Application of calcofluor staining to identify motile and coccoid stages of Symbiodinium (Dinophyceae)

Symbiotic dinoflagellates of the genus Symbiodinium (= zooxanthellae) are well known as symbionts of corals. Culture strains of Symbiodinium isolated from host animals show alternating morphological changes between the non-swimming coccoid stage and the flagellated motile stage with diel rhythmicity. While the coccoid stage is the representative form in hospite, the motile form is thought to be a key stage for approaching and infecting corals. When it is in culture, Symbiodinium stages are readily identified by observing swimming behavior. However, such swimming behavior cannot be observed in fixed specimens. This problem has limited our ability to identify their morphological status, especially in the field, when immediate examination is difficult. Therefore, we applied calcofluor staining, a method frequently used to stain dinoflagellate thecal plates (composed of cellulose complex), on fixed specimens to distinguish between motile and coccoid stages in Symbiodinium. The staining clearly separated the two morphological stages and compared well with the ratio that was determined through observation of swimming behavior and cell outline. This suggests that calcofluor staining method is useful for Symbiodinium, and will be potentially applicable for in situ investigation.