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 pets. 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.
Relatives and fossil record
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.
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. However, careful analysis of these fossils cannot definitively assign these specimens to Triops. 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.
Triops can be found in Africa, Australia, Asia, South America, Europe (including Great Britain), 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.
Life cycle
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 for 16 hours, whereas the adult cannot survive temperatures above for 24 hours or 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.
Taxonomy
The name Triops comes from the Greek () meaning "three" and () meaning "eye". The head of T. 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 , which means 'three-eye'. 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).