Quantifying the biomass of parasites to understand their role in aquatic communities

Abstract

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 the remarkable secondary production of some taxa, parasite biomass – although cryptic – can be comparable to other invertebrate and vertebrate groups. More resolved estimates of parasite biomass are therefore needed to understand parasite interactions, their consequences for host fitness, and potential influences on ecosystem energetics. We developed an approach to quantify the masses of helminth parasites and compared our results with those of biovolume-based approaches. Specifically, we massed larval and adult parasites representing 13 species and five life stages of trematodes and cestodes from snail and amphibian hosts. We used a replicated regression approach to quantify dry mass and compared these values with indirect biovolume estimates to test the validity of density assumptions. Our technique provided precise estimates (R2 from 0.69 to 0.98) of biomass across a wide range of parasite morphotypes and sizes. Individual parasites ranged in mass from 0.368 ± 0.041 to 320 ± 98.1 μg. Among trematodes, adult parasites tended to be the largest followed by rediae, with nonclonal larval stages (metacercariae and cercariae) as the smallest. Among similar morphotypes, direct estimates of dry mass and the traditional biovolume technique provided generally comparable estimates (although important exceptions also emerged). Finally, we present generalized length-mass regression equations to calculate trematode mass from length measurements, and discuss the most efficient use of limited numbers of parasites. By providing a novel method of directly estimating parasite biomass while also helping to validate more traditional methods involving length-mass conversion, our findings aim to facilitate future investigations into the ecological significance of parasites, particularly with respect to ecosystem energetics. In addition, this novel technique can be applied to a wide range of difficult-to-mass organisms.

Evaluation of three methods for biomass estimation in small invertebrates, using three large disparate parasite species as model organisms

“…Area By Depth By Density (Flat Section): This method was applied to C . oblonga and is based on Lambden & Johnson 21 . These authors squashed specimens in a microwell of known depth to obtain the ventral area of the organism by means of image acquisition and analysis software.…”

“…We measured the area (µm 2 ) of individuals in ventral view with Fiji-ImageJ version 1.51n 51 . As our specimens were mounted on permanent slides, their depth 21 was the mean thickness of each individual (measured as indicated above). To obtain individual body volume, we multiplied body area by the mean thickness of each individual (Fig.…”

Tracking the assembly of nested parasite communities: Using β‐diversity to understand variation in parasite richness and composition over time and scale

“…We found little evidence of heterogeneity among individual hosts; hosts within the same population shared significantly more parasites than expected by chance (Figure 2). Low β ‐diversity at this scale can result from high dispersal rates (‘propagule pressure’), which acts to homogenize communities (Leibold et al, 2004; Qian, 2009), and it is well‐established that dispersal to individual hosts can be high in pond ecosystems given the high biomass of infectious forms produced by trematodes (Lambden & Johnson, 2013; Preston, Orlofske, Lambden, & Johnson, 2013). Many parasites reached high prevalences within host populations, including numerous taxa that reached 100% prevalence (Appendix S1: Figure S4), indicating a lack of dispersal limitation to individual hosts.…”

Waterbird-Mediated Dispersal and Freshwater Biodiversity: General Insights From Bryozoans

“…Waterbird-mediated co-dispersal and establishment of parasites and hosts will almost certainly influence community interactions and food webs. Parasite biomass can be considerable (Kuris et al, 2008;Lambden and Johnson, 2013) and parasites contribute substantially to energy transfer and food web structure and stability (e.g., Dunne et al, 2013;Michalska-Smith et al, 2017). Parasites with complex life cycles may exploit hosts at different trophic levels.…”