THE EFFECT OF THE PATHOGENIC RHIZOPOD HYDRAMOEBA HYDROXENA (ENTZ) ON REPRODUCTION IN CHLOROHYDRA VIRIDISSIMA UNDER VARIOUS LEVELS OF TEMPERATURE

¹ Department of Zoology, University of North Carolina, Chapel Hill, N.C.

1. In prior studies on the spread of the parasite Hydramoeba hydroxena through populations of hydra, the density of host populations was kept fixed by removing buds and not allowing them to accumulate. Experimental analyses were undertaken, therefore, to determine the effect of this parasite on asexual reproduction in Chlorohydra viridissima under a range of temperatures from 15° to 30° C.

2. Budding rates were significantly different among light, heavy, and no (control) initial infections, and among four levels of temperature. The relationship between budding rate and temperature for the control group was curvilinear with the highest rate occurring around 20° to 25° and the lowest around 15° and 30° C.

3. When the cumulative number of buds produced by the host was considered over the 10-day span of the experiments, it was found that temperature and initial infection were not independent of one another. This lack of independence was due to the fact that the difference in bud production among the levels of infection was not the same for the levels of temperature. The converse of this latter statement is also true.

4. Hydra pseudoligactis, which is less resistant to the attack of the hydramoebae ceases budding almost immediately after infection.

5. In C. viridissima, significant correlations exist between the time the host stopped feeding and the day the host stopped budding, as well as between the time the host stopped feeding and the day it died from the attack of the hydramoebae. In addition, increases in temperature caused infected hosts to cease feeding sooner, a decrease in their period of budding, and a reduction in their length of life.

6. It appears that the destruction of the tentacles of the host by the hydramoebae renders the host incapable of feeding, and leads in turn to starvation and cessation of budding. The time at which the tentacles are rendered useless is dependent upon temperature and the size of the initial infection.

7. C. viridissima can tolerate the parasitic infection and continue budding at a lower temperature (i.e., 15° C.). It is postulated that this balanced relationship may be a mechanism for providing a reservoir for Hydramoeba hydroxena in the absence of a viable cyst. However, this balance does not always occur at this temperature, since the infection is frequently lost. Long-term experiments are required before much weight can be given to this hypothesis.