Frequency of binary fission in cultures of three strains of the symbiotic dinoflagellate Symbiodinium microadriaticum, isolated from the anemones Aiptasia tagetes, Heteractis lucida and the jellyfish Cassiopeia xamachana, was related to their diel patterns of motility. Under a 14:10 h light:dark cycle, karyokinesis, cytokinesis and motility occur diurnally. The highest frequency of cytokinesis preceded highest motility by 4 to 7 h. Karyokinesis preceded cytokinesis by 1 to 5 h. Generally, the ratio of number of dividing cells to the number of motile cells recorded in exponentially growing cultures was 1:2, suggesting that two motile cells arise from one newly divided zooxanthella. No motile cells were seen in stationary cultures. Algae grown at high irradiance (180/μE m−2 s−1) had higher growth rates and higher levels of motility than those grown at lower irradiance (15 μE m−2 s−1).

Under constant light or dark conditions, patterns of cytokinesis and motility were circadian. In darkness the rhythm tapered out after about 1 week. The patterns were evident for up to 7 days in constant dim light, but tended to dampen out with time.

These results suggest that current views of the life cycle of S. microadriaticum need to be modified, and we propose some alternatives which are consistent with our observations.