Anhydrobiosis Affects Thermal Habituation in the Bdelloid Rotifer, Adineta sp.

The bdelloid rotifer of the genus Adineta is a freshwater metazoan characterized by anhydrobiosis, a highly stable state of suspended animation induced by desiccation. This study investigated the influence of anhydrobiosis on the thermal habituation by use of an index, Activity Ratio (AR = the number of active rotifers at each experimental temperature/ number of active rotifers at the 25°C stage). In the first experiment, rotifers were divided into two groups: one group was cultivated at 25°C throughout experiment, and another group was transferred to 15°C for two days. AR was estimated during heating up to 40°C, or during cooling down to 5°C in each group. The largest difference in AR occurred at 35°C and 10°C, indicating that AR was changed depending on the pretreated medium temperature. In the next experiment, rotifers were maintained at 15°C, and were desiccated (anhydrobiosis). AR was estimated in the high temperature range (25°C to 40°C), using rotifers that had recovered from anhydrobiosis. AR was significantly different between the groups with and without desiccation, suggesting that thermal habituation at 15°C was completely cancelled by anhydrobiosis. Possible mechanisms on the influence of anhydrobiosis on the thermal habituation have been discussed in terms of neural changes and proteins.