Hatching phenology and resting eggs in tardigrades

Little is known about egg dormancy in tardigrades, except for their ability to survive desiccated for a long time. Our previous analyses of the life-history traits of a reared strain of the leaf litter-dwelling eutardigrade Paramacrobiotus richtersi revealed a variation in hatching phenology, suggesting the presence of diapause (resting) eggs in tardigrades. To study adaptive strategies in an unpredictable environment subject to stochastic variability, such as that colonized by tardigrades, we have analysed the hatching phenology of an apomictic triploid cytotype of P. richtersi. The first lab oviposition of mature females collected in the field in the spring and fall as well as cohorts of eggs laid by females born in the laboratory were used. The eggs of all samples, maintained under the same constant experimental conditions, had a high hatching percentage (from 75 to 93%) but a high variability occurred in the hatching time. Four patterns were identified. First, subitaneous eggs hatched within 30-40 days from oviposition. Second, delayed-hatching eggs hatched gradually over 41-62 days. Some eggs did not hatch within 90 days from oviposition when water was maintained in the culture. Within this group, 13% of eggs (diapause resting eggs; third category) do not hatch until they are subjected to desiccation, followed by rehydration, while 87% never complete their development (abortive eggs; fourth category). The four categories of eggs had no morphological differences. The high variability in the hatching time of tardigrade eggs might be considered a form of bet-hedging.