Localization of Symbiotic Dinoflagellate Cells within Tentacle Tissue of Aiptasia pallida (Coelenterata, Anthozoa)

The dinoflagellate symbiont of the sea anemone Aiptasia pallida occurs primarily within the tentacle gastrodermal tissue. Electron microscopy was used to identify the dinoflagellate symbiont and to determine whether it occurs within tentacle gastrodermal cells or in intercellular space between the gastrodermal cells. The ultrastructure of dinoflagellate vegetative cells appears identical to that of Symbiodinium microadriaticum (Freudenthal). Intact tentacles and enzymatically-digested tentacle gastrodermal tissue from infected and bleached animals were examined by scanning electron microscopy. Our observations provide evidence that dinoflagellate cells are located within vacuoles in tentacle gastrodermal cells. When the tentacles are expanded fully, as in full sunlight, gastrodermal cells are closely appressed to the mesoglea so that algal cells form essentially a monolayer. A possible significance of this algal monolayer is that it provides for maximum irradiation of the symbionts, thus optimizing photosynthesis. The significance of the intracellular location of the dinoflagellate symbiont is discussed.