Resting-egg hatching and early population development in rotifers: a review and a hypothesis for differences between shallow and deep waters

This paper reviews our very limited knowledge about resting-egg hatching and early population development in planktonic rotifers. Hatching of stem females from resting eggs may occur soon after resting eggs are produced, but perhaps usually it is delayed by a minimum obligate diapause, by a requirement for seasonal temperature changes, or by sinking to sediment environments that prevent hatching. In deep-water sediments, hatching probably is inhibited by low oxygen, darkness, or low temperature, so that eggs likely hatch following resuspension during water-column turnover. Populations should develop primarily by female parthenogenesis and have relatively low clonal diversity. By contrast, in shallow-water sediments, eggs are more likely to experience conditions conducive to hatching and to be resuspended into the water column. Populations may develop by massive emergence of stem females as well as by female parthenogenesis and thus have a very high clonal diversity. Stem females of some species are particularly fit for colonization of new habitat. First, compared to females hatched from parthenogenetic eggs, they can have a greater lipid reserve that enhances survival and reproduction. Second, amictic stem females can contain a transmissible factor that inhibits sexual reproduction and diapause for several to many successive generations, thus promoting rapid reproduction via female parthenogenesis.