Freshwater crab
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Freshwater crabs live in the tropics and subtropics and are divided into eight families . About 1300 species are known. In contrast to marine crabs, which release thousands of larvae in the form of plankton , freshwater crabs give birth to a small number of fully developed offspring that are cared for by the mother animals for a while. This fact limits the distribution of the species, so that their occurrence in small areas is endemic . If the environmental conditions in their habitat change, they have few options to move to other areas. The result is that a large proportion of the freshwater crabs must be listed on the Red List of Endangered Species as critically endangered or critically endangered.
Systematics
Among the total of 6,700 species of crab, there are more than 1,300 formally described species of freshwater crab. In fact, the total number of freshwater crab species is believed to be up to 65% higher, as many species have not yet been described. The crabs are divided into eight families, each of which has a limited range. Various crabs from other families belonging to the euryhalins are also able to tolerate a certain amount of freshwater, while other crabs have adapted to freshwater as a secondary habitat. The phylogenesis of these families is still the subject of scientific discussion, so it is unclear how many crab species can live in freshwater.
The eight families are:
| Superfamily | family | Occurrence | Remarks |
|---|---|---|---|
| Trichodactyloidea | Trichodactylidae | Central America and South America | |
| Potamoidea | Potamidae | Mediterranean and Asia | |
| Potamonautidae | Africa and Madagascar | ||
| Ceilingiidae | East Africa and Seychelles | Also as part of the Potamonautidae viewed | |
| Platythelphusidae | East Africa | Also as part of the Potamonautidae viewed | |
| Gecarcinucoidea | Gecarcinucidae | Asia | |
| Parathelphusidae | Asia and Australasia | ||
| Pseudothelphusoidea | Pseudothelphusidae | Central America and South America |
Against the background of a basically very small number of fossils formed from freshwater organisms, there are only a few of freshwater crabs. The oldest find is Tanzanonautes tuerkayi from the East African Oligocene . The development of freshwater crabs began with a high probability after the breakup of the supercontinent Gondwana .
Description and life cycle
The morphology of freshwater crabs varies very little, so the shape of the decapod crabs (especially a pelvic process modified for insemination ) is essential to their taxonomy . The developmental biology of freshwater crabs is characteristically direct: the eggs contain juveniles and the different stages of the larvae develop inside the egg . A brood comprises several hundred eggs, each with a diameter of about 1 mm.
The colonization of freshwater resources presupposed that the crabs changed their water balance. Freshwater crabs can reabsorb salt from their urine and have undergone various evolutionary adaptations to reduce their own water loss. In addition to their gills, they have pseudo-lungs in their gill cavity and can breathe in this way. These developments are a pre-adaptation of the crabs for a life on land, although they have to return regularly to the water in order to excrete ammonia .
Ecology and conservation
Freshwater crabs are found in all tropical and subtropical regions of the world. They live in a wide range of bodies of water , from fast flowing rivers to swamps , but also in tree trunks and caves . They are predominantly nocturnal and omnivorous . There are some specialized hunters, however - for example, the in-fed Lake Tanganyika living Platyhelphusa armata almost exclusively by snails . Some species are themselves an important source of food for other vertebrates . Some freshwater crabs are hosts for flukes of the genus Paragonimus , which cause lung fluke disease in humans when they consume raw crab meat .
The majority of the species are only endemic in very limited areas . This is at least partly related to their poor ability to spread and their low fertility . The fragmentation of their already small habitats by humans also plays a role. In West Africa , freshwater crabs are more widely distributed in savannah areas than in the rainforest , while in East Africa the species that live in the mountains live narrowly and are more widely distributed in the lowlands.
Every species of freshwater crab described so far has been assessed by the International Union for Conservation of Nature (IUCN). Of the species for which data are available, 32% were classified as endangered . For example, all but one of the 50 freshwater crabs living in Sri Lanka are endemic and more than half of them are critically endangered.
literature
- Neil Cumberlidge, Sadie K. Reed: Freshwater Crab Biology . In: Neil Cumberlidge (ed.): The freshwater crabs of West Africa: family Potamonautidae . Editions de l'IRD, Paris 1999, ISBN 2-7099-1433-6 ( faculty.nmu.edu ( memento of March 3, 2016 in the Internet Archive ) - in April 2009 modified content according to this book).
Individual evidence
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↑ a b c d e Darren CJ Yeo, Peter KL Ng, Neil Cumberlidge, Célio Magalhães, Savel R. Daniels, Martha R. Campos: Global diversity of crabs (Crustacea: Decapoda: Brachyura) in freshwater . In: Hydrobiologia . tape 595 , no. 1 , 2007, ISSN 0018-8158 , p. 275-286 , doi : 10.1007 / s10750-007-9023-3 . Estelle Virginia Balian, et al .: Freshwater animal diversity assessment (= Developments in Hydrobiology . Volume
198 ). Springer, Dordrecht 2008, ISBN 978-1-4020-8258-0 , Global diversity of crabs (Crustacea: Decapoda: Brachyura) in freshwater, p. 275-286 (reprint). - ↑ Sebastian Klaus, Darren CJ Yeo, Shane T. Ahyong: Freshwater crab origins - Laying Gondwana to rest . In: Zoologischer Anzeiger - A Journal of Comparative Zoology . tape 250 , no. 4 , 2011, ISSN 0044-5231 , p. 449–456 , doi : 10.1016 / j.jcz.2011.07.001 .
- ↑ a b c d e Michael Dobson: Freshwater crabs in Africa . (PDF) In: Freshwater Forum . 21, 2004, pp. 3-26.
- ↑ a b Ben Collen et al .: Broadening the coverage of biodiversity assessments . In: Jean-Christophe Vié, Craig Hilton-Taylor, Simon N. Stuart (Eds.): Wildlife in a Changing World: An Analysis of the 2008 IUCN Red List of Threatened Species . International Union for Conservation of Nature IUCN, 2009, ISBN 978-2-8317-1063-1 , p. 66-76 , here p. 72 ( books.google.co.uk ).
- ^ Holly T. Dublin: Broadening the coverage of biodiversity assessments . In: Jean-Christophe Vié, Craig Hilton-Taylor, Simon N. Stuart (Eds.): Wildlife in a Changing World: An Analysis of the 2008 IUCN Red List of Threatened Species . International Union for Conservation of Nature IUCN, 2009, ISBN 978-2-8317-1063-1 , p. vii – viii , Foreword ( portals.iucn.org [PDF]).