|GO Sars , 1867
The Notostraca , sometimes called back shell , or prehistoric crabs, are a species-poor order of leaf crabs. Only two genera currently live, Triops and Lepidurus , both are specialists in only briefly water-bearing, drying (called: ephemeral) standing water of fresh water or, more rarely, brackish water . Finds of fossil Notostraca show a morphology of the group that has remained almost unchanged for 360 million years, which is why they are counted among the " living fossils ".
Notostraca are relatively large leaf pods, the length of the carapace reaches about 15 to 30 millimeters. The total length of the body can hardly be given with certainty, since the segments of the abdomen can be stretched or telescoped into one another during life, the length is also highly variable within the species depending on the living conditions, usually lengths of 30 to 50, a maximum of 100 millimeters specified. The color of the animals is also extremely variable, mostly brownish, but there are also red (due to the blood pigment hemoglobin ) and green colored individuals. When viewed from above ( dorsal ), only a shield-shaped head section with carapace is visible, under which a rear body, which is divided into ring-shaped sections, protrudes with an end section ( called a telson ), which ends in two long tail appendages ( furca ). The shield-shaped carapace is attached to the head section at the back (in the neck region), it rests loosely on the trunk and abdomen section without any connection. When viewed from below, the body is clearly divided into segments . The number of body segments is also variable within the species, there are individuals from about 26 to 44 segments (plus the telson). The number of abdominal segments that protrude under the carapace when viewed from the dorsal view is different, usually 11 to 14 segments are covered by the carapace.
Head section and carapace
As in all crustaceans, the head section consists of five visible segments with appendages (extremities). In the Notostraca, the two pairs of antennae of the crustaceans are largely reduced, the first antennae are single-limbed, short pin-shaped appendages, the second antennae either short rudiments or completely receded. Their function is taken over by elongated extensions of the first pairs of trunk legs (thoracopods) called endites. A prominent upper lip (labrum) is formed on the front of the head. The mandibles are also large and distinctive, they serve, as an exception within the leaf crabs, as biting tools. Your palpus has regressed. The two pairs of maxillae are present, but small and inconspicuous. The eyes of the Notostraca are on the dorsal side. There are two, mostly kidney-shaped, relatively large complex eyes that are always close to one another. Between them sits the much smaller, single (unpaired) Nauplius eye . There is also a structure called the dorsal or nuchal organ , a chemical sensory organ, near the eyes . The head with the carapace is elongated oval to almost round when viewed from the dorsal point of view, with a recess (sulcus) at the rear end, which enables increased mobility of the trunk. The shield-shaped carapace, which is flattened in side view, has a longitudinal furrow in the middle on the upper side and various serrated keels, the shape and arrangement of the teeth is important for the determination of the species.
Torso and abdomen
The torso section of the Notostraca consists of eleven segments that all support legs. The last pair of torso legs of the females is transformed to carry the Eisackchen. The trunk legs or thoracopods show the same structure as one another, but are different in proportions. As is typical for leaf crabs, they are flattened and leaf-like. They serve both breathing (respiration) and swimming locomotion and are also involved in food intake. The leaf bones, designed as split legs, are markedly transformed in the group, both leg sections, endopodite and exopodite, are relatively small. However, each pair of leaf bones has five very elongated and multi-parted processes known as endites. The last (fifth) endites of the first pair of trunk legs are particularly elongated, they protrude well below the carapace and serve as sensory organs.
The anterior segments of the abdomen also carry pairs of legs, the basic structure of which does not differ from that of the thoracopods; but since their ends are much shorter, they have a different shape. Many abdominal segments do not carry a single leg, but rather several pairs of legs one behind the other, which is unusual in arthropods; possibly they go back to merged segments. On the dorsal side, the segments are usually markedly thorn, especially on the rear edge. The pairs of legs in the abdomen are becoming smaller and simpler towards the rear, a maximum of 72 pairs of legs were counted. The last abdominal segments are then completely legless. In them, the dorsal ( tergites ) and ventral ( sternites ) sclerorized plates are fused to form undivided rings.
The legs are usually moved one after the other in a wave-like movement from front to back, which allows the animals to swim well, but mostly they move in a swimming-running movement over the bottom of the water. At the beginning of the blow, numerous bristles are spread apart and form a collecting basket in which food particles and small prey are trapped. These are conveyed into a food channel in the middle between the basic limbs of the legs, which are designed as chewing blocks or gnatho bases, and are transported forward in this channel to the mouth.
At the rear end of the abdomen sits the telson with the two greatly elongated tail threads, the furca. In the genus Lepidurus , the telson is elongated at the rear into a long, tongue-shaped supraanal plate, which protrudes to the rear between the base of the furca branches over the tip of the abdomen, making it easy to distinguish from the genus Triops without a supraanal plate.
Reproduction and development
Notostraca are in principle separate sexes. In almost all species, however, populations are known in which either hermaphrodites (with male and female gonads) occur exclusively females that reproduce parthenogenetically ; these occur mainly in the northern parts of the range. Some populations of Triops show a peculiar mode of reproduction in which, in addition to predominantly hermaphroditic individuals, a small percentage of purely male individuals occur; this is known as "Androdiozian". Female individuals carry the fertilized eggs with them for a while in a brood pouch that is attached to the eleventh torso legs specially designed for this purpose. The eggs are mostly scattered individually, but in Lepidurus arcticus they are sometimes attached to mosses as a clutch. The eggs are extremely resistant to adverse environmental conditions and can dry out for several years without damage. If they are re-moistened, they develop very quickly. In Triops cancriformis , the first sexually mature individuals were registered as early as two weeks after hatching. The development takes place gradually over seven stages up to the adult animal.
The larvae hatch from the egg as metanauplius , with three functional pairs of extremities, but others that are already recognizable as an attachment. The Metanauplii do not ingest any food, they still feed on the yolk supply of the egg. Only the third larval stage begins to eat. As is typical for nauplius larvae, the developed extremities (all head appendages) of the planktonic larvae serve both for swimming locomotion and for food acquisition. In the early larval stages, the antennae serve as an essential drive. In the course of the development of the trunk extremities, in the later stages, these gradually become smaller and smaller and finally end as almost functionless rudiments. The larvae feed on unspecialized food particles that are relatively large in relation to their own size, which they grasp with their long bristle extremities.
Biology and way of life
All Notostraca live in small and shallow, puddle to puddle-like standing water bodies. They are absent in larger bodies of water such as lakes. The developing waters are always very shallow, but can range in size from small puddles to extensive shallow lakes. Most species are adapted to ephemeral waters in arid areas, which rarely fill with water during irregular heavy rainfall, but they occur on all continents from the tropics to arctic latitudes. Usually they only occur in bodies of water that regularly (periodically) dry out. The animals survive the dry periods in the form of the very resistant permanent eggs in the bottom mud. If the body of water fills up again as a result of rainfall or flooding, they can effectively use the time until they dry out again due to the extremely rapid development. Rice fields with wet rice crops, for example, offer favorable conditions for many species.
The Arctic species Lepidurus arcticus is an exception in its life cycle . Its developing waters do not dry out, but are frozen through most of the year and only thaw for a short time.
Phylogeny and Systematics
The Notostraca belong within the gill pods or Branchiopoda to the (real) leaf pods or Phyllopoda. There is as yet no certainty about their exact position within the leaf crabs. While the morphology speaks for a basal position, with all other phyllopods together as a sister group, according to genetic data, a sister group relationship to the Laevicaudata is also possible. All morphological and genetic data unanimously confirm the monophyly of the recent Notostraca.
Within the Notostraca, the genera Triops and Lepidurus are each monophyletic and sister groups, they are (formally) united in a family Triopsidae. Within the genres, however, the results showed an unexpectedly complex situation. The Notostraca are therefore, with high constancy of the general building plan over evolutionary long periods of time, morphologically very variable at the species level, with high variability within the species with little differences between different species. On the basis of slight morphological differences, about 70 species were initially described, which, due to the overlap of many characteristics and the lack of reliable differential characteristics, were then combined into 11 broad collective species worldwide during the revision by Alan Longhurst. Within the morpho-species, which can be distinguished by morphology, there are clear indications of genetically but hardly morphologically distinguishable crypto-species that each inhabit different regions. The species delimitation within the group is therefore currently in flux, which makes it difficult to provide reliable species numbers.
- Family Triopsidae
- Genus Triops
- Genus Lepidurus
Literature and Sources
- Alan L. Longhurst (1955): A review of the Notostraca. Bulletin of the British Museum (Natural History) Zoological Series 3 (1): 3-57.
- G. Fryer (1988): Studies on the functional morphology and biology of the Notostraca (Crustacea: Branchiopoda). Philosophical Transactions of the Royal Society London Series B 321: 27-124.
- Juan García-de-Lomas, Jordi Sala, Miguel Alonso (2015): Clase Branchiopoda, Orden Notostraca. Revista IDE @ - SEA, nº 71: 1-10. download
- E. Thenius (2007): "Living fossils" in the organism kingdom. Paleontology and molecular biology as the most important foundations. Denisia 20: 75-96.
- Jørgen Olesen & Ole Sten Møller: Notostraca. Chapter 7 in Joel W. Martin, Jørgen Olesen, Jens T. Høeg (editors): Atlas of Crustacean Larvae. Johns Hopkins University Press 2014, ISBN 978-1-4214-1197-2 .
- Martin Schwentner, Stefan Richter, D. Christopher Rogers, Gonzalo Giribet (2018): Tetraconatan phylogeny with special focus on Malacostraca and Branchiopoda: highlighting the strength of taxon-specific matrices in phylogenomics. Proceedings of the Royal Society B 285 (1885) doi: 10.1098 / rspb.2018.1524
- Jørgen Olesen & Stefan Richter (2013): Onychocaudata (Branchiopoda: Diplostraca), a New High-Level Taxon in Branchiopod Systematics. Journal of Crustacean Biology 33 (1): 62-65. doi: 10.1163 / 1937240X-00002121
- Stefan Richter, Jørgen Olesen, Ward C. Wheeler (2007): Phylogeny of Branchiopoda (Crustacea) based on a combined analysis of morphological data and six molecular loci. Cladistics 23: 301-336. doi: 10.1111 / j.1096-0031.2007.00148.x
- Michael Korn, Nicolas Rabet, Hemant V. Ghate, Federico Marrone, Anna K. Hundsdoerfer (2013): Molecular phylogeny of the Notostraca. Molecular Phylogenetics and Evolution 69: 1159-1171. doi: 10.1016 / j.ympev.2013.08.006
- Thomas C. Mathers, Robert L. Hammond, Ronald A. Jenner, Bernd Hänfling Africa Gómez (2013): Multiple global radiations in tadpole shrimps challenge the concept of 'living fossils'. PeerJ 1: e62; doi: 10.7717 / peerj.62
- Luc Brendonck, D. Christopher Rogers, Jorgen Olesen, Stephen Weeks, Walter R. Hoeh (2008): Global diversity of large branchiopods (Crustacea: Branchiopoda) in freshwater. Hydrobiologia 595: 167-176. doi: 10.1007 / s10750-007-9119-9
- D. Christopher Rogers, James H. Thorp (editors): Thorp and Covich's Freshwater Invertebrates: Volume 4: Keys to Palaearctic Fauna. Academic Press (Elsevier), London etc., 4th edition, 2019. ISBN 978-0-12-385024-9 , Chapter 16.2, pp. 661-662.
- Michael Korn & Anna K. Hundsdoerfer (2016): Molecular phylogeny, morphology and taxonomy of Moroccan Triops granarius (Lucas, 1864) (Crustacea: Notostraca), with the description of two new species. Zootaxa 4178 (3): 328-346. doi: 10.11646 / zootaxa.4178.3.2