Relatives and fossil record
The genus Triops can be distinguished from the only other genus of Notostraca, Lepidurus, by the form of the telson, which bears a pair of long, thin caudal extensions in Triops, while Lepidurus bears a central platelike process. Only 24 hours after hatching they already resemble miniature versions of the adult form.
Triops are sometimes called "living fossils". Fossils attributable to this genus have been found in rocks of Carboniferous age, an estimated , and one extant species, Triops cancriformis, has hardly changed since the Jurassic period (approximately ).
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.
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. 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.
The name Triops comes from the Greek τρία (tría) meaning "three" and ὤψ (ops) meaning "eye". The head of Triops longicaudatus bears a pair of dorsal compound eyes that lie close to each other and are nearly fused together. The compound eyes are generally sessile (not stalked). In addition, there is a naupliar ocellus (the "third eye") between them. The compound eyes are on the surface of the head, but the ocellus is deep within the head. All the eyes, however, are easily visible through the shell covering of the head.
Franz von Paula Schrank was the first author to use the genus name Triops, coining it in his 1803 work on the fauna of Bavaria. Their German name was Dreyauge, which means 'three-eye' in English. He collected and described specimens from the same locality in Regensburg from which Schäffer, another naturalist who had studied the Notostraca, obtained his specimens in the 1750s. However, other authors, starting with Louis Augustin Guillaume Bosc, had adopted the genus name Apus for the organisms Schrank had named Triops.
Ludwig Keilhack used the genus name Triops in his 1909 field identification key of the freshwater fauna of Germany. He suggested that the genus name Apus be replaced by Triops Schrank since an avian genus had already been described by Giovanni Antonio Scopoli under the name Apus. However, Robert Gurney preferred the name Apus Schäffer. He suggested that the name '...Triops Schrank, may be returned to the obscurity from which it was unearthed'. This controversy continued and was not resolved until the 1950s.
In his 1955 taxonomic review of the Notostraca, Alan R. Longhurst supported Keilhack's genus name Triops over Apus. Longhurst provided historical evidence to support this position. The International Commission on Zoological Nomenclature (ICZN) followed Longhurst in their 1958 ruling on the usage and origin of the genus names Triops and Apus. They rejected the genus name Apus and instead recognized the genus name Triops Schrank, 1803 (ICZN name no. 1246).
- Triops australiensis (Spencer & Hall, 1895)
- Triops baeticus Korn, 2010
- Triops cancriformis (Bosc, 1801)
- Triops emeritensis Korn & Pérez-Bote, 2010
- Triops gadensis Korn & García-de-Lomas, 2010
- Triops granarius (Lucas, 1864)
- Triops longicaudatus (LeConte, 1846)
- Triops mauritanicus Ghigi, 1921
- Triops newberryi Thomas, 1921
- Triops vicentinus Korn, Machado, Cristo & Cancela da Fonseca, 2010
- Denton Belk (2007). "Branchiopoda". In Sol Felty Light & James T. Carlton. The Light and Smith manual: intertidal invertebrates from central California to Oregon (4th ed.). University of California Press. pp. 414–417. ISBN 978-0-520-23939-5.
- Chip Hannum & Stuart Halliday. "An Introduction to Triops". MyTriops.com. Retrieved October 4, 2010.
- David A. Grimaldi & Michael S. Engel (2005). "Arthropods and the Origin of Insects". Evolution of the insects. Volume 1 of Cambridge Evolution Series. Cambridge University Press. pp. 93–118. ISBN 978-0-521-82149-0.
- Graham Bell (1982). The masterpiece of nature: the evolution and genetics of sexuality. Croom Helm applied biology series. Cambridge University Press. p. 243. ISBN 978-0-85664-753-6.
- Patrick L. Osborne (2000). "Hot deserts and environmental factors". Tropical ecosystems and ecological concepts. Cambridge University Press. pp. 18–49. ISBN 978-0-521-64523-2.
- John Scarborough (1992). "Crustacea". Medical and biological terminologies: classical origins. Volume 13 of Oklahoma Series in Classical Culture. University of Oklahoma Press. pp. 75–81. ISBN 978-0-8061-3029-3.
- Ole S. Møller, Jørgen Olesen & Jens T. Høeg (2003). "SEM studies on the early larval development of Triops cancriformis (Bosc) (Crustacea: Branchiopoda, Notostraca)" (PDF). Acta Zoologica 84 (4): 267–284. doi:10.1046/j.1463-6395.2003.00146.x.
- Chip Hannum. "A Brief Overview of the Species". MyTriops.com. Retrieved October 4, 2010.
- Michael Korn, Andy J. Green, Margarida Machado, Juan García-de-Lomas, Margarida Cristo, Luís Cancela da Fonseca, Dagmar Frisch, José L. Pérez-Bote & Anna K. Hundsdoerfer (2010). "Phylogeny, molecular ecology and taxonomy of southern Iberian lineages of Triops mauritanicus (Crustacea: Notostraca)" (PDF). Organisms Diversity and Evolution 10 (5): 409–440. doi:10.1007/s13127-010-0026-y.
- Michael Korn, Federico Marrone, Jose L. Pérez-Bote, Margarida Machado, Margarida Cristo, Luís Cancela de Fonseca & Anna K. Hundsdoerfer (2006). "Sister species within the Triops cancriformis lineage (Crustacea, Notostraca)" (PDF). Zoologica Scripta 35 (4): 301–322. doi:10.1111/j.1463-6409.2006.00230.x.