Growth of endosymbiotic algae in the green hydra, Hydra viridissima

Stable endosymbiosis depends upon balanced growth of the symbionts. In green hydra, coincident patterns of host and algal mitotic index suggest that coordinated reproduction provides for balanced growth. However, when hydra shrink during food shortage, the population of endosymbiotic algae in a green hydra must likewise decline in size. Thus far, no mechanism of reducing the size of the endosymbiont population has been described. Algal mitosis was found here to be stimulated by host feeding and clumped in its distribution among host cells, supporting the notion of some degree of control of algal mitosis exercised at the level of the host cell. However, comparisons of the rates of algal mitosis with the realized rates of algal population growth show that substantial numbers of algae disappear from hydra, in numbers in excess even of those necessary to accommodate host shrinkage. Only a small proportion of these lost algae was found to be expelled by hydra. Microscopic observations of the cells of macerated hydra show evidence of algal disintegration in nearly 50 % of the digestive epithelial cells of regularly fed hydra. Coincidence of remnants of algal cells and food-derived materials within the same vacuoles suggests that algae are digested by host cells.