Abstract
Host manipulation is a common strategy by which parasites alter the behaviour of their host to enhance their own fitness. In nature, hosts are usually infected by multiple parasites. This can result in a conflict over host manipulation. Studies of such a conflict in experimentally infected hosts are rare. The cestode Schistocephalus solidus (S) and the nematode Camallanus lacustris (C) use copepods as their first intermediate host. They need to grow for some time inside this host before they are infective and ready to be trophically transmitted to their subsequent fish host. Accordingly, not yet infective parasites manipulate to suppress predation. Infective ones manipulate to enhance predation. We experimentally infected laboratory-bred copepods in a manner that resulted in copepods harbouring (i) an infective C plus a not yet infective C or S, or (ii) an infective S plus a not yet infective C. An infective C completely sabotaged host manipulation by any not yet infective parasite. An infective S partially reduced host manipulation by a not yet infective C. We hence show experimentally that a parasite can reduce or even sabotage host manipulation exerted by a parasite from a different species.
Highlights
Many parasites possess the ability to modify their host’s behaviour or appearance to their needs
In complex life cycle parasites, many of the most prominent examples of host manipulation comprise cases in which parasites enhance the likelihood that their current host is consumed by a suitable subsequent host
In experiment I, we investigated either an intraspecific conflict between an infective C. lacustris and a not yet infective conspecific, or an interspecific conflict between an infective C. lacustris and a not yet infective S. solidus
Summary
Many parasites possess the ability to modify their host’s behaviour or appearance to their needs. We do not expect an interspecific conflict over host manipulation between S. solidus and C. lacustris of the same developmental stage but between different stages when the two parasite species co-occur. Such a conflict does not require clear manipulation by both developmental stages; if normal host behaviour fits one parasite’s need it should not manipulate itself but should interfere with counter-manipulation when the other parasite manipulates in the wrong direction. To analyse copepod activity (i.e. whether or not a copepod moved within each two second interval), we included the time point in the recording (i.e. after the simulated predator attack or after a recovery period) in the random effect. See the electronic supplementary material, tables S3 – S6 for all other statistics
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