Gammarus lacustris infected with cystacanths of Polymorphus paradoxus display an abnormal behavior. They respond to disturbance by swimming to the surface and clinging to a solid object (W. M. Bethel and J. C. Holmes. 1973. J. Parasitol. 59: 945–956). Uninfected gammarids injected with serotonin (1–20 μg/50 mg) but not other neurotransmitters also responded to mechanical stimulation by clinging, but the response lasted for a shorter time than in infected gammarids. Octopamine suppressed the clinging response in infected gammarids for several hours. The results suggest that cystacanths of P. paradoxus modulate the behavior of gammarids through the alteration of neural activity in some serotonin-sensitive or serotonergic central pathway, probably one involved in precopulatory clinging in male amphipods.
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Helluy et al. (1990) studied this question.