RECOGNITION OF SYMBIOTIC ALGAE BY HYDRA VIRIDIS. A QUANTITATIVE STUDY OF THE UPTAKE OF LIVING ALGAE BY APOSYMBIOTIC H. VIRIDIS

¹ Department of Zoology, University of California, Los Angeles, California 90024

1. An individual Hydra viridis (Florida Strain) harbors approximately 1.5 x 10⁵ unicellular green algae. These algae are normally found in the basal portion of gastrodermal digestive cells. An average of approximately 18 algae per digestive cell is encountered in the central region (stomach and budding zone) of the hydra under a given set of maintenance conditions.

2. The average number of algae per digestive cell may range from 7 to 22 depending on feeding schedules and photoperiod under which hydra cultures are maintained.

3. The symbiosis can be synthesized artifically by injecting symbiotic algae from H. viridis into the coelenteron of aposymbiotic H. viridis. Within 15 minutes after injection the algae are taken up by the digestive cells, apparently by phagocytosis. During the next hour, the algae move from the site of uptake to the basal part of the digestive cell.

4. Both green and aposymbiotic H. viridis can take up only about 8 ± 2 algae from an injected suspension of cells. Uptake may be limited by the extent to which the cell can sustain phagocytosis, rather than by availability of intracellular space. In aposymbiotic H. viridis, the full complement of 18 algae is attained by algal cell division and requires about 18 days.

5. An assortment of free-living and symbiotic algae were injected into H. viridis aposymbionts. All were rejected immediately with the exception of hydra algae and a symbiotic alga from Paramecium (NC64A) which formed a stable hereditary endosymbiosis with H. viridis. Growth rates of these hydra were virtually identical with those of normal green hydra.

6. The acquisition of algae by hydra digestive cells appears to involve several "phases" including contact , recognition, phagocytosis, and intracellular transport.

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