Daphnia pulex reproduces both sexually and asexually in a process called parthenogenesis, where male gametes are unnecessary. Parthenogenesis occurs mainly in the summer, so that during summer an entire population of Daphnia pulex will consist almost completely of females. This process begins in the female, which then molt the carapace to increase their size and develope anywhere from two to twenty eggs in their brood chamber. Even without fertilization from a male, these eggs will develope into immature females which are released after the next molting stage. The young that are produced in this way are more precocial or well-developed than in the process of producing altricial fertilized eggs. This stage of reproduction is most used for a rapid increase in Daphnia growth but requires more favorable conditions.
The sexual stage of Daphnia reproduction occurs mainly in the winter during less favorable conditions caused by overcrowding, accumulation of wastes, lower food availability, and lower temperatures. First, some of the eggs that were produced by parthenogenesis hatch into males instead of females. These males then copulate with the females to form fertilized eggs which are then kept in the female's brood chamber. After the female's next molt she releases these eggs which have the ability to overwinter. They can resist freezing and drying while encased in a purselike ephippium that protects the egg as it rests in the sediment at the bottom of the water body until spring. These eggs remain in this stage of arrested developement, lasting up to twenty years, until the conditions become more favorable for hatching.
Daphnia usually live about ten to thirty days and can live up to one hundred days if their environment is free of predators. An individual will generally have ten to twenty instars, or periods of growth, during their lifetime.
Key Reproductive Features: gonochoric/gonochoristic/dioecious (sexes separate); sexual ; asexual ; fertilization (Internal ); oviparous
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