Quantitative estimation of the cost of parasitic castration in a Helisoma anceps population using a matrix population model.

Larval trematodes frequently castrate their snail intermediate hosts. When castrated, the snails do not contribute offspring to the population, yet they persist and compete with the uninfected individuals for the available food resources. Parasitic castration should reduce the population growth rate lambda, but the magnitude of this decrease is unknown. The present study attempted to quantify the cost of parasitic castration at the level of the population by mathematically modeling the population of the planorbid snail Helisoma anceps in Charlie's Pond, North Carolina. Analysis of the model identified the life-history trait that most affects lambda, and the degree to which parasitic castration can lower lambda. A period matrix product model was constructed with estimates of fecundity, survival, growth rates, and infection probabilities calculated in a previous study. Elasticity analysis was performed by increasing the values of the life-history traits by 10% and recording the percentage change in lambda. Parasitic castration resulted in a 40% decrease in lambda of H. anceps. Analysis of the model suggests that decreasing the size at maturity was more effective at reducing the cost of castration than increasing survival or growth rates of the snails. The current matrix model was the first to mathematically describe a snail population, and the predictions of the model are in agreement with published research.

Life history cost of trematode infection in Helisoma anceps using mark-recapture in Charlie's Pond.

Parasitism has the potential to affect key life history traits of an infected host. Perhaps the most studied interactions are in snail-trematode systems, where infection can result in altered growth rates, survival, and/or fecundity of the individual. Positive correlations between host size and parasite prevalence are often attributed to changes in growth rates or mortality, which have been observed in the laboratory. Extending lab-based conclusions to the natural setting is problematic, especially when environmental conditions differ between the laboratory and the field. The present study uses reproduction experiments and mark-recapture methods to directly measure key life history traits of the pulmonate snail Helisoma anceps in Charlie's Pond. Based on previous laboratory and field experiments on H. anceps, we predict a significant reduction in fecundity, but not growth rate or survival, of infected snails. Individual capture histories were analyzed with multistate models to obtain estimates of survival and infection probabilities throughout the year. Recaptured individuals were used to calculate specific growth rates. Trematode infection resulted in complete castration of the host. However, neither survival nor growth rates were found to differ between infected and uninfected individuals. The probability of infection exhibited seasonal variation, but it did not vary with size of the snail. These results suggest that the correlation between host size and trematode prevalence is not due to differential mortality or changes in growth rates. Instead, the infection accumulates in large snails via the growth of smaller, infected individuals.

Long-term analysis of Charlie's Pond: fecundity and trematode communities of Helisoma anceps.

Charlie's Pond (North Carolina) harbors a diverse community of trematodes that infect the planorbid snail Helisoma anceps. Research at the Pond began in 1984 and serves as a foundation on which to investigate long-term changes in trematode communities. In 2002, 2005, and 2006 average size and fecundity of H. anceps were calculated each month, and seasonal trends analyzed with randomization tests. Concomitantly, trematode infections were recorded, and the community composition compared to those from previous studies. Helisoma anceps in 2002, 2005, and 2006 were smaller and less fecund than snails in 1984. The trematode community was consistently diverse, with 11 species recovered in 2006 versus 7 in 1984. However, the prevalence of Halipegus occidualis was much lower than previously observed (60% in 1984) and never exceeded 20% during the latter years. The decline of emergent vegetation is likely contributing to these changes. Aquatic macrophytes increase the surface area for growth of periphyton, the food source of these snails. Limited food supplies result in lower snail growth rates and fecundity. Similarly, emergent vegetation creates foci of transmission for H. occidualis between the frog definitive host and the snail intermediate host. When these areas are lost from the Pond, probability of transmission is reduced, and prevalence in the snail declines.

An examination of the infracommunities and component communities from impala (Aepyceros melampus) in the Kruger National Park, South Africa.

The intestinal helminth parasites of the impala from the Kruger National Park, South Africa, were examined to describe the parasite community structure. Demographic variation and the associated differences in behavior were used to further investigate the patterns of community composition. Monte Carlo simulations were performed to test for differences in species richness and mean abundance between the various demographic groups, and nonmetric multidimensional scaling ordination was used to compare community composition. Seventeen species of nematodes, totaling more than 1.3 million worms, were recovered. Males harbored a greater number of nematode species than did females, but adult females were more heavily infected than their male counterparts. Lambs acquired infections early in life, and their parasite community composition rapidly approached that of the older animals. The parasite community in the juvenile and adult males was significantly different from the community of the adult females. These data suggest that social and feeding behavior of the different age-sex classes structure the parasite component community of impala. Additionally, the distinction between common and rare parasites, and their classification in other herbivores, implies complex transmission dynamics that includes extensive species sharing within the Kruger National Park.

Patterns of association, nestedness, and species co-occurrence of helminth parasites in the greater kudu, Tragelaphus strepsiceros, in the Kruger National Park, South Africa, and the Etosha National Park, Namibia.

The helminth parasites of the greater kudu from the Kruger National Park (KNP), South Africa, and the Etosha National Park (ENP), Namibia, were examined to determine the major patterns of spatial and demographic variation in community structure and to evaluate nonrandomness in parasite community assembly. Nonmetric multidimensional scaling ordination procedures were used to test for differences in parasite community composition between hosts of the 2 parks and between hosts of different demographic groups within KNP. Infracommunities within KNP were also examined for patterns of nonrandomness using 2 null models, i.e., nestedness and species co-occurrence. Infracommunities of KNP and ENP were significantly different from each other, as were infracommunities of different host demographic groups within KNP. Parasite species in the greater kudu from KNP displayed significant levels of nestedness and were found to co-occur less frequently than expected by chance; however, this lack of co-occurrence was significant only when all demographic groups were considered. When restricted to any particular age class, co-occurrence patterns could not be distinguished from random. Overall, these data suggest that biogeography and host demographics are important factors in determining community organization of helminth parasites in the greater kudu.

Effect of temperature on the infectivity of metacercariae of Zygocotyle lunata (Digenea: Paramphistomidae).

As a test of the energy limitation hypothesis (ELH), we predicted that temperature would have a significant influence on the infectivity of metacercariae of the digenetic trematode Zygocotyle lunata. Snails infected with Z. lunata were collected from ponds near Crawfordsville, Indiana, isolated at room temperature, and examined for the release of cercariae. Newly encysted metacercariae were collected and incubated 1-30 days at 1 of 5 temperatures (0, 3, 25, 31, 37 C). Twenty-five cysts were fed to each of 5 or 10 mice per treatment group (temperature). At 17 days postinfection, mice were killed and worms were recovered; data were collected on levels of infection in each group and the total body area of each worm. No worms were found in mice fed cysts that had been held at 0 C or 37 C (after 30 days). There were no differences in prevalence, infectivity, or mean intensity among the 3, 25, and 31 C treatments. Infectivity of metacercariae incubated at 37 C for 1 day was significantly greater than in all other treatments, while infectivity of metacercariae in the 37 C/15-day treatment was significantly lower than in all others. Mean body area of worms at 37 C/15 days was significantly greater than at other temperatures, suggesting density-dependent increases in growth. These results, particularly those from the 37 C treatments, are consistent with the ELH; infectivity was lower at high temperatures or when incubated for more time at 1 temperature (fewer energy reserves). It is suggested that microhabitat conditions experienced by metacercariae of Z. lunata could contribute to longer larval life, thus influencing this parasite's temporal dispersal.