Water crabs
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Water crabs ( Bathynellacea ) |
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| Chappuis , 1915 |
The water crabs , scientific name Bathynellacea, are an order of crustaceans and the smallest Malacostraca . Almost all species are specialized groundwater animals and rarely and exceptionally occur in surface waters. Water crabs are common all over the world. More than 200 species are known and new species are still regularly described.
The German name "Brunnenkrebse" is traced back to the first finding of an animal of the order, the species Bathynella natans , in a fountain in Prague in 1880.
description
Well crabs are small, worm-like elongated, colorless or white colored crabs. Most species reach a body length of around 1 millimeter, the largest a little more than 5 millimeters. Due to the subterranean way of life, their physique is characterized by regressions: For example, the eyes, most swimming legs or pleopods and the tail fan are missing . The body is divided into the head, eight limb-bearing trunk segments and a pleon of six segments , the last of which is fused with the telson to form a unit, so that five free somites are formed. No trunk segment is fused with the head, and a carapace is completely missing.
The head has symmetrically built mandibles , which always lack a movable part (the lacinia mobilis). The first and second maxillae are simple, with a different number of long-thorny endites. The first antennae have three basic members (pedunculus) and only one flagellum, the second is reduced to a small button or a rod-shaped appendix. The eight pairs of limbs of the narrow and elongated, straight trunk section, the thoracopods, are two-branched split legs , with the exception of the eighth (in some genera the seventh pair of legs is regressed). In the males this is transformed into a mating organ, in the females it is more or less reduced and rudimentary. The others, which are designed as walking legs, consist of a four-limbed endopodite and a mostly one-piece exopodite, in addition one or two tiny epipodites may be present. Pleopods occur in some species on the first two segments of the pleon, often one or both pairs are regressed. They are single-branch, at most two-part, small attachments that cannot be used for swimming. On the pleotelson (consisting of the telson and the last pleon segment fused with it) there is always a pair of uropods , they have rod-shaped, single-limbed exo- and endopodites. There is also a furca , which consists of two small, thorny plates (rami). The Bathynellacea are the only higher crustaceans that have a furca in the sexually mature stage.
Reproduction
Well crabs are separate sexes. The females lay eggs, which are relatively large and rich in yolks, in relation to their body size, and egg sacs or similar structures are always missing. The nauplius and the first subsequent larval stages are still passed through in the egg shell. The hatching young animals are, depending on the genus, in different stages of development, with one, two or three developed thoracopods. Up to sexual maturity, 8 to 11 stages are then passed through, each stage separated by a moult. The first stages are considered to be larvae , the larval form is a Parazoea , as the body structure still differs greatly from the adult; The long antennae serve as organs of locomotion, long bristles develop on the furca, which support the animal when it is at rest. The following stages of youth correspond in their physique to the sexually mature animals, they are called bathynellids. Some researchers take the view that the adult stage of the water crayfish actually corresponds more closely to the last larval stage of the other Decapoda, in which case it is a case of neoteny .
Biology and way of life
Almost all well crabs live underground, mostly in the groundwater , some also in the gap system in the sediment below surface water ( hyporheic interstitial ). With the help of the thoracopods, you move in a meandering motion between running and swimming. They can swim awkwardly in open water. Almost all species are freshwater inhabitants , but some euryhaline members of the Parabathynellidae family are also known from brackish water, sometimes even from seawater, one genus lives exclusively in salty groundwater. Some types are specified from thermal springs at 55 ° C, others are considered cold stenothermic. Two species, Bathynella baicalensis and Baicalobathynella magna , are only known from deeper layers of water in Lake Baikal . With a body length of over five millimeters, they are the largest well crabs.
Almost nothing is known about nutrition, based on the structure of the mouthparts and similar species, an unspecialized ingestion of bacteria and detritus is usually assumed. Two genera, Iberobathynella and Brevisomabathynella , predate other small crustaceans .
distribution
Water crabs are common worldwide and occur on all continents (with the exception of Antarctica).
So far, seven species have been identified in Germany, including one that was only newly described in 2012. There is only one species in Great Britain and Ireland ( Antrobathynella stammeri ).
Systematics
The order Bathynellacea includes two families:
- Family Bathynellidae Grobben, 1904 with 27 genera
- Family Parabathynellidae Noodt, 1965 with 39 genera
The number of species is difficult to give as new species are regularly described. In addition, in an Australian and a Spanish investigation, numerous genetically strongly separated but morphologically indistinguishable cryptospecies were found, the presence of which is suspected from other regions. For 2008/2009 around 220 to 240 species are given worldwide.
Sister group of Bathynellacea is the order Anaspidacea , which is only widespread in the southern hemisphere , with which (and some only fossil-known representatives) it forms the superorder Syncarida . The Syncarida are considered a relic group, the remainder of a kinship widespread in the Paleozoic. However, fossil species that could have been assigned to the well crabs themselves have never been found.
There are two theories about the evolution of the group. Some scientists assume that they colonized the groundwater directly from marine habitats. Others are of the opinion that today's forms stem from species that originally lived in limnic surface waters, so that the transition to freshwater would be primary here.
Individual evidence
- ^ Andreas Fuchs, Hans Jürgen Hahn, Klaus-Peter Barufke: Groundwater monitoring program. Survey and description of the groundwater fauna in Baden-Württemberg. published by the LUBW State Institute for the Environment, Measurements and Nature Conservation Baden-Württemberg. Groundwater protection series, Volume 32. Karlsruhe, 2006.
- ^ A b Hans-Eckhard Gruner: Class Crustacea. In: HE Gruner, M. Moritz, W. Dunger (editor): Textbook of special zoology (founded by Alfred Kaestner). Volume I: Invertebrates, Part 4 Arthropoda (without Insecta). 4th edition 1993. Gustav Fischer Verlag, Jena. ISBN 3-334-60404-7 . on page 750 ff.
- ↑ a b Nicole Coineau & Ana I. Camacho: Super Order Syncarida Packard, 1885. In: Frederick Schram, Mireille Charmantier-Daures (editors): Treatise on Zoology - Anatomy, Taxonomy, Biology. The Crustacea, Volume 4, Part A. Brill Scientific Publishers, Leiden 2013. ISBN 978-90-04-17809-0
- ↑ Andreas Fuchs, Hans Juergen Hahn, Joo-Lae Cho (2012): Parabathynella badenwuerttembergensis n. Sp., The First Record of Parabathynellidae Noodt, 1965 (Malacostraca: Bathynellacea) from Germany. Journal of Crustacean Biology 32 (4): 655-663. doi: 10.1163 / 193724012X630651
- ↑ Maria G. Asmyhr, Grant trousers, Peter Graham, Adam J. Stow (2013): Fine-scale genetics of subterranean syncarids. Freshwater Biology 59 (1): 1-11. doi: 10.1111 / fwb.12239
- ^ Ana I. Camacho, Beatriz A. Dorda, Isabel Rey (2012): Undisclosed Taxonomic Diversity of Bathynellacea (Malacostraca: Syncarida) in the Iberian Peninsula Revealed by Molecular Data. Journal of Crustacean Biology 32 (5): 816-826. doi: 10.1163 / 193724012X638473
- ↑ AI Camacho & AG Valdecasas (2008): Global diversity of syncarids (Syncarida; Crustacea) in fresh water. Hydrobiologia 595: 257-266. doi: 10.1007 / s10750-007-9021-5
- ^ ST Ahyong, JK Lowry, M. Alonso, RN Bamber, GA Boxshall, P. Castro, S. Gerken, GS Karaman, JW Goy, DS Jones, K. Meland, DC Rogers, J. Svavarsson (2011): Subphylum Crustacea Brünnich, 1772 In: Zhang, Z.-Q. (Editor) Animal biodiversity: An outline of higher-level classification and survey of taxonomic richness. Zootaxa 3148: 165-191.
- ↑ Horst Kurt Schminke (2014): Freshwater origin of Bathynellacea (Malacostraca). Crustaceana 87 (10): 1225-1242. doi: 10.1163 / 15685403-00003322