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Abstract A parasite can change its host’s behavior in spectacular ways. When the saltmarsh amphipod Orchestia grillus (Bosc, 1802) is infected with the trematode Levinseniella byrdi (Heard, 1968) it is bright orange and is found in the open unlike uninfected individuals. I tested the hypothesis that infected amphipods are found in the open because L. byrdi reverses their innate photophobia. During daytime treatments and when placed in a dark chamber, 0% of the uninfected and 20% of the infected amphipods, on average, moved to the light chamber after 30 minutes. When placed in a light chamber, 91% of the uninfected and 53% of the infected amphipods, on average, went to the dark side after 30 minutes. These results clearly indicate that O. grillus is normally photophobic, but not drawn to light when infected with L. byrdi. Instead, L. byrdi appears to neutralize the amphipod’s photophobia. Uninfected O. grillus are typically found under vegetation. I hypothesize that O. grillus with L. byrdi infections wander into open, unvegetated habitats randomly. In addition, 94% of infected amphipods could be touched by a finger in the field suggesting they can be easily caught by predators. Levinseniella byrdi infects at least three other amphipod hosts, Chelorchestia forceps (Smith & Heard, 2001), Uhlorchestia spartinophila (Bousfield & Heard, 1986), and U. uhleri (Shoemaker, 1930). The parasite-manipulation hypothesis suggests that the parasite-induced changes (conspicuous body color and neutralized light response) are adaptive for L. byrdi to make amphipod hosts more susceptible to bird predators, the definitive hosts. This hypothesis remains to be tested.
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Parasitic manipulation or by-product of infection: an experimental approach using trematode-infected snails
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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- Award ID(s):
- 1906465
- PAR ID:
- 10339732
- Date Published:
- Journal Name:
- Journal of Helminthology
- Volume:
- 96
- ISSN:
- 0022-149X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1017/s0022149x21000699
