Growth of algal symbionts in regenerating hydra

BETWEEN 10 and 20 photosynthetic algae are maintained per digestive cell in green hydra, the freshwater polyp. In normal conditions the number of symbionts maintained within the host remains stable—the algae do not overgrow the host nor do the animals outgrow the algae 1 . In experiments described here we found that when the upper two-thirds of a hydra was removed, there was a pronounced increase in the number of algae per digestive cell in the remaining peduncle of the animal. The observed increase in algal numbers was not a function of starvation and could be abolished by grafting a head, from a donor animal, on to the peduncle. We believe that our experiments are the first to show that the reproduction of symbiotic algae may be influenced by the host and that the presence of a head, thought to be responsible for maintenance of polarity and other morphological phenomena in hydra 2 , may also influence the reproduction of symbiotic algae.