Critical Periods in Development of Ichthyophthirius multifiliis (Ciliophora) Populations1

ABSTRACT. Three periods in development that strongly influence population dynamics of Ichthyophthirius multifiliis were identified in experimental infections of channel catfish. The first occurred upon establishment within the host, 0 to 10 min postexposure (PE), when the parasite population that gained entrance declined 50%. Survival from 10 to 45 min PE, however, was constant. The second period identified came after I. multifiliis left the host and the free-living tomont encysted. The third occurred during reproduction. Although survival of encysting tomonts approached 100% among individuals departing after three to five days residence in the host, theront production varied significantly with parasite size, culture temperature during development, and length of residence by the trophont in the host. Theront production per tomont increased daily and on days three, four, and five PE was significantly higher for parasites developing at 24°C than for those at 21°C. At five days PE, mean production was 562 theronts/tomont and 240 theronts/tomont, respectively, and production by tomonts of equal size was greater for parasites maintained at 24°C.