A comparative study of tentacle regeneration and number in symbiotic and aposymbiotic Hydra viridis: Effect of zoochlorellae

Abstract

AbstractThe 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 ± 1.05 and 7.48 ± 0.86 in the Swiss strain, and 8.62 ± 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.