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Ceratium ranipes: The Shape Shifter

Ceratium species are part of the phytoplankton, for the most part microscopic single-cell organisms which rely largely on photosynthesis like plants do. The species Ceratium ranipes 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. It was first was described by P. T. Cleve in 1900 from material collected near the Azores.  He called it  a "rare and remarkable form". Although found in low abundances, it is relatively common in tropical and subtropical oligotrophic waters throughout the world oceans. Forms of Ceratium ranipes with fingers of different length or without fingers have long been reported and termed varieties. The distinct morphotypes, sometimes spatially and temporally separated, and sometimes co-occurring, suggested the existence of a species complex, or forms with thermal preferences (Sournia 1967). However, it was discovered that Ceratium ranipes has highly unusual diurnal polymorphism; it has a day morphology with fingers and a night morphology without fingers. Thus, the different morphologies, previously thought to represent varieties, are exhibited by the same, "shape shifter" individual.

How was this discovered? Cultures, maintained on a 12:12 photoperiod, when examined during the dark period were composed of 'finger-less' cells; examined later, after a few hours of illumination, the same cultures were composed of cells with 'fingers'.  The fingerless night forms appeared to be more active, constantly in motion, compared to the lethargic daylight forms. 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. The species then has distinct night and day morphologies. While diurnal rhythms, reversible changes in morphology, and autotomy have been described separately in other species they occur together in a single remarkable phenomenon in Ceratium ranipes. For details and more illustrations see Pizay et al. 2009.

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© John Dolan

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