COST OF STENOSTOMUM-INDUCED MORPHOLOGICAL DEFENSE IN THE CILIATE EUPLOTES-OCTOCARINATUS

Several predatory organisms induce a defensive morphological response in the freshwater ciliate Euplotes octocarinatus. The morphological ch anges have demographic cost besides an adaptive advantage. The generat ion time of E. octocarinatus increased from 40 +/- 1 h (SDM) to 47 +/- 2 h when the ciliate changes its morphology in response to the presen ce of Stenostomum sphagnetorum (Turbellaria). It remained at a high le vel in the protective ''winged'' form of the ciliates (45 +/- 1 h). Th e population growth rate therefore decreased by 15 % (from r = 0.60/d to r = 0.51/d). When predators had disappeared the duration of the fol lowing reproduction cycle decreased to 38 +/- 1 h. A longer growth pha se of winged ciliates was related to their lowered reproduction rate. The length of the G1-phase of the cell cycle increased from 25 +/- 1 h to 34 +/- 2 h (SD); the length of the S phase remained constant (13 /- 1 h or 12 +/- 3 h). Synthesis of proteins was necessary for predato r-induced morphological changes. The protein content of Euplotes octoc arinatus increased from 1.1 +/- 0.1 pg/ciliate to 1.7 +/- 0.2 pg (SD) during morphological transformation. The protein synthesis inhibitor c ycloheximid (greater-than-or-equal-to 50 muM) inhibited morphological changes. These results indicate that additional growth of the ciliate is necessary for morphological changes. The growth process leads as a cost to an increased generation time of Euplotes. The feeding rate of winged forms of the ciliate had slightly increased compared to the fee ding rate of the typical form. Therefore, food limitation is not cruci al for the cost of predator-induced defence; however, it may be benefi cial for the winged form.