Role of Symbiotic Algae on Gemmule Germination of a Freshwater Sponge, Radiospongilla cerebellata

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Annandale sponges (Radiospongilla cerebellata) have chlorellae as endosymbiotic algae and are distributed throughout the freshwaters of southwestern Japan. Gemmules of this sponge hatch only under an illuminated condition. The hatch is perfectly inhibited in total darkness. Although illumination is necessary for the gemmule hatching, the intensity of the light is very low and the illuminating period to induce the hatching is very short. Gemmules also fail to hatch unless the surface of incubating media is freely exposed to the atmosphere. Photosynthetic inhibitors strongly suppressed the hatching even under the optimal temperature and fully illuminated condition. To understand role of air components and photosynthesis on the gemmule hatch, we examined gaseous elements of the air and found that oxygen and carbon dioxide were responsible for the gemmule hatch. These gases are also evolved or consumed by photosynthesis. When oxygen was evacuated from the incubating media, no gemmules were hatched. On the other hand, gemmules incubated in a medium containing excess carbon dioxide did not germinate even under the optimal illuminated condition. This suggests that gemmule germination of the Annandale sponges seems to be initiated by oxygen and suppressed by carbon dioxide. This suggests that the both gasses might be evolved in or consumed from hibernating gemmule cells by photosynthesis of symbiotic algae to induce germination in these animal cells.

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