| Notostraca Temporal range: Carboniferous–Recent |
|
|---|---|
| Triops australiensis | |
| Lepidurus apus | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Subphylum: | Crustacea |
| Class: | Branchiopoda |
| Subclass: | Phyllopoda |
| Order: | Notostraca G. O. Sars, 1867 |
| Family: | Triopsidae Keilhack, 1909 |
| Genera | |
The order Notostraca comprises the single family Triopsidae, containing the tadpole shrimp[1] or shield shrimp.[2] The two genera, Triops and Lepidurus, are considered living fossils, having not changed significantly in outward form since the Triassic. They have a broad, flat carapace, 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. The telson 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 are omnivores living on the bottom of temporary pools and shallow lakes.
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Notostracans are 2–10 centimetres (0.8–3.9 in) long, with a broad carapace at the front end, and a long, slender abdomen.[1] This gives them a similar overall shape to a tadpole, from which the common name tadpole shrimp derives.[1] The carapace is dorso-ventrally flattened, smooth, and bears no rostrum; it includes the head, and the two sessile compound eyes are located together on top of the head.[1] The two pairs of antennae are much reduced, with the second pair sometimes missing altogether.[2] The mouthparts comprise a pair of uniramous mandibles and no maxillipeds.[2]
The trunk comprises a large number of "body rings", which appear to be body segments, but do not always reflect the underlying segmentation.[1] The first eleven body rings compose the thorax and bear one pair of legs each, the last of which also bears the genital opening.[1] 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 remaining segments form the abdomen. The number of body rings is variable both within and between species,[1] and the number of pairs of legs per body ring can rise as high as six.[4] The legs become progressively smaller along the abdomen,[3] with the last segments being legless.[1]
The abdomen ends in a telson and a pair of long, thin, multi-articulate caudal rami.[5] The form of the telson varies between the two genera: in Lepidurus, a rounded projection extends between the caudal rami, while in Triops there is no such projection.[1]
Within the Notostraca, and even within species, there is variation in the mode of reproduction, with some populations reproducing sexually, some showing self-fertilisation of females, and some showing a mix of the two.[1] The frequency of males in populations is therefore highly variable.[3] In sexual population, the sperm leave the male's body through simple pores, there being no penis. The eggs are released by the female and then held in the cup-like brood pouch.[3] The eggs are only retained by the female for a short time before being laid,[4] and the larvae develop directly, without passing through a metamorphosis.[2]
Notostracans are omnivorous, eating small animals such as fishes and fairy shrimp.[1] They are found worldwide in freshwater, brackish water or saline pools, as well as in shallow lakes, peat bogs and moorland.[2] In Californian rice paddies, the species Triops longicaudatus is considered a pest, because it stirs up sediment, preventing light from reaching the rice seedlings.[6]
The fossil record of Notostraca is extensive, occurring in a wide range of geological deposits, and reaching back to the Carboniferous.[7] The lack of major morphological change since 250 million years ago has led to Notostraca being described as living fossils.[8] Kazacharthra, a group known only from Jurassic fossils from Kazakhstan, are closely related to notostracans, and may belong within the order Notostraca.[9]
The "central autapomorphy" of the Notostraca is the abandonment of filter 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.[4] 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, the adductor muscle is missing.[4] Notostracans retain the plesiomorphic condition of having two separate compound eyes, which abut, but have not become united, as seen in other groups of Branchiopoda.[4]
The order Notostraca is composed of a single family, Triopsidae, and only two genera, Triops and Lepidurus.[8]
The phenotypic plasticity shown by notostracan species make identification to the species level difficult.[8] Many putative species have been described based on morphological variation, such that by the 1950s, as many as 70 species were recognised.[8] Two important revisions – those of Linder in 1952[10] and Longhurst in 1955[11] – synonymised many taxa, and resulted in the recognition of only 11 species in the two genera. This taxonomy was accepted for decades,[8] "even attaining the status of dogma".[12] More recent studies, especially those employing molecular phylogenetics, have shown that the eleven currently recognised species conceal a greater number of reproductively isolated populations.[8]
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