Ichthyophthirius multifilis invasive stages 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 cer cariae. These multicellular organisms respond with complex behavior pattern s to sequences of different chemical host signals. However, little is known on the host-finding of protozoan parasites. We therefore studied how the i nvasive theronts of Ichthyophthirius multifiliis find their fish hosts by o ffering them various fractions and chemical modifications of fish skin surf ace. Free swimming theronts showed at least 6 different behavior patterns, 4 of them were stimulated by fish skin components. The most evident respons es, i.e. "stop and orientation" and "change of direction" were mainly stimu lated by amino acids of fish skin surface. Theronts approached towards soli d substrates ("stop and go ahead") in response to macromolecular glycoprote ins of fish skin, and they were repelled ("stop and turn away") by other ma cromolecular glycoconjugates. After contact with solid surfaces, they repea tedly dipped on the surface ("dipping contact") in response to glycoconjuga tes of fish skin and they swam along the surface ("body parallel") when it contained proteins of fish skin. The results demonstrate that also a protoz oan fish-parasite without nervous system and cellular chemoreceptors may sh ow very ifferent host-finding behavior patterns (at least 11) and may respo nd to different chemical host cues (at least 4).