Quantifying the biomass of parasites in amphibian hosts to understand their role in aquatic food webs

Background/Question/Methods

By infecting multiple host species and acting as a food resource, parasites can affect food-web topography and contribute to ecosystem energy transfer. Owing to their remarkable secondary production, parasite biomass – while cryptic – can be comparable to other invertebrate and vertebrate groups. Within individual hosts, parasite species can vary substantially in both abundance and biomass, such that more resolved estimates of parasite biomass are needed to understand parasite interactions, their consequences for host fitness, and potential influence on ecosystem energetics. However, relatively few studies have empirically measured parasite mass, particularly across the multiple life stages and hosts inherent to many macroparasites. Here, we developed and applied an approach to accurately quantify the masses of different parasites and compared our results with those of more traditional biovolume-based approaches. Specifically, we isolated varying numbers and species of freshly collected larval and adult parasites (ten species and four life stages) onto pre-dried and weighed filters and measured mass change following 48 hrs of drying at 60°C using a microbalance. We used a replicated regression approach to quantify mass, which entailed a range of replicated parasite quantities such that mass could be estimated using the slope of observed relationships.

 Results/Conclusions

We present data on the following: cercariae, meso/metacercariae, rediae and adult Echinostoma trivolvis, Ribeiroia ondatrae, Cephalogonimus spp., Alaria mustelae, Clinostomum attenuatum, Fibricola spp., Halipegus spp., Haematoloechus spp., Megalodiscus temperatus & Gorgoderina spp. On average, the mass of each group was as follows: cercariae (0.62±0.25µg), metacercariae (36.0±33.0µg), adults (280.0±87.0µg), and rediae (5.0±1.1µg). However, there was four orders of magnitude variation among stages and species, ranging from 0.37±0.04µg (E. trivolvis cercariae) to 501.0±37.0µm (Halipegus spp. adults). In general, our methods provided robust estimates of mass, and the use of a regression approach further validated our measurements with R² values of 0.91 to 0.99. We compare these results with those obtained using more typical biovolume estimation techniques and show that biovolume can yield accurate biomass estimates but for some species/lifestages, it may be off by 2-3x. Finally, we present some insights that a metric of parasite biomass can provide. By incorporating measurements of host mass and parasite infection intensity within amphibian hosts, we estimate that parasite biomass averages 0.061% (±0.00016) and can reach as high as 5.8% of total mass, which provides a valuable foundation for subsequent studies focused on parasite community interactions and the role of parasites in aquatic energy transfer.