Abstract
Tardigrades can cope with adverse environmental conditions by turning into anhydrobiotes with a characteristic tun shape. Tun formation is an essential morphological adaptation for tardigrade entry into the anhydrobiotic state. The tun cell structure and ultrastructure have rarely been explored in tardigrades in general and never in Hypsibius exemplaris. We used transmission electron microscopy to compare cellular organization and ultrastructures between hydrated and anhydrobiotic H. exemplaris. Despite a globally similar cell organelle structure and a number of cells not significantly different between hydrated and desiccated tardigrades, reductions in the sizes of both cells and mitochondria were detected in dehydrated animals. Moreover, in anhydrobiotes, secretory active cells with a dense endoplasmic reticulum network were observed. Interestingly, these anhydrobiote-specific cells are in a close relationship with a specific extracellular structure surrounding each cell. It is possible that this rampart-like extracellular structure resulted from the accumulation of anhydrobiotic-specific material to protect the cells. Interestingly, after five hours of rehydration, the number of secretory cells decreased, and the specific extracellular structure began to disappear. Twenty-four hours after the beginning of rehydration, the cellular structure and ultrastructure were comparable to those observed in hydrated tardigrades.
Highlights
Among all adaptative survival strategies that organisms have developed in response to harsh environmental conditions, cryptobiosis must be considered one of the most extreme
We investigated the structure and ultrastructure of cells and organelles of anhydrobiotic Hypsibius exemplaris specimens by electron microscopy and compared them to the ultrastructure of active hydrated specimens
The contraction of the body in the tun was observed by confocal microscopy with differential interference contrast (DIC) (Fig. 1a–f), revealing that hydrated tardigrades measured 164 +/− 32 μm, while tuns measured 101 +/− 12 μm (Fig. 1g)
Summary
Among all adaptative survival strategies that organisms have developed in response to harsh environmental conditions, cryptobiosis must be considered one of the most extreme. While most animals have a limited ability to survive dehydration[3], a few species in invertebrate taxa are able to survive complete water loss[4] These organisms include bdelloid rotifers, nematodes and tardigrades, which have the ability to enter anhydrobiosis at any developmental stage[1,5,6,7], and some dipteran larvae, such as those of the chironomid Polypedilum vanderplanki[8]. Some studies report low levels of this sugar, while others cannot identify trehalose at all in the same species[20,21,22,23,24] It has not yet been studied in Hypsibius exemplaris. To characterize the process in greater detail, we enlarged our study to individuals rehydrated for 5 and 24 hours
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