The effect of symbiotic zoochlorellae on tentacle number and budding in Swiss and Israeli strains of Hydra viridis in the light and in the dark, in normally grown and in regenerating specimens has been studied. The mean tentacle number in symbiotic and aposymbiotic hydra was 8.65 f 1.05 and 7.48 t-0.86 in the Swiss strain, and 8.62 t-1.29 and 6.66 + 0.72 in the Israeli strain. In aposymbiotic hydra reinfected with zoochlorellae isolated from either strain, the number of tentacles increased to that of symbiotic hydra. Decapitated symbiotic hydra regenerated more tentacles than aposymbionts, and regenerating Swiss symbionts also formed more buds than their respective aposymbiotic hydra. The algal enhancing effect was found in the light and in the dark. Regeneration rate was initially higher in the symbiotic hydra, and the ratio between the number of tentacles formed by the regenerating symbiotic and aposymbiotic hydra varied daily during the experimental period of 7 days. We conclude that in H. viridis the endosymbiotic zoochlorellae exert a significant stimulatory and morphogenic effect in the light and in the dark, We do not know yet how the algal effect is mediated to their host. We suggest that a distinction should be made between the stimulatory effect on regeneration rate and the morphogenic effect on the final number of tentacles or buds.
Recent studies have shown that tentacle numbers in hydra are influenced by temperature, certain ions, feeding regime, age, and rate of budding (Shostak et al., '78; Shostak, '79, '81). A "head activator" formed by hydra was shown to control the number of tentacles formed by regenerating specimens (Schaller, '73; Schaller et al., '80).
We observed that symbiotic algae-hosting Hydra viridis of Swiss and Israeli strains (Rahat and Reich, 'go), have a significantly greater tentacle number compared to aposymbiotic hydra of the same strains.
Our present study aimed to answer the following questions: (1) Is there a correlation between the presence of endosymbiotic zoochlorellae and the greater tentacle number in H. uiridis? (2) Does this difference persist in regenerating hydra? (3) Is there a correlation between the regeneration rate and the final tentacle and bud numbers in symbiotic and aposymbiotic H. viridis?