Parasites and their freshwater snail hosts maintain their nutritional value for essential fatty acids despite altered algal diets

Assessing the Combined Effects of Host and Parasite Exposure to Forever Chemicals in an Amphibian-Echinostome System

Per- and polyfluoroalkyl substances (PFAS) are environmental contaminants of growing concern due to their potential negative effects on wildlife and human health. Per- and polyfluoroalkyl substances have been shown to alter immune function in various taxa, which could influence the outcomes of host-parasite interactions. To date, studies have focused on the effects of PFAS on host susceptibility to parasites, but no studies have addressed the effects of PFAS on parasites. To address this knowledge gap, we independently manipulated exposure of larval northern leopard frogs (Rana pipiens) and parasites (flatworms) via their snail intermediate host to environmentally relevant PFAS concentrations and then conducted trials to assess host susceptibility to infection, parasite infectivity, and parasite longevity after emergence from the host. We found that PFAS exposure to only the host led to no significant change in parasite load, whereas exposure of parasites to a 10-µg/L mixture of PFAS led to a significant reduction in parasite load in hosts that were not exposed to PFAS. We found that when both host and parasite were exposed to PFAS there was no difference in parasite load. In addition, we found significant differences in parasite longevity post emergence following exposure to PFAS. Although some PFAS-exposed parasites had greater longevity, this did not necessarily translate into increased infection success, possibly because of impaired movement of the parasite. Our results indicate that exposure to PFAS can potentially impact host-parasite interactions. Environ Toxicol Chem 2024;43:1537-1546. © 2024 SETAC.

Species-specific oxylipins and the effects of ontogeny and predation on their emission from freshwater snails

Chemical cues play important roles in mediating ecological interactions. Oxylipins, oxygenated metabolites of fatty acids, are one signalling molecule type that influences the physiology and function of species, suggesting their broader significance in chemical communication within aquatic systems. Yet, our current understanding of their function is restricted taxonomically and contextually making it difficult to infer their ecological significance. Snails and leeches are ubiquitous in freshwater ecosystems worldwide, yet little is known about their oxylipin profiles and the factors that cause their profiles to change. As snails and leeches differ taxonomically and represent different trophic groups, we postulated oxylipin profile differences. For snails, we hypothesized that ontogeny (non-reproductive vs reproductive) and predation (non-infested vs leech-infested) would affect oxylipin profiles. Oxylipins were characterized from water conditioned with the snail Planorbella duryi and leech Helobdella lineata, and included three treatment types (snails, leeches, and leech-infested snails) with the snails consisting of three size classes: small (5-6 mm, non-reproductive) and medium and large (13-14 and 19-20 mm, reproductive). The two species differed in the composition of their oxylipin profiles both in diversity and amounts. Further, ontogeny and predation affected the diversity of oxylipins emitted by snails. Our experimental profiles of oxylipins show that chemical cues within freshwater systems vary depending upon the species emitting the signals, the developmental stage of the species, as well as from ecological interactions such as predation. We also identified some candidates, like 9-HETE and PGE2, that could be explored more directly for their physiological and ecological roles in freshwater systems.

Consumption of trematode parasite infectious stages: from conceptual synthesis to future research agenda

Given their sheer cumulative biomass and ubiquitous presence, parasites are increasingly recognized as essential components of most food webs. Beyond their influence as consumers of host tissue, many parasites also have free-living infectious stages that may be ingested by non-host organisms, with implications for energy and nutrient transfer, as well as for pathogen transmission and infectious disease dynamics. This has been particularly well-documented for the cercaria free-living stage of digenean trematode parasites within the Phylum Platyhelminthes. Here, we aim to synthesize the current state of knowledge regarding cercariae consumption by examining: (a) approaches for studying cercariae consumption; (b) the range of consumers and trematode prey documented thus far; (c) factors influencing the likelihood of cercariae consumption; (d) consequences of cercariae consumption for individual predators (e.g. their viability as a food source); and (e) implications of cercariae consumption for entire communities and ecosystems (e.g. transmission, nutrient cycling and influences on other prey). We detected 121 unique consumer-by-cercaria combinations that spanned 60 species of consumer and 35 trematode species. Meaningful reductions in transmission were seen for 31 of 36 combinations that considered this; however, separate studies with the same cercaria and consumer sometimes showed different results. Along with addressing knowledge gaps and suggesting future research directions, we highlight how the conceptual and empirical approaches discussed here for consumption of cercariae are relevant for the infectious stages of other parasites and pathogens, illustrating the use of cercariae as a model system to help advance our knowledge regarding the general importance of parasite consumption.

The Symbiotic Relationship between the Antarctic Limpet, Nacella concinna, and Epibiont Coralline Algae

The Antarctic limpet, Nacella concinna, is one of the most abundant benthic marine invertebrates found in the intertidal zone of King George Island, Antarctica. The shell of N. concinna is often encrusted with the coralline algae Clathromorphum obtectulum. In this study, to reveal the relationship between the limpet and coralline algae, we examined how the coralline algae affect the physical condition (survival and health) and morphology of the limpet. We cultured the limpets for 22 days and compared mortality, weight, condition factor (CF), fatty acid content, and the structure of the shell surface between limpets both with and without coralline algae in the laboratory. We also measured the environmental factors (i.e., temperature, pH, and salinity) of the seawater at each sampling site and the CF of the limpets and correlated them with coverage of coralline algae. The presence of coralline algae significantly increased the mortality of the limpets by 40% and the shell weight by 1.4-fold but did not affect the CF. Additionally, coralline algae altered the fatty acid profiles related to the limpet’s lipid metabolism (saturated fatty acids (SFA) and some polyunsaturated fatty acids (PUFA)). Specifically, C16:0, C17:0, C18:0, and total SFA increased, whereas C18:2 and C18:3 decreased. However, observations with a scanning electron microscope showed that shell damage in limpets with coralline algae was much less than in limpets without coralline algae, suggesting that coralline algae may provide protection against endolithic algae. The area of coralline algae on the limpet shell was positively correlated with the pH and temperature of the seawater. The results suggest that although coralline algae are generally assumed to be parasitical, the relationship between N. concinna and coralline algae may change to mutualism under certain conditions.

Stable isotopes unravel the feeding mode-trophic position relationship in trematode parasites

Stable isotopes have been sporadically used over the last two decades to characterise host-parasite trophic relationships. The main reason for this scarcity is the lack of an obvious pattern in the ratio of nitrogen stable isotope values (δ¹⁵ N) of parasites in comparison to their host tissues, which would be key to understand any host-parasite system dynamics. To circumvent this, we focused on a single snail host, Zeacumantus subcarinatus, and three of its trematode parasites. We used stable isotopes to investigate each host-trematode trophic relationship and shed light on the mechanisms utilised by the parasite to reroute its hosts' biomass. All our trematodes were found to be ¹⁵ N-enriched compared to their host, with their δ¹⁵ N values strongly related to their feeding behaviours: passive versus active. It was possible to 'rank' these parasite species and assess their 'relative' trophic position using δ¹⁵ N values. We also demonstrated that including a broader range of samples (e.g. host food and faeces, multiple parasite life stages) helped understand the metabolic mechanisms used by the various participants, and that using carbon stable isotope values and C:N ratios allowed to identify an important lipid requirement of these trematode parasites. Finally, we show how critical it is to not ignore parasitic infections as they can have a great influence on their host's trophic position. We have shown that by focussing on a single host species and a single taxonomic group of parasites, we can remove a certain amount of variation recorded by broader isotope studies. We hope that these data will ultimately improve our ability to place parasites in food webs, and thus improve our understanding of the connections and interactions that dictate food web dynamics.