Bdelloid rotifers are aquatic microinvertebrates able to cope with the loss of environmental water by entering dormancy, and are thus capable of living in temporary habitats. When water is evaporating, bdelloids contract into "tuns", silence metabolism and lose water from the body, a condition known as anhydrobiosis. Under controlled conditions, a bdelloid species (Macrotrachela quadricornifera) was made anhydrobiotic, and its morphology was studied by light, confocal and electron microscopy. A compact anatomy characterizes the anhydrobiotic rotifer, resulting in a considerable reduction of its body volume: the internal organs, precisely packed together, occupy the body cavity almost completely and the lumen of hollow organs disappears. Remarkable ultrastructural changes characterize the anhydrobiotic condition. The mitochondria are wholly surrounded by a ring of electron-dense particles, and the epidermal pores, open in the hydrated specimens, become gradually closed by structures similar to epithelial junctions. The cilia are densely packed: microtubules are still identifiable, but the axonemal organization appears disrupted. This is the first extensive comparative study on the morphological changes associated with the anhydrobiosis process in a rotifer, providing the basis for an improved understanding of the processes involved in this extreme adaptation. (C) 2010 Elsevier Inc. All rights reserved.