Tough guys

The cells in the ovaries become capable of producing males as a result of several factors such as deviations in water temperature, dissolved gas concentration, the quality of their diet and also of the photoperiod. Otherwise, during the rest of the year, female water fleas produce only genetically identical daughters of themselves. Under these deteriorating conditions, the younger generation of females is able to form a rigid sheath around the eggs, called an ephippium.

 Ephippias are often formed during harsh months to protect diploid zygotes, which are the product of gamete fusion of both sexes. In such a capsule, the embryos can survive winter, anoxic conditions in mud, prolonged drought, and even pass through the intestinal tract intact. The embryos in the ephippium remain almost inactive in the early stages of development and can stay viable for decades. They will hatch when the conditions are favourable again. Even such resilient propagules can be threatened – predators such as Triops can completely damage the ephippium by digging up the eggs while feeding. 

The presence of males is therefore really important for a specific reproductive strategy to overcome a bad period. However, we know of cases in which individuals could produce and subsequently use ephippia despite the absence of males. How, why? These females simply produce clonal eggs and encapsulate them in a rigid sheath because sometimes they don't have enough time to reproduce sexually – male has to meet his mate. If only clones are produced almost immediately after oviposition, without a resting phase, the population wouldn't survive with the same probability as with eggs being deposited in ephippias if they should quickly prepare to overcome the bad period by escaping through time.

Snyman, M., Xu, S. (2023): Transcriptomics and the origin of obligate parthenogenesis. Heredity 131: 119–129. Dostupné z: https://doi.org/10.1038/s41437-023-00628-3

G.O. Sars (1885) v Petrusek (2022): When Crustaceans Get a Suntan. Živa 4: 192–194. Dostupné z: https://ziva.avcr.cz/files/ziva/pdf/kdyz-se-korysi-opaluji