PARASITE-INDUCED CHANGES IN BEHAVIOR AND COLOR MAKEGAMMARUS PULEXMORE PRONE TO FISH PREDATION

Abstract

The acanthocephalan parasite Pomphorhynchus laevis is transmitted by crustaceans such as Gammarus pulex to its paratenic or final hosts, fish. The conspicuous orange-yellow parasite is visible through the transparent cuticle of G. pulex. Infected gammarids are significantly less photophobic than uninfected ones. When hungry three-spined sticklebacks (Gasterosteus aculeatus), one of the hosts of this parasite, were offered equal numbers of uninfected and infected prey, G. pulex infected with P. laevis were eaten significantly more often. We tested experimentally whether parasite color and parasite-induced changes in host behavior affected the predation rate of G. pulex. Color effects were tested with uninfected G. pulex by painting an orange spot on their cuticle that simulated infection. Behavioral effects were tested with infected G. pulex by covering the place through which the orange parasite was visible with inconspicuous brown paint. We showed for the first time that both parasite color and changed intermediate host behavior promote the transmission of P. laevis to its next host. The evolution of orange parasite color, and why sticklebacks do not avoid infected prey, are discussed.