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1. Trematode parasites exceed aquatic insect biomass in Oregon stream food webs

   https://doi.org/10.1111/1365-2656.13409  

   Preston, Daniel L. ; Layden, Tamara J. ; Segui, Leah M. ; Falke, Landon P. ; Brant, Sara V. ; Novak, Mark ; Hoye, ed., Bethany ( January 2021 , Journal of Animal Ecology)

   Although parasites are increasingly recognized for their ecosystem roles, it is often assumed that free‐living organisms dominate animal biomass in most ecosystems and therefore provide the primary pathways for energy transfer.

   To examine the contributions of parasites to ecosystem energetics in freshwater streams, we quantified the standing biomass of trematodes and free‐living organisms at nine sites in three streams in western Oregon, USA. We then compared the rates of biomass flow from snailsJuga pliciferainto trematode parasites relative to aquatic vertebrate predators (sculpin, cutthroat trout and Pacific giant salamanders).

   The trematode parasite community had the fifth highest dry biomass density among stream organisms (0.40 g/m²) and exceeded the combined biomass of aquatic insects. Only host snails (3.88 g/m²), sculpin (1.11 g/m²), trout (0.73 g/m²) and crayfish (0.43 g/m²) had a greater biomass. The parasite ‘extended phenotype’, consisting of trematode plus castrated host biomass, exceeded the individual biomass of every taxonomic group other than snails. The substantial parasite biomass stemmed from the high snail density and infection prevalence, and the large proportional mass of infected hosts that consisted of trematode tissue (M = 31% per snail).

   Estimates of yearly biomass transfer from snails into trematodes were slightly higher than the combined estimate of snail biomass transfer into the three vertebrate predators. Pacific giant salamanders accounted for 90% of the snail biomass consumed by predators.

   These results demonstrate that trematode parasites play underappreciated roles in the ecosystem energetics of some freshwater streams.

    

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2. Predators enhance resilience of a saltmarsh foundation species to drought

   https://doi.org/10.1111/1365-2745.13525  

   Sharp, Sean J. ; Angelini, Christine ; Smale, ed., Dan ( October 2020 , Journal of Ecology)

   Disturbances are increasing in size and frequency with climate change, facilitating species that opportunistically exploit areas where habitat‐forming foundation species have been removed. Although it is well‐recognized that consumers, disease and weedy space‐holders can affect foundation species’ resistance to and recovery from disturbance, how predators influence their resilience is less clear.

   In salt marshareas de‐vegetated by drought and intensive snailLittoraria irroratagrazing (hereafter, ‘die‐offs’), we monitored bird use and experimentally manipulated bird and nekton access to the vegetated borders of die‐off mudflats across periods of both vegetation die‐off and regrowth to explore how these predators mediate the resilience of cordgrassSpartina alterniflora, the foundation species that structures US Atlantic coast salt marshes.

   Surveys revealed that birds, especially probers that agitate soils, forage year‐round for invertebrates in die‐off mudflats in our study area but not in adjacent vegetated areas.

   During periods of die‐off, cordgrass borders accessible to bird and nekton predators retreated >3‐times slower and snail densities were halved, relative to predator exclusion cages. In predator‐accessible plots, slower border retreat corresponded to greater snail infection by a bird host‐dependent trematode parasite. During recovery, cordgrass borders revegetated more quickly, and snail densities declined faster over time in unmanipulated controls relative to predator exclusions.

   Synthesis. These findings suggest that birds, through their transmission of parasites to snails, appear to act synergistically with snail‐consuming nekton to slow cordgrass loss after drought‐snail disturbances. Predator access also corresponds to faster cordgrass recovery as environmental conditions improve, although the mechanisms behind this need further investigation. Thus, predators that opportunistically forage within disturbances have the potential to suppress consumer impacts through multiple mechanisms, including consumption and disease transmission, thereby bolstering foundation species’ resilience and modulating whole ecosystem responses to climate change.

    

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3. Infectivity of the parasite Metschnikowia bicuspidata is decreased by time spent as a transmission spore, but exposure to phycotoxins in the water column has no effect

   https://doi.org/10.1111/fwb.14082  

   Sánchez, Kristel F. ; Zhong, Baili ; Agudelo, Jorge A. ; Duffy, Meghan A. ( April 2023 , Freshwater Biology)

   Transmission from one host to another is a crucial component of parasite fitness. For some aquatic parasites, transmission occurs via a free‐living stage that spends time in the water, awaiting an encounter with a new host. These parasite transmission stages can be impacted by biotic and abiotic factors that influence the parasite's ability to successfully infect or grow in a new host.

   Here we tested whether time spent in the water column and/or exposure to common cyanobacterial toxins impacted parasite transmission stages. More specifically, we tested whether the infectivity, within host growth, and virulence of the fungal parasiteMetschnikowia bicuspidatachanged as a result of time spent in the water or from exposure to cyanotoxins in the water column. We exposed parasite transmission spores to different levels of one of two ecologically important cyanotoxins, microcystin‐LR and anatoxin‐a, and factorially manipulated the amount of time spores were incubated in water. We removed the toxins and used those same spores to infect one genotype of the common lake zooplanktonDaphnia dentifera.

   We found that cyanotoxins did not impact parasite fitness (infection prevalence and spore yield per infected host) or virulence (host lifetime reproduction and survivorship) at the tested concentrations (10 and 30 μg/L). However, we found that spending longer as a transmission spore decreased a spore's chances for successful infection: spores that were only incubated for 24 hr infected approximately 75% of exposed hosts, whereas spores incubated for 10 days infected less than 50% of exposed hosts.

   We also found a negative relationship between the final spore yield from infected hosts and the proportion of hosts that became infected. In treatments where spores spent longer in the water column prior to encountering a host, infection prevalence was lower (indicating lower per spore infectivity), but each infected host yielded more spores at the end of infection. We hypothesise that this pattern may result from intraspecific parasite competition within the host.

   Overall, these results suggest that transmission spores of this parasite are not strongly influenced by cyanotoxins in the water column, but that other aspects of spending time in the water strongly influence parasite fitness.

    

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4. Parasitic manipulation or by-product of infection: an experimental approach using trematode-infected snails

   https://doi.org/10.1017/s0022149x21000699  

   Namias, Alice ; Delph, Lynda F. ; Lively, Curtis M. ( January 2022 , Journal of Helminthology)

   Abstract Natural selection should favour parasite genotypes that manipulate hosts in ways that enhance parasite fitness. However, it is also possible that the effects of infection are not adaptive. Here we experimentally examined the phenotypic effects of infection in a snail–trematode system. These trematodes ( Atriophallophorus winterbourni ) produce larval cysts within the snail's shell ( Potamopyrgus antipodarum ); hence the internal shell volume determines the total number of parasite cysts produced. Infected snails in the field tend to be larger than uninfected snails, suggesting the hypothesis that parasites manipulate host growth so as to increase the space available for trematode reproduction. To test the hypothesis, we exposed juvenile snails to trematode eggs. Snails were then left to grow for about one year in 800-l outdoor mesocosms. We found that uninfected males were smaller than uninfected females (sexual dimorphism). We also found that infection did not affect the shell dimensions of males. However, infected females were smaller than uninfected females. Hence, infection stunts the growth of females, and (contrary to the hypothesis) it results in a smaller internal volume for larval cysts. Finally, infected females resembled males in size and shape, suggesting the possibility that parasitic castration prevents the normal development of females. These results thus indicate that the parasite is not manipulating the growth of infected hosts so as to increase the number of larval cysts, although alternative adaptive explanations are possible. 

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

   https://doi.org/10.1017/S0022149X23000111  

   Koprivnikar, J. ; Thieltges, D.W. ; Johnson, P.T.J. ( January 2023 , Journal of Helminthology)

   Abstract 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. 

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