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Digeneans have important roles within ecosystems; however, it is estimated that only 14% of the species have been described. Therefore, before being able to detail their role, digenean species' identification and the diversity present in the ecosystems must be known. In this study, the diversity and the temporal-spatial dynamics of larval digeneans in the freshwater snail Heleobia parchappii were analysed in a shallow lake. Specimens of H. parchappii were collected seasonally at three points during one year and a total of 2871 molluscs were analysed. A total of 23 species of digenea were registered and both the overall prevalence and the composition of the assemblages presented temporal and spatial variations, responding to the differential environmental conditions characteristics (anthropic effect, presence of native forests, and differential use of the habitat by the definitive hosts) of three sampled sites. The assemblages of larval digenean in their first intermediate host support the idea that this area is of great importance in biodiversity, and could be endemic areas of some species of digenean that use reptiles, amphibians and bats as hosts, groups that are at risk of conservation. Protection of these environments is a fundamental pillar in the policies for the conservation of wild flora and fauna.

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Temporary ponds are ecologically valuable habitats and useful sites for studies of diversity, ecology, evolution, and climate change. Organisms inhabiting these environments have developed strategies to ensure their survival. However, little is known about the temporal dynamics and strategies of parasites in these habitats. A 4-year study was carried out in a temporary pond to analyze the larval digenean assemblage in Biomphalaria peregrina, a potential vector of Schistosoma mansoni, a zoonosis of global importance. This environment had intermittent and irregular hydroperiods, and the overall prevalence showed differences throughout the years. After drought seasons, eight out of a total of nine species of digeneans were observed, and almost half of the snails were parasitized. In addition, six of the nine species of digeneans showed some type of abbreviation of their life cycles. These results suggest that digeneans have certain degree of plasticity in their life cycles in response to environmental changes in this pond, and the abbreviation of the digenean life cycle could be regarded as a parasite strategy to resist prolonged periods of desiccation.

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The larval digenean assemblages in snail hosts are influenced by biotic and abiotic factors and by environmental disturbances due to human activities. Given their susceptibility to different types of impact, they can be useful tools for the study of temporal variations in parasite communities. The study of temporal dynamics in prevalence and species richness of larval digenean assemblages of Heleobia parchappii (DOrbigny, 1835), in two Pampean lakes, provides a good opportunity to test the supposition that seasonal variations in digenean assemblages are determined by a series of biotic and abiotic factors that operate interdependently. To analyze the temporal variations, and to evaluate how local factors influence these digenean assemblages in Los Padres and La Brava lakes (Buenos Aires province, Argentina), 2,400 specimens of H. parchappii were collected seasonally, during one year. The digenean assemblages in Los Padres and La Brava lakes were composed of 13 and 12 species respectively. These larval digenean assemblages showed similarities in the number and in the seasonal frequency of occurrence of species (eight of which were present in both lakes). However, despite these similarities, the assemblages showed different temporal dynamics in La Brava and Los Padres lakes and, also, different correlations with the abiotic and biotic factors analyzed during this study (temperature, high of the water column and snail relative abundance). Temporal dynamics in prevalence and species richness of larval digenean assemblages of H. parchappii in both lakes seems to be influenced by a series of factors (diversity and abundance of definitive hosts, intermediate and definitive host location, snail relative abundance and anthropogenic disturbances). Our results highlight the importance of analyzing the role and interaction of local factors, especially in studies dealing with spatiotemporal fluctuations or with distance decay of similarities in species richness of parasite assemblages.(AU)

Assuntos

RESUMO

The larval digenean assemblages in snail hosts are influenced by biotic and abiotic factors and by environmental disturbances due to human activities. Given their susceptibility to different types of impact, they can be useful tools for the study of temporal variations in parasite communities. The study of temporal dynamics in prevalence and species richness of larval digenean assemblages of Heleobia parchappii (DOrbigny, 1835), in two Pampean lakes, provides a good opportunity to test the supposition that seasonal variations in digenean assemblages are determined by a series of biotic and abiotic factors that operate interdependently. To analyze the temporal variations, and to evaluate how local factors influence these digenean assemblages in Los Padres and La Brava lakes (Buenos Aires province, Argentina), 2,400 specimens of H. parchappii were collected seasonally, during one year. The digenean assemblages in Los Padres and La Brava lakes were composed of 13 and 12 species respectively. These larval digenean assemblages showed similarities in the number and in the seasonal frequency of occurrence of species (eight of which were present in both lakes). However, despite these similarities, the assemblages showed different temporal dynamics in La Brava and Los Padres lakes and, also, different correlations with the abiotic and biotic factors analyzed during this study (temperature, high of the water column and snail relative abundance). Temporal dynamics in prevalence and species richness of larval digenean assemblages of H. parchappii in both lakes seems to be influenced by a series of factors (diversity and abundance of definitive hosts, intermediate and definitive host location, snail relative abundance and anthropogenic disturbances). Our results highlight the importance of analyzing the role and interaction of local factors, especially in studies dealing with spatiotemporal fluctuations or with distance decay of similarities in species richness of parasite assemblages.

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ABSTRACT The larval digenean assemblages in snail hosts are influenced by biotic and abiotic factors and by environmental disturbances due to human activities. Given their susceptibility to different types of impact, they can be useful tools for the study of temporal variations in parasite communities. The study of temporal dynamics in prevalence and species richness of larval digenean assemblages of Heleobia parchappii (DOrbigny, 1835), in two Pampean lakes, provides a good opportunity to test the supposition that seasonal variations in digenean assemblages are determined by a series of biotic and abiotic factors that operate interdependently. To analyze the temporal variations, and to evaluate how local factors influence these digenean assemblages in Los Padres and La Brava lakes (Buenos Aires province, Argentina), 2,400 specimens of H. parchappii were collected seasonally, during one year. The digenean assemblages in Los Padres and La Brava lakes were composed of 13 and 12 species respectively. These larval digenean assemblages showed similarities in the number and in the seasonal frequency of occurrence of species (eight of which were present in both lakes). However, despite these similarities, the assemblages showed different temporal dynamics in La Brava and Los Padres lakes and, also, different correlations with the abiotic and biotic factors analyzed during this study (temperature, high of the water column and snail relative abundance). Temporal dynamics in prevalence and species richness of larval digenean assemblages of H. parchappii in both lakes seems to be influenced by a series of factors (diversity and abundance of definitive hosts, intermediate and definitive host location, snail relative abundance and anthropogenic disturbances). Our results highlight the importance of analyzing the role and interaction of local factors, especially in studies dealing with spatiotemporal fluctuations or with distance decay of similarities in species richness of parasite assemblages.

RESUMO

Penis anatomy is used to discriminate species of gastropods belonging to the family Cochliopidae; however, this characteristic may be affected by the presence of parasites. To evaluate the possible effect of parasites on penis length and number of papillae in Heleobia parchappii, 195 males were collected from the Nahuel Rucá Lagoon, Argentina. Male snails were only infected by trematode digeneans (total prevalence 45.13%). Three out of 9 species of digeneans registered showed prevalence values higher than 10%: Microphallus szidati, M. simillimus, and Notocotylidae sp. 1. The penis length of non-parasitized males and those parasitized by M. szidati and M. similimus increased with increased snail length; however, this increase was lower in infected snails. In the case of snails infected with Notocotylidae sp. 1, no relationship between shell length and penis length was apparent. Differences in the life cycles of these 3 digeneans could explain the null or lower penis growth rate in relation to host body growth. In contrast, no change was observed in the number of penial papillae of H. parchappii when these snails were infected by larval digeneans compared to those that were not infected. This indicates that penial papillae may be a more stable characteristic than penis length to discriminate between species within the Cochliopidae. The study of penial papillae should be central in the taxonomy and identification of new species within the Cochliopidae, as well as in previously described species.

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The microphallids Microphallus szidati and Levinseniella cruzi parasitized at least three crustacean species as second intermediate host in Mar Chiquita coastal lagoon: the crabs Neohelice granulata and Cyrtograpsus angulatus and the grass shrimp Palaemonetes argentinus. Palaemonetes argentinus acts as host for both metacercariae in brackish and freshwater environments; however the seasonal variations of these digeneans in freshwater habitats remain unexplored. The aim of the present study was to analyze the seasonal dynamic of the larval digenean populations parasitizing P. argentinus in a lentic environment. From the total sample of 600 P. argentinus collected seasonally, 28.370 and 194 metacercariae of M. szidati and L. cruzi, respectively, were recovered. Microphallus szidati was characterized by high prevalence, 100% in all seasons. In contrast, L. cruzi presented low prevalence values, which increased towards summer. The two species of digeneans exhibit seasonal fluctuations in intensity of infection, which was related to host size. Temporal variation of larval digenean assemblage in the crustacean host is probably associated with the presence of the other hosts included in their life cycles. The high values of prevalence found for M. szidati in Nahuel Rucá lagoon suggest that P. argentinus could favor the contact between the larval digenean and the definitive hosts, establishing the presence of M. szidati also in freshwater environments and, thus, increasing the presence of the parasite in different areas of Mar Chiquita coastal lagoon.

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The importance of small-scale heterogeneity in local factors which overrides that of larger-scales factors, suggest that local factors play a major role in determining the richness and prevalence of larval digeneans in intertidal gastropods. The restricted distribution of the snail Heleobia australis along a 500 m transect in Cangrejo creek (Mar Chiquita, Argentina) provides a good opportunity to test the assumption that a study at spatial scale of 100s meters can detect spatiotemporal fluctuations of a larval digenean assemblage. To analyze that, 3600 specimens of H. australis were collected seasonally during the year 2011. A quantitative variation and a space-time interaction between sampling points and seasons of the year for the total prevalence of larval digeneans and snail's densities were found, as well as a positive correlation with abiotic factors. These results revealed that the fluctuations in the community of larval digeneans of the snail H. australis can be detected at small spatial scale, using its natural distribution of 500 m. This study also highlights the importance of seasonality as a factor that must be considered in studies focused on the search for patterns structuring the communities of larval digeneans, at medium and large scales.

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The nonparametric estimators of species richness are some of the most widely used extrapolation methods in studies of biodiversity. These studies need relative large samples to achieve total diversity (including rare species), and estimators are routinely used to alleviate this problem. In the existing literature, parasite communities have received little attention. In fact, the present study is the first dealing with parasites in invertebrate hosts. Real data on seasonal diversity of larval digeneans in the snails Heleobia conexa and Heleobia australis from Mar Chiquita coastal lagoon (Argentina) were used to evaluate seven nonparametric estimators of species richness, observing the behavior of each method and its ability to estimate the richness using different subsample sizes. In addition, estimated species richness was compared with the previously observed species richness in the parasite component communities of both snail hosts, and the minimum sample size, necessary to include all the observed species of larval digeneans, was estimated using the nonparametric estimators. The bootstrap method had the best overall performance in the two communities. Although this estimator required large subsamples to estimate sample richness, it was in general the least biased and most accurate, and it presented the most similar curve shapes with lack of erratic behavior through the seasons. The results obtained from the comparison between the minimum sample sizes estimated in previous studies of both snail hosts and those suggested by the estimators also support the use of species richness estimators as methods to calculate the minimum sample size in studies of diversity.

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