Water loss and morphological changes during desiccation in Macrotrachela quadricornifera (Rotifera, Bdelloidea)

Bdelloid rotifers are aquatic microinvertebrates able to cope with the loss of environmental water and can, thus, live in temporal habitats. When water is evaporating, bdelloids contract into ‘tuns’, silence metabolism and lose water from the body. This condition is known as anhydrobiosis. Under controlled conditions, the bdelloid rotifer Macrotrachela quadricornifera was made anhydrobiotic and the variation of body volume, of metabolic activity and the adjustment of the internal structures were recorded and compared in hydrated and dry rotifers. Rotifers were observed by light (LM and CLSM) and electron microscopy (SEM and TEM), to know the volume of the body and the spatial distribution of the internal structures (muscles, tissues, cavities). The volume of the body was estimated comparing images at SEM and CLSM, and the loss of volume at desiccation was found to be about 50%. Images at LM and TEM did not show cavities in the dry rotifer body, while cavities were visible in the hydrated animal. We hypothesize that the volume loss is mostly due to the clearing of body cavities. The dry condition of the rotifer is characterised by a very compact organization of its anatomy and, also, by condensation of the fine structure of its tissues. We found also that the tun shape is attained by the contraction of longitudinal muscles, whereas the same muscles seem loose during anhydrobiosis suggesting that the tun shape is not maintained by muscle contraction. This agrees well with the apparent shut-down of the bdelloid metabolism during anhydrobiosis as observed in previous experiments.