It's a worm‐eat‐worm world: Consumption of parasite free‐living stages protects hosts and benefits predators

Abstract

1. Predation on parasites is a common interaction with multiple, concurrent outcomes. Free-living stages of parasites can comprise a large portion of some predators' diets and may be important resources for population growth. Predation can also reduce the density of infectious agents in an ecosystem, with resultant decreases in infection rates. While predator-parasite interactions likely vary with parasite transmission strategy, few studies have examined how variation in transmission mode influences contact rates with predators and the associated changes in consumption risk. To understand how transmission mode mediates predator-parasite interactions,we examined associations between an oligochaete predator Chaetogaster limnaei that lives commensally on freshwater snails and nine trematode taxa that infect snails. Chaetogaster is hypothesized to consume active (i.e. mobile), freeliving stages of trematodes that infect snails (miracidia), but not the passive infectious stages (eggs); it could thus differentially affect transmission and infection prevalence of parasites, including those with medical or veterinary importance.Alternatively, when infection does occur, Chaetogaster can consume and respond numerically to free-living trematode stages released from infected snails (cercariae). These two processes lead to contrasting predictions about whether Chaetogaster and trematode infection of snails correlate negatively ('protective predation') or positively ('predator augmentation').3. Here, we tested how parasite transmission mode affected Chaetogaster-trematode relationships using data from 20,759 snails collected across 4 years from natural ponds in California. Based on generalized linear mixed modelling, snails with more Chaetogaster were less likely to be infected by trematodes that rely on active transmission. Conversely, infections by trematodes with passive infectious stages were positively associated with per-snail Chaetogaster abundance. 4. Our results suggest that trematode transmission mode mediates the net outcome of predation on parasites. For trematodes with active infectious stages, predatory Chaetogaster limited the risk of snail infection and its subsequent pathology (i.e. castration). For taxa with passive infectious stages, no such protective effect was observed. Rather, infected snails were associated with higher Chaetogaster

The enemy of my enemy is my friend: Consumption of parasite infectious stages benefits hosts and predators depending on transmission mode

“…Similarly, a grazing predator reduced the abundance of bottom-dwelling cercariae, but only a filter-feeder significantly removed free-swimming cercariae(Selbach et al, 2019). Relatively large and motile cercariae are also more likely to be consumed(Born-Torrijos et al, 2021;McDevitt-Galles et al, 2021) Hobart et al (2021). expand our understanding regarding the circumstances under which consumption of free-living infectious propagules can reduce the transmission of multiple parasites over time by considering the extent to which two host snail species were parasitized by nine trematode taxa with either an actively or passively transmitted infectious stage over a 4year period.…”

“…. Hobart et al (2021) elegantly explore the association between predator abundance and host parasitism by considering an episymbiont that consumes free-living infectious stages.…”

“…Both consumers and predators could therefore gain from predation upon infectious propagules, but are such benefits broad, or limited to particular parasites? Hobart et al (2021) consider predation upon trematode (flatworm) infectious stages based on their transmission mode, specifically whether they 'actively' or 'passively' infect their hosts.…”

“…This being said, predators must come into contact with potential prey, and parasite transmission mode may play a role. Based on a multi-year field study that examined over 20,000 freshwater snails, Hobart et al (2021) report that the greater the per snail abundance of a mantle-dwelling episymbiont (the oligochaete Chaetogaster limnaei) known to consume both miracidia and cercariae (e.g. Hopkins et al, 2013;Khalil, 1961), the less likely snails were infected by trematodes with active transmission.…”

“…This provides strong support for a mechanistic pathway leading to reduced transmission considering that these hosts and predators are in extremely close proximity-an otherwise unlikely scenario. In this way,Hobart et al (2021) were able to examine how predation upon infectious propagules could 'protect' individual hosts against parasitism by quantifying the abundance of C. limnaei for each snail, an approach that complements existing population-and community-level studies.As with exclusively free-living prey and their predators, encounter between the latter and parasite/pathogen infectious stages present within the environment is key. Recent papers have demonstrated that parasite characteristics play an important role, particularly their microhabitat use and behaviours.…”