SOME FACTORS AFFECTING THE GROWTH AND DISTRIBUTION OF THE ALGAL ENDOSYMBIONTS OFHYDRA VIRIDIS

1. Under conditions of continuous illumination and daily feeding, individual specimens of Hydra viridis possess approximately 1.5 x 105 endosymbiotic algae.2. The endosymbiotic algae are found in greatest abundance in those digestive cells constituting the central growth region (Zone 2) of the hydra (≈ 19 algae/ digestive cell) with fewer algal cells (≈ 12 algae/digestive cell) residing in the hypostome, tentacles (Zone 1) and stalk and basal disc (Zone 3).3. Under steady-state growth in the light, the algae reproduce at approximately the same rate as the animal hosts—K = 0.380 and 0.385, respectively.4. Endosymbiotic algae require light for maximum reproduction and green hydra grown in the dark show a decrease in the number of their algal symbionts. The rate of algal decline in the dark is believed to result from a dilution of the standing crop by the continuous growth of the animal tissues. Support for this theory is based on the observation that the rate of algal decline in the dark (K = 0.258) approximates the rate of animal growth (K = 0.288).5. The algae in hydra maintained in the dark exhibit a different pattern of distribution in the host than those in the light. Apparently loss of symbionts in the dark takes place differentially with proportionately more algae being lost from the tentacles, hypostome and growth region than from the stalk and basal disc.6. When hydra that have been maintained in the dark are returned to the light, their endosymbiotic algae undergo a rapid multiplication and repopulate the host in approximately two days.