The order Notostraca, containing the single family Triopsidae, is a group of known as tadpole shrimp
Description
Notostracans are long, with a broad
carapace at the front end, and a long, slender abdomen.
This gives them a similar overall shape to a
tadpole, from which the
common name tadpole shrimp derives.
The carapace is dorso-ventrally flattened, smooth, and bears no rostrum; it includes the head, and the two sessile
are located together on top of the head.
The two pairs of antennae are much reduced, with the second pair sometimes missing altogether.
The mouthparts comprise a pair of uniramous mandibles and no maxillipeds.
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-developed arthropod leg and the genital opening on the eleventh segment. In the female, it is modified to form a "brood pouch".
The somites on thorax II are fused into "rings", which varies in number between species and gender and appear to be body segments, but do not always reflect the underlying segmentation.[ Evolution and Phylogeny of Pancrustacea: A Story of Scientific Method - 44. Notostraca and Allies (Calmanostraca)]
The limbless abdomen ends in a telson and a pair of long, thin, multi-articulate Caudal ramus.
Life cycle
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
Androdioecy.
[ Self-Fertilization and the Role of Males in Populations of Tadpole Shrimp (Branchiopoda: Notostraca: Triops)] The frequency of males in populations is therefore highly variable.
In sexual populations, 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.
The eggs are retained by the female only for a short time before being laid,
and the larvae develop directly, without passing through a
metamorphosis.
Ecology and distribution
Notostracans are
omnivore, eating small animals such as
and
fairy shrimp.
They are found worldwide in
freshwater,
brackish water, or
Saline water pools, as well as in shallow
,
, and
moorland.
The species
Triops longicaudatus is considered an agricultural pest in
California Paddy field, because it prevents light from reaching the rice seedlings by stirring up sediment.
Evolution and fossil record
The
fossil record of Notostraca is extensive, occurring in a wide range of geological deposits.
The oldest known notostracan is the species
Strudops from the Late
Devonian (
Famennian ~ 365 million years ago) of Belgium.
The lack of major morphological change since has led to Notostraca being described as
.
, a group known only from
Triassic and
Jurassic fossils from
Kazakhstan and Western
China,
are closely related to notostracans, and may belong within the order Notostraca,
or alternatively are placed as their sister group within the clade Calmanostraca.
The "central autapomorphy" of the Notostraca is the abandonment of filter feeder 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.
Taxonomy
The extant members of order Notostraca composed a single family, Triopsidae, with only two
genus,
Triops and
Lepidurus.
The problematic Middle Ordovician fossil
Douglasocaris has been erected and placed in its own family Douglasocaridae by Caster & Brooks 1956, and may be ancestral to Notostraca.
The phenotypic plasticity shown by notostracan species make identification to the species level difficult.
Genera list
-
Apudites, (Formerly "Notostraca" minor, often referred to as Triops cancriformis minor, or " Triops" minor in historic literature) Lower Triassic, Grès à Voltzia, Vosges Mountains, France; Hassberge Formation, Germany, Late Triassic (Carnian)
-
Brachygastriops Dabeigou Formation, China, Late Jurassic or Early Cretaceous
-
Chenops Yixian Formation, China, Early Cretaceous (Aptian)
-
Dikelocephala, Lower Triassic of North China
-
Discocephala, Lower Triassic of North China
-
Heidiops, Lower Permian of the Lodève Basin, France
-
Jeholops Yixian Formation, China, Early Cretaceous (Aptian)
-
Lynceites Germany, Canada, Carboniferous
-
Prolepidurus, Late Jurassic?-Lower Cretaceous, Transbaikal, Russia
-
Strudops Strud locality, Belgium, late Devonian (Fammenian)
-
Thuringiops, Upper Oberhof Formation, Thuringian Forest Basin, Carboniferous Germany
-
Weichangiops Dabeigou Formation, China, Late Jurassic or Early Cretaceous
-
Xinjiangiops Kelamayi Formation, China, Middle Triassic
Incertae sedis species
-
"Notostraca" oleseni Yixian Formation, China, Early Cretaceous (Aptian)
-
"Calmanostraca" hassbergella Hassberge Formation, Germany, Late Triassic (Carnian)
See also
External links
