypes of the same species and several host species. To execute this study, we developed a synthetic workflow which combines the use of a fluorescent dye and standard molecular techniques to study host-parasite interactions and host specificity. The utility of the fluorescent dye, BIODIPY FL C12, was evaluated to label and track larval trematodes during experimental infections using the Cerithidea californica-trematode host-parasite system. Our results showed that low dye concentrations (200 nM) did not significantly affect survival or infectivity of Acanthoparyphium spinulosum and proved to be useful for labeling cercariae. Parasites were genotyped based on sequences from cytochrome oxidase subunit 1 (COI) and the nuclear internal transcribed spacer 1 (ITS1) prior to labeling and experimental infections. Samples with low COI PCR product yield were reamplified using the M13 tails to obtain enough material for sequencing. Three parasite genotypes were recovered and results from experimental infections demonstrated varying levels of host specificity. Of the three host species used (C. californica, Polydora nuchalis, Tagelus californianus), genotype B was unable to infect P. nuchalis. Genotype A individuals were less likely to infect P. nuchalis than the other host species. Additionally, genotype C was unable to infect any host offered in this study. These findings reflect possible suboptimal pairings between parasite genotype and host species. Furthermore, the present study provides procedures that are useful for exploring parasite ecology at the molecular level.

Digenean metacercariae of fishes from the lagoon flats of Palmyra Atoll, Eastern Indo-Pacific.

Although many studies on the taxonomy of digenean trematodes of marine fishes have been completed in the Eastern Indo-Pacific (EIP) marine ecoregion, only a few have considered metacercarial stages. Here, the results are presented of a taxonomic survey of the digenean metacercariae of fishes from Palmyra Atoll, a remote and relatively pristine US National Wildlife Refuge located 1680 km SSW of Hawaii. Up to 425 individual fish were collected, comprising 42 fish species, from the sand flats bordering the lagoon of the atoll. Quantitative parasitological examinations of each fish were performed. Morphological descriptions of the encountered digenean metacercariae are provided, together with their prevalence, mean intensities, host and tissue-use. Up to 33,964 individuals were recovered representing 19 digenean metacercaria species from eight families. The species composition of digeneans in lagoon fishes at Palmyra Atoll is a subset of what has previously been reported for the EIP. Further, the large diversity and abundance of metacercariae reported in this study highlight the utility of including this group in future ecological research in the EIP marine ecoregion.

Ecology of the brain trematode Euhaplorchis californiensis and its host, the California killifish ( Fundulus parvipinnis ).

We describe the distribution and abundance of the brain-encysting trematode Euhaplorchis californiensis and its second intermediate host, the California killifish (Fundulus parvipinnis), in 3 estuaries in southern California and Baja California. We quantified the density of fish and metacercariae at 13-14 sites per estuary and dissected 375 killifish. Density (numbers and biomass) was examined at 3 spatial scales, i.e., small replicate sites, habitats, and entire estuaries. At those same scales, factors that might influence metacercaria prevalence, abundance, and aggregation in host individuals and populations were also examined. Metacercaria prevalence was 94-100% among the estuaries. Most fish were infected with 100s to 1,000s of E. californiensis metacercariae, with mean abundance generally increasing with host size. Although body condition of fish did not vary among sites or estuaries, the abundance of metacercariae varied significantly among sites, habitats, estuaries, and substantially with host size and gender. Metacercariae were modestly aggregated in killifish (k > 1), with aggregation decreasing in larger hosts. Across the 3 estuaries, the total populations of killifish ranged from 9,000-12,000 individuals/ha and from 7-43 kg/ha. The component populations of E. californiensis metacercariae ranged from 78-200 million individuals/ha and from 0.1-0.3 kg/ha. Biomass of E. californiensis metacercariae constituted 0.5-1.7% of the killifish biomass in the estuaries. Our findings, in conjunction with previously documented effects of E. californiensis, suggest a strong influence of this parasite on the size, distribution, biomass, and abundance of its killifish host.

Parasite manipulation of brain monoamines in California killifish (Fundulus parvipinnis) by the trematode Euhaplorchis californiensis.

California killifish (Fundulus parvipinnis) infected with the brain-encysting trematode Euhaplorchis californiensis display conspicuous swimming behaviours rendering them more susceptible to predation by avian final hosts. Heavily infected killifish grow and reproduce normally, despite having thousands of cysts inside their braincases. This suggests that E. californiensis affects only specific locomotory behaviours. We hypothesised that changes in the serotonin and dopamine metabolism, essential for controlling locomotion and arousal may underlie this behaviour modification. We employed micropunch dissection and HPLC to analyse monoamine and monoamine metabolite concentrations in the brain regions of uninfected and experimentally infected fish. The parasites exerted density-dependent changes in monoaminergic activity distinct from those exhibited by fish subjected to stress. Specifically, E. californiensis inhibited a normally occurring, stress-induced elevation of serotonergic metabolism in the raphae nuclei. This effect was particularly evident in the experimentally infected fish, whose low-density infections were concentrated on the brainstem. Furthermore, high E. californiensis density was associated with increased dopaminergic activity in the hypothalamus and decreased serotonergic activity in the hippocampus. In conclusion, the altered monoaminergic metabolism may explain behavioural differences leading to increased predation of the infected killifish by their final host predators.

Small estuarine fishes feed on large trematode cercariae: lab and field investigations.

In aquatic ecosystems, dense populations of snails can shed millions of digenean trematode cercariae every day. These short-lived, free-living larvae are rich in energy and present a potential resource for consumers. We investigated whether estuarine fishes eat cercariae shed by trematodes of the estuarine snail Cerithidea californica. In aquaria we presented cercariae from 10 native trematode species to 6 species of native estuarine fishes. Many of these fishes readily engorged on cercariae. To determine if fishes ate cercariae in the field, we collected the most common fish species, Fundulus parvipinnis (California killifish), from shallow water on rising tides when snails shed cercariae. Of 61 killifish, 3 had recognizable cercariae in their gut. Because cercariae are common in this estuary, they could be frequent sources of energy for small fishes. In turn, predation on cercariae by fishes (and other predators) could also reduce the transmission success of trematodes.

Histopathological effects of trypanorhynch metacestodes in the digestive gland of a novel host, Carcinus maenas (Decapoda).

The green crab Carcinus maenas was introduced to Australian temperate waters in the late 1800s, has since become established, and is now considered to be a pest. We undertook an extensive parasite survey to find potential natural enemies of C. maenas and found it to be infected in Australia by 2 species of larval trypanorhynch tapeworm, Trimacracanthus aetobatidis and Dollfusiella martini. We describe the gross pathology and histopathology of the parasites' new host (C. maenas) and note that the plerocercoid larvae are located in the lumen of the digestive gland tubules. The presence of D. martini in C. maenas with low population numbers suggests that either D. martini has an impact (direct or indirect) on the survival of C. maenas, or that the parasite may be an indicator of high predation pressure. If the former were true, this would contribute to the control of this introduced pest species.

Parasites in the thoracic ganglion of Pachygrapsus marmoratus (Brachyura: Grapsidae) from the coast of Portugal.

We examined 149 marbled shore crabs, Pachygrapsus marmoratus, from the coast of Portugal for parasites. In particular, we focused our effort on the crab thoracic ganglion. The thoracic ganglion is the largest concentration of nervous tissue in a crab and thus, parasites associated with this organ are well situated to influence host behavior. We found metacercariae of two microphallid trematode species in the thoracic ganglion. We also found a microsporan and an apicomplexan associated with the thoracic ganglion. Other parasites not associated with the thoracic ganglion included gregarine trophozoites which were present in the digestive diverticulae in some of the crabs and the entoniscid isopod, Grapsion cavolini. Metacercariae of one of the trematodes (probably Microphallus pachygrapsi (Deblock and Prevot)), may influence the mortality of its host.

Parasites and marine invasions.

Introduced marine species are a major environmental and economic problem. The rate of these biological invasions has substantially increased in recent years due to the globalization of the world's economies. The damage caused by invasive species is often a result of the higher densities and larger sizes they attain compared to where they are native. A prominent hypothesis explaining the success of introduced species is that they are relatively free of the effects of natural enemies. Most notably, they may encounter fewer parasites in their introduced range compared to their native range. Parasites are ubiquitous and pervasive in marine systems, yet their role in marine invasions is relatively unexplored. Although data on parasites of marine organisms exist, the extent to which parasites can mediate marine invasions, or the extent to which invasive parasites and pathogens are responsible for infecting or potentially decimating native marine species have not been examined. In this review, we present a theoretical framework to model invasion success and examine the evidence for a relationship between parasite presence and the success of introduced marine species. For this, we compare the prevalence and species richness of parasites in several introduced populations of marine species with populations where they are native. We also discuss the potential impacts of introduced marine parasites on native ecosystems.

Selective predation on parasitized prey--a comparison between two helminth species with different life-history strategies.

In Lake Fjellfrøsvatn, northern Norway, the larval helminths Cyathocephalus truncatus and Cystidicola farionis use Gammarus lacustris as intermediate hosts and Arctic charr (Salvelinus alpinus) as final hosts. There was sampled 1,433 live G. lacustris from the lake and 1,964 G. lacustris from stomach contents of the charr. Prevalence of infection were, respectively, 0.49% and 3.72% for C. truncatus, and 0.21% and 0.20% for C. farionis. Usually, only 1 parasite was present in each host, and the 2 parasite species never co-occurred. Gammarus lacustris amphipods parasitized by C. truncatus were positively selected by the Arctic charr and were consumed approximately 8 times as often as were the unparasitized amphipods or the amphipods infected with C. farionis. This suggests that G. lacustris amphipods infected with C. truncatus larvae are more susceptible to predation than noninfected specimens, probably because of parasite-induced alterations in behavior or visibility. Alternatively, this could also be explained by selection toward the largest G. lacustris specimens observed, which are also the most frequently parasitized amphipods. However, the data show clearly that this was not a result of size-selective predation by the charr. In contrast, the presence of C. farionis did not increase the susceptibility to predation of its intermediate host. The discrepancy between the 2 helminth species supports the hypothesis that parasite-increased susceptibility to predation is related to the life history strategies of the parasites.

Impact of the crayfish Procambarus clarkii on Schistosoma haematobium transmission in Kenya.

The Louisiana red swamp crayfish, Procambarus clarkii, which was introduced into east Africa in the 1950s or 1960s, has since widely dispersed. Previous work by our group has shown that P. clarkii can reduce populations of the molluscan intermediate hosts of human schistosomes through predatory and competitive interactions. Here, we investigate whether crayfish can reduce populations of Bulinus africanus and consequently, Schistosoma haematobium prevalence in school children. Children from 6 primary schools in the Machakos and Kitui Districts of Kenya were selected for study. Schools were divided into 3 experimental-control pairs. At experimental schools, crayfish were introduced into nearby aquatic habitats harboring Bulinus africanus snails and serving as S. haematobium transmission sites. Snail habitats near control schools did not receive crayfish. Six months after crayfish introduction, all infected children were treated with praziquantel. Children were then monitored quarterly for 2 years, at which time infection and reinfection rates were compared statistically between the paired schools. In one such pair, crayfish failed to establish, resulting in neither snail control nor a reduction in transmission. At the second pair of schools, the numbers of snails were decreased by the presence of crayfish, but a clear difference in infection rates in children could not be detected, primarily because drought conditions kept overall transmission rates low. At the third school pair, crayfish established well in experimental habitats, snail numbers decreased precipitously, and children at the experimental school were significantly less likely to acquire S. haematobium infections than children at the control school. Our results indicate that under certain environmental circumstances, P. clarkii exerts a significant impact on the transmission of human schistosomiasis in Kenya. Important questions remain regarding the impact of P. clarkii on Kenyan freshwater ecosystems, not the least of which is its potential to significantly influence the epidemiology of schistosomiasis in east Africa.

Morphological and biological notes on Polymorphus (Profilicollis) sphaerocephalus and Corynosoma stanleyi (Polymorphidae: Acanthocephala).

Polymorphus (Profilicollis) sphaerocephalus (Bremser in Rudolphi, 1819) Van Cleave, 1947 (Polymorphidae) cystacanths were recovered from 5 species of grapsid crabs (Paragrapsus gaimardii (Milne Edwards, 1837), Paragrapsus laevis (Dana, 1852), Paragrapsus quadridentatus (Milne Edwards, 1837), Brachynotus spinosus (Milne Edwards, 1853), and Cyclograpsus granulosus (Milne Edwards, 1853)) and 1 species of portunid crab (Nectocarcinus integrifrons (Linnaeus, 1766)) from intertidal zones in southern temperate waters of Australia. Cystacanths of Corynosoma stanleyi Smales, 1986 (Polymorphidae) were also recovered from P. gaimardii, P. quadridentatus, and C. granulosus. Polymorphus (P.) sphaerocephalus was the most prevalent (100%) in C. granulosus at Flinders I. and C. stanleyi was most prevalent (59.1%) in C. granulosus at Dunally Channel, Tasmania.

Infestation of an introduced host, the European green crab, Carcinus maenas, by a symbiotic nemertean egg predator, Carcinonemertes epialti.

The recent introduction of the European green crab, Carcinus maenas, to the west coast of the U.S. has provided an opportunity for host transfer of the symbiotic nemertean egg predator, Carcinonemertes epialti, from its native shore crab host, Hemigrapsus oregonensis to the exotic C. maenas. Two surveys of C. maenas in Bodega Harbor, California, revealed that, in March 1995 prevalence of C. epialti on C. maenas was significantly lower than on H. oregonensis (11% versus 74%), but in November 1995 there was no significant difference between the 2 species (79% versus 98%). Only juvenile C. epialti were recovered from C. maenas in March 1995. However, in November 1995, ovigerous C. maenas were harboring actively feeding adult worms. Prevalence in both crab species significantly differed from March to November. Laboratory studies revealed that C. epialti fed and reproduced on eggs of C. maenas. The feeding rate of C. epialti on C. maenas eggs (2.5 eggs/trial) was not significantly different from that on H. oregonensis eggs (3.6 eggs/trial). Our findings suggest that this nemertean may have less host specificity than was previously thought. If C. epialti causes brood mortality of C. maenas in nature, it could potentially impact populations of this exotic crab.

Predation and control of laboratory populations of the snail Biomphalaria glabrata by the freshwater prawn Macrobrachium rosenbergii.

Laboratory experiments were conducted on predation by the giant freshwater prawn, Macrobrachium rosenbergii, on Biomphalaria glabrata, a snail intermediate host of human schistosomiasis. Prawns greater than 22 mm carapace length could consume snails of any size. Smaller prawns exhibited a size-specific upper limit on the size of snail they could kill. Below the maximum size of prey consumed, little selectivity for prey size was demonstrated. All prawns heavier than 2.5 g killed the largest size of snail offered (16 mm diameter). Consumption, in terms of the numbers of snails killed, and the snails' wet weight and percentage of prawn body weight eaten per day, increased with prawn size. Large prawns consumed snails at a high rate (39% of body weight per day). Population experiments demonstrated that a prawn of 25 mm carapace length could eliminate 95% of a population of 80 snails in a 20-1 aquarium within 20 days and all snails by day 40. A prawn of 15 mm carapace length could not eliminate all snails. Large snails (24% of the initial population) had some protection from predation because of their size. Although the snails bred continuously, no snails were able to recruit to these populations in the presence of small prawns. In contrast, snail populations in control aquaria without prawns expanded to a mean of 919 snails by the end of the experiment (day 70). Since M. rosenbergii are voracious predators on B. glabrata, exhibit considerable habitat overlap with the snail prey, and are likely to treat snails as highly preferred food, further experimentation on these prawns in the context of biological control of schistosomiasis is warranted. The ready availability of prawns through established prawn hatcheries and the synergistic use of these prawns in aquaculture may ensure the reliability of stocking procedures and meet the standards of availability and cost-effectiveness required of a biological control agent. Polyculture of Macrobrachium with fin fish (Tilapia) indicates that these prawns may be a particularly useful control agent in aquacultural environments.

Echinostome cercarial penetration and metacercarial encystment as mortality factors for a second intermediate host, Biomphalaria glabrata.

Cercarial penetration and metacercarial encystment of Echinostoma liei in Biomphalaria glabrata caused high mortality of juvenile snails (3-8 mm in diameter) after 4 to 6 days of continuous exposure to about 150 cercariae per snail per day. Larger snails (10-13 mm) withstood cercarial penetration longer, significant mortality appearing 16 days after exposure. Life-table analysis showed that the snail mortality rate per age interval increased with increasing exposure to echinostome penetration. Single exposures of 3- and 6-mm snails to 500 cercariae caused 97% mortality for the former and 84% for the latter within 2 days. Snails surviving the first 2 days did not suffer further mortality when infected with as many as 488 metacercariae. Thus, mortality seems to result from penetration and early stages of encystment, rather than the presence of large numbers of metacercariae in the pericardial region. Growth of 3-mm snails exposed to 10 or 100 cercariae and of 6-mm snails to 500 cercariae was significantly less than the growth of unexposed controls after 20 days. Cercariae rapidly located the snails, 90% penetrating within 1 hr when individual, 6-mm snails were exposed to 100 cercariae in 5 ml of water. The high rate of cercarial penetration needed to cause snail mortality suggests that echinostome penetration and encystment will not cause much mortality of snail intermediate hosts in nature unless the density of echinostome cercariae is high.

Effect of limb regeneration on size increase at molt of the shore crabs Hemigrapsus oregonensis and Pachygrapsus crassipes.

Size increase at molt is reduced following multiple limb regeneration in the shore crabs, Hemigrapsus oregonensis and Pachygrapsus crassipes. Limb loss per se does not influence postmolt size. Effect of increasing number of regenerating limbs is additive. Postmolt size is programmed early in the premolt period of the preceding instar and is probably not readily influenced by water uptake mechanics at ecdysis. A simple model for growth, molting, and regeneration in heavily calcified Crustacea is developed from the viewpoint of adaptive strategies and energetic considerations.