Triops
Triops is a genus of small crustaceans in the order Notostraca (tadpole shrimp). The long-lasting resting eggs of several species of Triops are commonly sold in kits as a pet. The animals hatch upon contact with fresh water. Most adult-stage Triops have a life expectancy of up to 90 days and can tolerate a pH range of 6 to 10. In nature, they often inhabit temporary pools.[1]
The genus Triops can be distinguished from the only other living genus of Notostraca, Lepidurus, by the form of the telson (the end of its 'tail'), which bears only a pair of long, thin caudal extensions in Triops, while Lepidurus also bears a central platelike process. Only 24 hours after hatching they already resemble miniature versions of the adult form.[1]
Triops are sometimes called "living fossils", since fossils that have been attributed to this genus have been found in rocks hundreds of millions of years old.[2] However, careful analysis of these fossils cannot definitively assign these specimens to Triops.[3] Molecular clock estimates suggest that Triops split from Lepidurus during the Triassic or Jurassic, between 152.3–233.5 million years ago. The earliest diverging lineages of living Triops are found in areas that are part of the former supercontinent Gondwana, suggesting Triops originated in Gondwana.[4]
Triops can be found in Africa, Australia, Asia, South America, Europe (including Great Britain[5]), and in some parts of North America where the climate is right. Some eggs stay unhatched from the previous group and hatch when rain soaks the area. Triops are often found in vernal pools.
Most species reproduce sexually, but some populations are dominated by hermaphrodites which produce internally fertilised eggs. Reproduction in T. cancriformis varies with latitude, with sexual reproduction dominating in the south of its range, and parthenogenesis dominating in the north.[6]
Triops eggs enter a state of extended diapause when dry, and will tolerate temperatures of up to 98 °C (208 °F) for 16 hours, whereas the adult cannot survive temperatures above 34 °C (93 °F) for 24 hours or 40 °C (104 °F) for 2 hours.[7] The diapause also prevents the eggs from hatching too soon after rain; the pool must fill with enough water for the dormancy to be broken.[7]
