Parasites as Prey

Abstract

Parasites are usually considered to use their hosts as a resource for energy. However, there is increasing awareness that parasites can also become a resource themselves and serve as prey for other organisms. Here we describe various types of predation in which parasites act as prey for other organisms: (1) predation of nonhosts on infected hosts (concomitant predation), (2) predation on free‐living parasite life cycle stages, (3) predation on ectoparasites in form of grooming or cleaning and (4) predation or hyperparasitism by other parasites. In many cases, these types of predation significantly reduce the numbers of parasites and thus affect parasite population dynamics. In contrast, predation on parasites is often beneficial for the hosts as they are released from parasite burden. Finally, when parasites act as prey they may contribute to the nonhost predator's diet, in some cases constituting a significant proportion of energy intake.

Key Concepts:

  • There is increasing awareness that parasites can become a resource themselves by serving as prey for other organisms.

  • Predation on parasites can have significant effects on parasite population dynamics, for example, by interfering with the transmission of infective stages.

  • For hosts, parasite predation may result in a reduced parasite burden, the so‐called dilution effect.

  • When nonhosts prey on parasites or infected prey, this may have an effect on their energy uptake by mediating the energetic value of their prey.

  • Future studies on the levels of predation on parasites in food webs will increase our understanding of disease dynamics as well as the topology and functioning of food webs.

Keywords: parasites; food web; trophic transmission; predation; hyperparasitism; biological control; ectoparasites

Figure 1.

Example of a typical three‐host life cycle of a trematode, showing the sequence of different hosts and free‐living stages involved (black arrows) and illustrating the different types of predation on them (blue arrows). In this case, the definitive host is a bird, from which the parasites release free‐living eggs into the water together with the bird's faeces. The eggs infect a first intermediate host (a gastropod), in which the parasite reproduces asexually in so‐called sporocysts or rediae and releases a second free‐living stage into the water (cercariae). These cercariae infect a second intermediate host (a cockle) in which the parasites encyst (metacercariae). Finally, when the second intermediate host is consumed by the definitive host, the life cycle of the parasite is completed. Parasites can become prey either when infected hosts are consumed by nonhosts (concomitant predation; filled blue arrows) or when their free‐living stages are consumed (open blue arrows). Note that parasite consumption is also an essential part of the parasite's life cycle when the infected second intermediate host is eaten by the definitive host.

Figure 2.

A ‘zombie snail’. The snail is infected by a trematode that invades the snail's tentacles, leading to swollen and colourful tentacles. This mimics the appearance of a caterpillar, which is the prey of the parasite's definitive bird hosts. Hence, the phenotypic changes caused by the parasite increase its transmission. Reproduced by permission of Marek Snowarski (www.atlas‐roslin.pl).

Figure 3.

Partial predation on infected hosts by a nonhost predator. A trematode species infecting the New Zealand cockle impairs its burying ability so that cockles are exposed on the sediment, resulting in an increased predation by the definitive host compared to uninfected cospecifics (a case of host manipulation). However, fish also partially prey on exposed infected cockles by nipping off the tip of the cockles’ foot. Most parasites are concentrated in the foot tip and, since the fishes do not serve as down‐stream hosts, a significant proportion of the parasites are lost via this concomitant predation.

Figure 4.

Examples of the levels of reduction of infective free‐living stages of parasitic nematodes by various organisms from different ecosystems (indicated by different colours): earth worms (Waghorn et al., ) and dung beetles (English, ) preying on nematode eggs and larvae in dung, copepods preying on nematode larvae in fresh water systems (Achinelly et al., ), mites preying on nematode larvae in sandy soils (Karagoz et al., ) and nematophagous fungi on nematode larvae on livestock pasture (Ferreira et al., ).

Figure 5.

Typical ectoparasites of salmon in aquaculture and their consumption by cleaner fishes: (a) salmon lice (parasitic copepods, the brown ectoparasites) infect salmon and cause damage to the skin, often leading to secondary infections. Reproduced by permission of Bruce MacGregor Sandison. (b) Salmon farm, where the high stocking densities are ideal habitats for the transmission of the copepods (with a direct life cycle) from one host to another. Reproduced by permission of The Atlantic Salmon Trust. (c) A cod with a cleaner fish, preying on ectoparasites like copepods. Reproduced by permission of Alexandra Grutter.

Figure 6.

Example of a parasitoid, introduced to control a plant pest, which then became host to native parasitoids, thus becoming hyperparasitoids. The cassava plant is defoliated by the South American mealybug, an unwanted herbivore. As a control for this pest, a parasitoid from the order Hymenoptera was introduced. This parasitoid, in turn, was exploited by 10 native species of parasitoids.

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Further reading

Johnson PTJ, Dobson A, Lafferty KD et al. (2010) When parasites become prey: ecological and epidemiological significance of eating parasites. Trends in Ecology and Evolution 25: 362–371.

Keesing F, Belden LK, Daszak P et al. (2010) Impacts of biodiversity on the emergence and transmission of infectious diseases. Nature 468: 647–2010.

Keesing F, Holt RD and Ostfeld RS (2006) Effects of species diversity on disease risk. Ecology Letters 9: 485–498.

Thieltges DW, Jensen KT and Poulin R (2008) The role of biotic factors in the transmission of free‐living endohelminth stages. Parasitology 135: 407–426.

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Goedknegt, Anouk, Welsh, Jennifer, and Thieltges, David W(Sep 2012) Parasites as Prey. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0023604]