The sexes may be distinct, but more frequently parthenogenetic females are dominant. In some species the male form has never been observed. Where male forms do occur they are typically smaller than the females and have a reduced longevity. During sex females are impregnated by specialised male copulatory organs, and fertilisation occurs internally. An unusual feature of rotiferan reproduction is that a variety of different egg types may be produced.
Parthenogenetic females produce only amictic eggs which have a characteristically thin shell.
Amictic eggs are already diploid, and cannot be fertilised. They develop and hatch rapidly. The mictic egg is also thin shelled, however it is haploid, and can be fertilised. Should fertilisation occur, the egg secretes an additional, thick shell layer and becomes known as a dormant egg . The amictic egg is produced when environmental conditions are favourable, for example during a wet season.