Theronts of Ichthyophthirius multifiliis find their fish hosts with complex behavior patterns and in response to different chemical signals.

How parasites find and recognize fish hosts is best known for trematode cercariae. These multicellular organisms respond with complex behavior patterns to sequences of different chemical host signals, although these invasive stages are produced in large numbers. However, little is known on the host-finding of protozoan parasites. We therefore studied how the infective theronts of I. multifiliis find their fish hosts by offering them various fractions and chemical modifications of fish skin surface. Free swimming theronts showed at least 6 different behavior patterns, 4 of them were stimulated by fish skin components, e.g. by amino acids. Theronts approached towards solid substrates ("stop and go ahead") in response to macromolecular glycoproteins of fish skin, and they were repelled ("stop and turn away") by other macromolecular glycoconjugates. After contact with solid surfaces, they repeatedly dipped on the surface ("dipping contact") in response to glycoconjugates of fish skin and they swam along the surface ("body parallel") when it contained proteins of fish skin. The results demonstrate that also a protozoan fish-parasite without nervous system and cellular chemoreceptors may show very diverse host-finding behavior patterns (at least 11) and may respond to different chemical host cues (at least 5).