| Notostraca | |
|---|---|
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| Triops australiensis | |
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| Lepidurus apus | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Class: | Branchiopoda |
| Superorder: | Calmanostraca |
| Order: | Notostraca G. O. Sars, 1867 |
| Genera | |
The orderNotostraca, containing the single familyTriopsidae, is a group ofcrustaceans known astadpole shrimp[1] orshield shrimp.[2] The two genera,Triops andLepidurus, are consideredliving fossils, with similar forms having existed since the end of theDevonian, around 360 million years ago. They have a broad, flatcarapace, which conceals the head and bears a single pair of compound eyes. The abdomen is long, appears to be segmented and bears numerous pairs of flattened legs. Thetelson is flanked by a pair of long, thin caudal rami.Phenotypic plasticity within taxa makes species-level identification difficult, and is further compounded by variation in the mode of reproduction. Notostracans areomnivores living on the bottom oftemporary pools and shallow lakes.
Notostracans are 2–5 centimetres (0.8–2.0 in) long, with a broadcarapace at the front end, and a long, slender abdomen.[1] This gives them a similar overall shape to atadpole, from which thecommon nametadpole shrimp derives.[1] The carapace is dorso-ventrally flattened, smooth, and bears norostrum; it includes the head, and the two sessilecompound eyes are located together on top of the head.[1] The two pairs ofantennae are much reduced, with the second pair sometimes missing altogether.[2] Themouthparts comprise a pair of uniramousmandibles and no maxillipeds.[2]
The trunk consists of three regions; thorax I, thorax II and the abdomen. Thorax I is made up of 11 segments, each with a pair of well-developedlimbs and the genital opening on the eleventh segment. In the female, it is modified to form a "brood pouch".[3] The first one or two pairs of legs differ from the remainder, and probably function as sensory organs.[3]
The somites on thorax II are fused into "rings", which varies in number between species and gender and appear to bebody segments, but do not always reflect the underlying segmentation.[1] Each ring is made up of 2–6 complete or partial fused segments, and the number of legs on each body ring match its number of segments.[4][1] The legs become progressively smaller posteriorly,[3] with the last segments being legless.[1]
The limbless abdomen ends in atelson and a pair of long, thin, multi-articulatecaudal rami.[5] The form of the telson varies between the two genera: inLepidurus, a rounded projection extends between the caudal rami, while inTriops there is no such projection.[1]
Within the Notostraca, and even within species, there is variation in the mode of reproduction, with some populationsreproducing sexually, some showingself-fertilisation of females, and some showing amix of the two.[1][6] The frequency of males in populations is therefore highly variable.[3] In sexual populations, thesperm leave the male's body through simple pores, there being nopenis. The eggs are released by the female and then held in the cup-like brood pouch.[3] The eggs are retained by the female only for a short time before being laid,[7] and thelarvae develop directly, without passing through ametamorphosis.[2]
Notostracans areomnivorous, eating small animals such asfishes andfairy shrimp.[1] They are found worldwide infreshwater,brackish water, orsaline pools, as well as in shallowlakes,peat bogs, andmoorland.[2] The speciesTriops longicaudatus is considered anagricultural pest inCaliforniarice paddies, because it prevents light from reaching the rice seedlings by stirring up sediment.[8]
Thefossil record of Notostraca is extensive, occurring in a wide range of geological deposits.[9] The oldest known notostracan is the speciesStrudops goldenbergi from the LateDevonian (Famennian ~ 365 million years ago) of Belgium.[10] The lack of major morphological change since250 million years ago has led to Notostraca being described asliving fossils.[11]Kazacharthra, a group known only fromTriassic andJurassic fossils fromKazakhstan and WesternChina,[12] are closely related to notostracans, and may belong within the order Notostraca,[13] or alternatively are placed as their sister group within the clade Calmanostraca.
The "central autapomorphy" of the Notostraca is the abandonment offilter feeding in open water, and the development of a benthic lifestyle in muddy waters, taking up food from particles of sediment and preying on small animals.[7] A number of other characteristics are correlated with this change, including the increased size of the animal compared to its relatives, and the loss of the ability to hinge the carapace; although a central keel marks the former separation into two valves, theadductor muscle is missing.[7] Notostracans retain theplesiomorphic condition of having two separate compound eyes, which abut, but have not become united, as seen in other groups of Branchiopoda.[7]
The extant members oforder Notostraca composed a singlefamily, Triopsidae, with only twogenera,Triops andLepidurus.[11]The problematic Middle Ordovician fossilDouglasocaris has been erected and placed in its own family Douglasocaridae by Caster & Brooks 1956, and may be ancestral to Notostraca.
Thephenotypic plasticity shown by notostracan species make identification to the species level difficult.[11] Many putative species have been described based on morphological variation, such that by the 1950s, as many as 70 species were recognised.[11] Two important revisions – those of Linder in 1952[14] and Longhurst in 1955[15] –synonymised many taxa, and resulted in the recognition of only 11 species in the two genera. This taxonomy was accepted for decades,[11] "even attaining the status of dogma".[16] More recent studies, especially those employingmolecular phylogenetics, have shown that the eleven currently recognised species conceal a greater number of reproductively isolated populations.[11]
Incertae sedis species
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Families of classBranchiopoda | ||
|---|---|---|
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| Anostraca |  | |
| Notostraca | ||
| Laevicaudata | ||
| Spinicaudata | ||
| Cyclestherida | ||
| Cladocera | ||
| Authority control databases: National |
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