Freshwater crab

There are about 1,300 different species of freshwater crabs that live in the tropics and subtropics and are divided into eight families. In contrast to marine living crabs, which release thousands of larvae in the form of plankton, the freshwater crabs give birth to a small number of already fully developed offspring that are cared for by their mothers for some time. This fact limits the range of the species, so that their deposits are regularly endemic to small areas. As a direct consequence, a large proportion of freshwater crabs are threatened with extinction.

System

Among the total of 6,700 species of crabs, there are more than 1,300 formally described species of freshwater crabs. In fact, it assumes, however, that the total number of freshwater crab species up to 65 % higher, as many types have not yet been described. The shrimp are divided into eight families, each of which has a limited propagation. Various crabs of other families, which are among the euryhaline, are also able to tolerate a certain amount of fresh water content, while, in turn, have adapted to freshwater as a secondary habitat other crabs. The phylogeny of these families is still the subject of scientific debate, so it is unclear how many crab species in total can live in fresh water.

The eight families are:

Against the backdrop of generally very small number of freshwater organisms resulting from fossils, there are only a few of freshwater crabs. The oldest find is a Tanzanonautes tuerkayi from the East African Oligocene. The development of the freshwater crabs began with high probability after the breakup of the supercontinent Gondwana.

Description and life cycle

The morphology of the freshwater crabs varies only to a very limited extent, so that the shape of the decapods (particularly for insemination, a modified abdominal appendage ) is essential for their taxonomy. Developmental biology of freshwater crabs is characteristic directly: The eggs contain Juvenile and the different stages of the larvae develop inside the egg. A brood includes several hundred, with a diameter of about 1 mm each relatively large eggs.

The colonization of freshwater required that the crabs changed their water balance. Freshwater crabs can absorb salt from their urine and have different evolutionary adaptations to reduce their own water loss. They have in addition to their gills on pseudo - lungs in their gill cavity and can breathe in this way. These developments are a preadaptation of crabs for a life on land, although they must return to the water regularly to excrete ammonia.

Ecology and conservation

Freshwater crabs are found in all tropical and subtropical regions of the world. They live in a wide variety of waters ranging from fast flowing rivers to marshes, but also in trunks of trees and caves. They are mostly nocturnal and omnivorous. However, there are some specialized hunters - for example, the living in Lake Tanganyika Platyhelphusa armata feeds almost exclusively on snails. Some species even turn provide an important food source for other vertebrates dar. Some freshwater crabs are hosts for trematodes of the genus Paragonimus that cause in humans after consumption of raw crab meat the Lungenegelkrankheit.

The majority of the species occurs only in very limited areas endemic. This is due at least in part related to their lack of opportunities and the spread of their low fertility. The fragmentation of their already small habitats by humans also plays a role. In West Africa, freshwater crabs are redistributed in savanna areas than in the rainforest, while in East Africa living in the mountains species live strictly limited and are redistributed in the lowlands.

Any kind of freshwater crabs previously described has been assessed by the International Union for Conservation of Nature ( IUCN). Of the species for which data are available, 32 % were classified as at risk. For example, all endemic to one of the 50 people living in Sri Lanka freshwater crabs and more than half of them are threatened with extinction.