Environmental Adaptations: Encystment and Cyclomorphosis

Stressful environmental conditions generally limit animal survival, growth, and reproduction and may induce dormancy in the form of various resting stages. Tardigrades represent one of a few animal phyla in which different forms of dormancy are frequently encountered. One of these forms, cryptobiosis, a quick response to sudden changes in the environment, has gained a great deal of attention, whereas much less is known of the slower emerging form of dormancy, diapause. In this review we present the current knowledge of diapause in tardigrades. Diapause in tardigrades, represented by encystement and cyclomorphosis, is likely controlled by exogenous stimuli, such as temperature and oxygen tension, and perhaps also by endogenous stimuli. These stimuli initiate and direct successive phases of deep morphological transformations within the individual. Encystment is characterized by tardigrades that lie dormant—in diapause—within retained cuticular coats (exuvia). The ability to form cysts is likely widespread but presently only confirmed for a limited number of species. In tardigrades, cyclomorphosis was first reported as a characteristic of the marine eutardigrade genus Halobiotus. This phenomenon is characterized by pronounced seasonal morphological changes and in Halobiotus involves stages with an extra protecting cuticle. Cyst formation in moss-dwelling limnic species may also occur as part of a seasonal cyclic event and can thus be viewed as part of a cyclomorphosis. Therefore, whereas diapause generally seems to be an optional response to environmental changes, it may also be an obligate part of the life cycle. The evolution of encystment and cyclomorphosis finds its starting point in the molting process. Both phenomena represent an adaptation to environmental constraints. Notably, the evolution of diapause is not necessarily an alternative to cryptobiosis, and some tardigrades may enter both forms of dormancy. The simultaneous occurrence of several adaptive strategies within tardigrades has largely increased the resistance of these enigmatic animals toward extreme environmental stress.