Aspects of Light in the Biology of Green Hydra
A wide variety of cnidarians maintain algal symbionts within their tissues. These symbionts are photosynthetically active when illuminated and translocate large percentages of their photosynthetic products to the host (see Muscatine, 1974, for a comprehensive review). In addition to the light requirements for photosynthesis, the symbionts, like other plants, appear to have light requirements for cell division, cell development, and other processes. By altering light intensities, the wavelength of radiant energy and the duration of exposure (photoperiod), the experimenter can attempt to selectively manipulate some of the symbiont’s biology and thereby dissect or characterize subsystems functioning within the intact association. Using this strategy I have examined the effects of light on the green hydra symbiosis and have found that symbiont multiplication and ultrastructure exhibit a light dependency, and that high-intensity light may cause expulsion of symbionts. In terms of the physiology of the association, the light-dependent photosynthetic reactions may alter the association’s metabolic and respiratory processes. Finally, the light-stimulated and light-dependent processes of the symbiotic algae may be superimposed on the host’s normal light-sensitive behavior or phototaxis. What follows is a brief description of the green hydra symbiosis followed by a review of some recent work on the role of light in various aspects of the association ending with a description of current experiments on green hydra phototaxis.
KeywordsLight Requirement Digestive Cell Symbiotic Alga Algal Symbiont Positive Phototaxis
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