Behavioral defense against parasites: California killifish move, dart, and scratch more during trematode cercaria exposure and attack.

With the ubiquity of parasites, many hosts have been selected to decrease parasite infection success by employing behavioral defenses, such as avoidance of infected habitats/conspecifics, grooming, grouping, altering swimming behavior, or even self-inducing behavioral fevers. California killifish, Fundulus parvipinnis – common to southern California and Baja California estuaries – are typically exposed to several trematode species that use them as 2nd intermediate hosts. At least one of these trematodes substantially impacts killifish fitness. We also know killifish likely perceive trematode infectious propagules (cercariae). However, we do not know whether they employ behavioral defenses. We experimentally exposed killifish (originating from two San Diego wetlands) individually and in groups to two of their trematode species: Euhaplorchis californiensis – which infects the killifish brain and manipulates host behavior to increase predation rates – and Small Cyathocotylid, which infects connective and muscle tissues. To assess killifish behavioral response to exposure, we quantified several behavioral traits: average number/type of potential defensive behaviors (PDBs), activity, vertical position in the water column and group size before and during exposure to parasites. Our results showed that killifish individually-exposed (both previously infected and naïve) to parasites increase their average number of PDBs, but not their activity. However, in groups, parasite-exposed killifish increase both their average number of PDBs and activity. Conversely, neither average vertical position (in either experiment) nor group size was influenced by parasite exposure. In sum, parasites can alter killifish behavior after infection, but also during anti-parasite behavioral defense, with implications for host-parasite coevolution, and host social and ecological interactions