In sexual reproduction new individuals are produced by the fusion of haploid gametes to form a diploid zygote. Sperm are male gametes, ova (ovum singular) are female gametes. Meiosis produces cells that are genetically distinct from each other; fertilization is the fusion of two such distinctive cells that produces a unique new combination of alleles, thus increasing variation on which natural selection can operate.

Rotifers will reproduce asexually when conditions are favorable by having females produce eggs by mitosis. When conditions deteriorate, rotifers will reproduce sexually and encase their zygotes inside a resistant shell. Once conditions improve, these eggs hatch into diploid individuals. Rotifers thus use sexual reproduction as way to survive a deteriorating environment.

Sexual reproduction offers the benefit of generating genetic variation among offspring, which enhances the chances of the population's survival. Costs of this process include the need for two individuals to mate, courtship rituals, as well as a number of basic mechanisms described later.

Information provided by: http://gened.emc.maricopa.edu