It is thought that when "Gammarus lacustris" is infected with a thorny-headed worm, the parasite causes serotonin to be massively expressed. Serotonin is a neurotransmitter involved in emotions and mood. Researchers have found that during mating "Gammarus lacustris" expresses high levels of serotonin. Also during mating, the male "Gammarus lacustris" clamps down on the female and holds on for days. Researchers have additionally found that blocking serotonin releases clamping. Another experiment found that serotonin also reduces the photophobic behavior in "Gammarus lacustris". Thus, it is thought that the thorny-headed worm physiologically changes the behavior of the "Gammarus lacustris" in order to enter the bird, its final host.