The tapeworm Ligula intestinalis alters the behavior of the fish intermediate host Engraulicypris sardella, but only after it has become infective to the final host

•Fish host infected with parasite differed in behavior from uninfected ones.•Were caught easily, active in swimming and stayed higher up in the water column.•The parasite had a significant impact on the fish host behavior at the infective stage.•Less developed parasite had little effect on the fish host behavior.•This parasite may affect the fish host’s behavior and makes it vulnerable to predation. Ligula intestinalis is a tapeworm using copepods and cyprinid fish as intermediate hosts and fish-eating birds as final hosts. Since some parasites can increase their own fitness by manipulating the behavior of the intermediate host, we explored if this parasite affected predator avoidance, swimming activity and depth preference of the fish intermediate host, Engraulicypris sardella. We found that when L. intestinalis had reached a developmental stage that is able to establish in the bird host, it had a significant impact on E. sardella behavior, while the tapeworm that was not fully developed had little effect and fish hosts showed a behavior more similar to uninfected fish. These results are discussed with respect to two different processes: the manipulation hypothesis and the energy drain hypothesis.