Night and day morphologies in a planktonic dinoflagellate.

We describe remarkable diurnal changes in the morphology of the planktonic dinoflagellate Ceratium ranipes. The species is distinguished by the unusual appendages, known as toes or fingers, protruding from its horns. Varieties have been described based on the characteristics of the fingers. We discovered that cultures, maintained on a 12:12 photoperiod, when examined during the dark period were composed of 'finger-less' cells. Monitoring of isolated cells revealed a diurnal cycle of distinct changes in morphology with daytime cells showing appendages, well-stocked with chlorophyll, and the absence of the fingers at the end of the photoperiod. Fingers are absorbed at the end of the light period and re-grown at the end of the dark period. Sampling the Bay of Villefranche, we found the distinct morphologies in a natural population at night and during the day.