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 6–10. 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). Although the taxonomy of the genus has not been reviewed since 1955 , the following species are recognised: "T. mauritanicus" was considered a subspecies of "T. cancriformis" by Longhurst in 1955, but was given full species status again by Korn "et al." in 2006. Note that for several of these species there are different varieties, some of which have recently been suggested as subspecies and even separate species. "T. longicaudatus", for example, may actually be several species lumped together, and "T. cancriformis" is generally recognized as having three subspecies: "T. cancriformis cancriformis", "T. c. mauretanicus", and "T. c. simplex". Also, the albino form has the special name of "T. cancriformis" var. "Beni-Kabuto  Ebi". Relationship with humans. The species is considered a human ally against the West Nile virus, as the individuals consume "Culex" mosquito larvae. They also are used as a biological pest control in Japan, eating weeds in rice paddies. The Beni-Kabuto Ebi Albino variant of "T. cancriformis" is particularly valued for this purpose. In Wyoming, the presence of "T. longicaudatus" usually indicates a good chance of the hatching of spadefoot frogs. Dried eggs of "T. longicaudatus" are sold in kits to be raised as aquarium pets, sold under the name of "aquasaurs", "trigons" or "triops". Among enthusiasts, "T. cancriformis  is also common. Other species often encountered in captivity include T. australiensis", "T. newberryi" and "T. granarius". Captive "Triops" are frequently kept in aquaria and fed a diet consisting mainly of carrots, shrimp pellets and dried shrimp. Often they are also given living shrimp and "Daphnia" as live prey. Because they can feed on just about anything they are also fed lunch meat, crackers, potatoes etc. In California, "T. longicaudatus" has emerged as a significant pest of rice cultivation, due to its digging behaviour uprooting young rice seedlings.