Cladocerans can reproduce both sexually and asexually; the mode of reproduction depends on environmental conditions, but most species reproduce asexually most of the time. Re production begins as water temperatures rise in the spring, but the population declines in the summer due to overcrowding and competition for food. A second peak of reproduction and population growth may occur in autumn.
Cladoceran eggs develop in about two days in the female's brood chamber, which is located between the body and the carapace. In most cladoceran species, the clutch size increases with body size, while the development time of the eggs is inversely related to temperature. Some eggs produced asexually develop into young immediately. If conditions are unfavorable, both sexually- and asexually-produced eggs may enter a resting state called diapause, during which they are resistant to heat, desiccation (drying out), and freezing. Most eggs develop into females. The production of male offspring is triggered by such environmental signals as crowding, changes in food concentrations, or decreasing day length.
During episodes of sexual reproduction, males use their first antennae to locate female mates. After finding a receptive female, the male grasps the edge of her carapace. The female continues to swim, carrying the male with her. After some time, the swimming ceases and both extend their postabdominal region, presumably to allow the male to eject his sperm as close to the female's brood pouch as possible. Mating may take anywhere between 15 minutes and several hours to complete.
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