Desiccation tolerance in embryonic stages of the tardigrade

Desiccation tolerance commonly found among tardigrades allows them to cope with temporal variation of available water. Although the long-term survival of adults has been demonstrated in several species, desiccation tolerance of eggs and embryos is less well studied, however it is an important aspect from an ecological and evolutionary point of view. For the first time we evaluated the desiccation tolerance and subsequent hatching success of five different developmental stages of the tardigrade species Milnesium tardigradum , when rehydrated following drying at eight different humidity levels (10, 20, 31, 40, 54, 59, 72, 81%). Humidity level and developmental stage are significant factors in determining successful hatch rates. The results showed that the less developed stages were quite sensitive to desiccation. Low humidity levels during the first 3 days of development lead to a decrease in hatch rates following rehydration. Later developmental stages showed higher hatch rates than embryos dried at earlier stages. However, fast drying at low humidity levels resulted in delayed development and lower hatch rates following rehydration. In general, further developed embryos exhibit a better survival capacity compared with younger stages.