Simultaneous recording of cytosolic Ca2+ levels inDidinium andParamecium during aDidinium attack onParamecium

SummaryThe carnivorous ciliateDidinium nasutum captures prey such asParamecium by discharging extrusomes, known as toxicysts, while the attackedParamecium defensively discharges trichocysts. Several authors have suggested that both discharges, the toxicysts ofDidinium and the trichocysts ofParamecium, are evoked by the rise in cytosolic Ca2+ level in each cell. However, these putative increases in cytosolic Ca2+ levels have not as yet been recorded simultaneously in these cells during aDidinium attack onParamecium. We injected the fluorescent Ca2+ indicator Ca-Green 1 dextran into bothDidinium andParamecium, and simultaneously observed the cytosolic Ca2+ levels in these cells asDidinium attackedParamecium. When aParamecium came into contact with theDidinium proboscis, theDidinium showed a significant rise in cytosolic Ca2+ in the basal portion of the proboscis. One video frame (33 ms) after the onset of the Ca2+ rise inDidinium, theParamecium also showed an increase in cytosolic Ca2+. This is the first simultaneous recording of changes in the Ca2+ level during a predator-prey interaction in ciliates. The possible roles of these Ca2+ increases are discussed in relation to the discharge of toxicysts during theDidinium attack and of trichocysts as a defensive behavior ofParamecium.