A cryptomonad symbiont of Aiptasia pallida Verrill (Anthozoa: Cnidaria).

It has been the purpose of this study to determine the nature of the algal symbiont of the anemone, Aiptasia pallida Verrill. Cultures have been made of the alga and its life cycle and culture characteristics studied. Note has been made of the behavior of the anemone with its symbiont in attempts to correlate activities of the coelenterate with needs of the alga. The algal symbiont of A. pallida exists within the tissues of the host as a vegetative cell virtually indistinguishable from the zooxanthellae of other coelenterates. When cultured and subjected to lowered levels of calcium and phosphate, a typically cryptomonad flagellated form appears and assumes a free-swimming condition in the medium. Attempts to identify this form with previously described cryptomonads have been unsuccessful due to discrepancies in available keys. Both vegetative and motile cells of the algal symbiont were observed to divide through longitudinal fission. In addition, a secondary sexual reproductive cycle was observed for the vegetative cells. In this process, a series of rapid divisions produces a sixteen-celled, membrane bound palmella from which weekly motile uniflagellate cells are released. The fusion of these cells results in the development of vegetative cells identical to the original. Light requirements of the alga and the behavior of the anemone in response to light indicate that the symbiosis is mutualistic. Migrations and contraction-expansion responses in the coelenterate parallel the light requirements of cultured algal cells. Almost total retention of alga within the tissues during prolonged periods of total darkness indicates that it is a facultative parasite.