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SYNOPSIS. Zooxanthellae in pure culture, exposed to continuous light, release free O 2 in amounts varying with time and cell number. Zooxanthellae exposed to alternate light and dark produce essentially as much O 2 as in continuous light. Those in continuous dark do not yield free O 2 ; rather, they draw from the medium's residual O 2 . Both the anemone Condylactis and the scyphozoan Cassiopeia are markedly phototactic. Kept in total darkness, both species show a dramatic numerical diminution in zooxanthellae. During 24 days of darkness, the mean total number of zooxanthellae in the individual Condylactis body was reduced from 26 million to ∼ 1 million. Specimens so bleached lost phototaxis. It is suggested that each species of zooxanthella host animal has its own specific light needs, which it caters to in various ways: ( a ) early selection of position, as with planulae or other freely motile larval forms; ( b ) gross body movement or posturing, as with anemones, medusae, and worms; ( c ) possession of light‐filtering pigments, as with colored corals and molluscs; ( d ) possession of light‐concentrating devices, as with tridacnids; ( e ) possession of highly contractile and differentially light‐absorbing tissues, as with most coelenterates and some molluscs. Susceptibility of marine invertebrates to zooxanthellae infection may be related to the known paucity of nitrates and phosphates in tropical seas, a situation possibly inducing the free‐swimming gymnodinioid forms to enter animal tissues where catabolic products are available to them. Zooxanthellae are reported from sessile marine invertebrates taken at depths of 100–116 fathoms.
Zahl et al. (Sun,) studied this question.