be anciently asexual. recent transitions: loss of sexual reproduction can be inherited in a simple mendelian fashion in the monogonont rotifer brachionus calyciflorus: this species can normally switch between sexual and asexual reproduction (cyclical parthenogenesis), but occasionally gives rise to purely asexual lineages (obligate parthenogens). these lineages are unable to reproduce sexually due to being homozygous for a recessive allele. resting eggs resting eggs enclose an embryo encysted in a three layered shell that protects it from external stressors. they are able to remain dormant for several decades and can resist adverse periods (e.g., pond desiccation or presence of antagonists). when favourable conditions return and after an obligatory period of diapause which varies among species, resting eggs hatch releasing diploid amictic females that enter into the asexual phase of the life cycle. anhydrobiosis bdelloid rotifer females cannot produce resting eggs, but many can survive prolonged periods of adverse conditions after desiccation. this facility is termed anhydrobiosis, and organisms with these capabilities are termed anhydrobionts. under drought conditions, bdelloid rotifers contract into an inert form and lose almost all body water; when rehydrated they resume activity within a few hours. bdelloids can survive the dry state for long periods, with the longest well-documented dormancy being nine years. while in other anhydrobionts, such as the brine shrimp, this desiccation tolerance is thought to be linked to the production of trehalose, a non-reducing disaccharide (sugar), bdelloids apparently cannot synthesise trehalose. in bdelloids, a major cause of the resistance to desiccation, as well as resistance to ionizing radiation, is a highly efficient mechanism for repairing the dna double-strand breaks induced by these agents. this repair mechanism likely involves mitotic recombination between homologous dna regions. predators rotifers fall prey to many animals, such as copepods, fish (e.g. herring, salmon), bryozoa, comb jellies, jellyfish, starfish, and tardigrades. genome size the genome size of a bdelloid rotifer, adineta vaga, was reported to be around 244 mb. the genomes of monogononts seem to be significantly smaller than those of bdelloids. in monogononta the nuclear dna content (2c) in eight different species of four different genera ranged almost fourfold, from 0.12 to 0.46 pg. haploid "1c" genome sizes in brachionus species range at least from 0.056 to 0.416 pg. gallery pair of lepadella rotifers from pond water locula of the rotifer keratella cochlearis