SYMBIOSIS IN THE PLANKTONIC FORAMINIFER,ORBULINA UNIVERSA,AND THE ISOLATION OF ITS SYMBIOTIC DINOFLAGELLATE,GYMNODINIUM BÉII SP. NOV.1

The symbiotic association of the spinose planktonic foraminifer, Orbulina universa, with the dinoflagellate, Gymnodinium béii sp. nov., was examined with light and electron microscopy, and the symbiont was isolated into unialgal culture. The intact association is characterized by a diurnal movement of the symbionts from the distal regions of the spines during the day, to perialgal vacuoles within the host cytoplasm at night. This diurnal migration involves a daily endo- exocytotic cycle. Gymnodinium béii is non-motile and spindle-shaped within the host, whereas it is motile and gymnodinoid in shape when in culture. Ultrastructural examination revealed two or more stalked pyrenoids penetrated by lamellae, a typical dinokaryon nucleus and no trichocysts. A distinct ‘flange’projects over the sulcus from the hypocone. The swimming behavior of this dinoflagellate was characterized by intermittent darting events. Swimming speeds during a dart reached velocities of 770 μm. s^?1 as compared to a mean, non-darting swimming velocity of 126 μm. s^?1. Gymnodinium béii is eurythermal and division rates ranged between 0.16 and 0.65 divisions day^?1 for culture temperatures between 6.5 and 25° C respectively.