THE EFFECT OF FAST, AND REGENERATION IN LIGHT VS DARK, ON REGULATION IN THE HYDRA-ALGAL SYMBIOSIS

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Abstract
Green hydra are able to regenerate tentacles after fast durations which cause brown, i.e., asymbiotic, hydra to fail completely, but the presence of endosymbiotic algae does not always enhance regeneration in fasted hydra. Green hydra whose nutritional state falls below some threshold, exhibit a light induced inhibition of regeneration. That is, hydra, fasted in the light, then randomly assigned to light or dark after decapitation, regenerate better in the dark. This effect of light does not appear to be present either in brown hydra or in normally green hydra from which the algae were removed. In a large strain of Chlorohydra viridissima after fasts of intermediate duration (10 and 15 days), this light induced inhibition of regeneration is associated with an increase in the number of algae per gastric cell in regenerating hydra relative to nonregenerating controls. This algal increment was not associated with an increase in hydra gastric cell mitosis for animals fasted 9 days prior to decapitation and allowed to regenerate 48 h in the light.