Australian Freshwater Crabs
Freshwater crabs belong to the Order Decapoda, the crustacean group that also includes freshwater shrimp (Atyidae), prawns (Palaemonidae) and crayfish (Parastacidae), which share the characteristic presence of five pairs of thoracic legs (pereiopods). In freshwater crabs the first pereiopods are modified as pincers (chelipeds), and the remaining four pairs are relatively unspecialised walking legs. The general body plan of freshwater crabs consists of a head, thorax and abdomen, with the head and thorax (cephalothorax) covered by a broad carapace, and the abdomen reduced, flattened and flexed under the thoracic sternum. In adults, the male abdomen is slim and narrow, and is either triangular or T-shaped, while the female abdomen is broad and round and covers nearly the entire thoracic sternum. Adult males bear two pairs of abdominal appendages (pleopods) that are modified into copulatory structures known as gonopods. Gonopod structure is taxonomically important, especially because the external morphology of freshwater crabs tends to be rather conservative.
Australian species were revised by Riek (1951), and then more comprehensively by Bishop (1963) who recognised six species. Short (1994) described a new species (Austrothelphusa tigrina), increasing the currently recognised number of species to seven. These species range from the Pygmy Crab (Austrothelphusa wasselli), whose carapace is less than 2.5 cm wide to Austrothelphusa angustifrons reputedly with shells 10 cm across. There are several undescribed species that occur in areas from south-western Cape York Peninsula around the Gulf of Carpentaria and across the Northern Territory to the Kimberley, and the real number is probably well in excess of the currently described species. Similarly, in New Guinea, a great number of freshwater species have yet to be identified and have not been adequately evaluated. All species are in need of taxonomic study and research.
Austrothelphusa differs from Holthuisana most significantly by the shape of the male abdomen which in Austrothelphusa species is comparatively broader and less obviously T-shaped (those of several species appearing almost triangular), and in particular the distal segments are broader and less elongated. Other less obvious features are a more convex and smoother carapace that has the epigastric and postorbital cristae comparatively weaker.
Austrothelphusa is confined to Australia and New Guinea. They are squarish crabs with the lateral margin of the carapace approximately straight and the lateral margins without prominent teeth. These freshwater crabs occur across a wide area of northern and inland Australia where they can be found in creeks, swamps, river pools and dams. Given the great extent of morphological similarity between most of the Australian species, and the fact that most species have been found on Cape York Peninsula, a reasonable hypothesis is that the Australian species radiated from New Guinea during a period of glacial land-bridge connection. All of them, however, are believed to have derived from ancestral, Southeast Asian forms.
Freshwater crabs complete their life cycles exclusively in freshwater habitats. A key attribute of all true freshwater crabs is direct development whereby larval stages are lacking and the eggs hatch directly into juvenile crabs. The broad, shallow female sterno-abdominal cavity and the equally broad abdomen together form a brood pouch for the relatively small number of large eggs and the hatchling crabs.
The biogeographic importance of the freshwater crabs arises from their restriction to freshwater habitats, and their relatively poor powers of dispersal. This is because freshwater crabs lack the dispersive planktonic larval stage seen in most marine crabs. This means the geographic range of freshwater crab species is in part limited by their low dispersal capabilities, their low fecundity and their restricted range of habitats. As a consequence, freshwater crabs become isolated relatively easily and they tend to exhibit high rates of endemism. It is common for a relatively small geographic area to support a high species diversity of freshwater crabs.
The distribution of the described freshwater crabs is mostly restricted to relatively small areas except the broadly distributed Austrothelphusa transversa. All described Australian species except one are restricted to northern Queensland. In general terms Austrothelphusa agassizi is only known from Ravenshoe west to the Walsh River, and north to Iron Range in the east, and to the Archer River in the west. Austrothelphusa angustifrons is known only from the Torres Strait south to the Normanby River. Austrothelphusa raceki is known only from the Wenlock River and Archer River on the western coast and the Stewart River in the east.
Austrothelphusa tigrina is only known from One Mile Creek, an upstream tributary of the Alice River (Mitchell River Basin). Austrothelphusa valentula is known only from west and east flowing catchments in the vicinity of Coen. Austrothelphusa wasselli is known only the Norman River catchment, north to the Coen River, in the west, and east to the upper Barron River, and north to the Stewart River in the east.
Austrothelphusa transversa is the most widely distributed species. It has been reported from the Fitzroy River in Western Australia, across northern Australia to west of Chillagoe, Queensland, and south to the Lake Eyre catchment and down the Murray-Darling system to about Nyngan in New South Wales. It has also been reported as occurring east of the Great Dividing Range between Townsville and Gladstone. However, considering that the type locality of A. transversa (Thelphusa transversa) is Cape York Peninsula, any freshwater crabs currently recognised as Austrothelphusa transversa, are in all probability, undescribed species.
Habitat & Ecology
Freshwater crabs can be found in sheltered areas particularly amongst submerged water plants and leaf litter; under stones, rocks or woody debris. They are found in a variety of habitats; floodplains, large rivers and lakes. They can be found in both flowing and standing waters. All species prefer shallow areas where they can easily reach the waters surface. Most require atmospheric air for their spongy gill structures within the branchial (gill) chambers. They possess both normal gill material and other gill material for atmospheric air exchange - the ratio of each probably varying according to species and its habitat. Most species are assumed to be water dependant, but the ecology of Australian species remains unknown. Freshwater crabs in Australia have been barely studied at all.
The most studied species is the inland freshwater/land crab, currently known as "Austrothelphusa transversa". They are widespread in the arid and semi-arid areas of inland Australia and are well adapted to living in ephemeral streams. They are frequent found in freshwater rivers, swamps and billabongs in the Murray-Darling and Lake Eyre catchments. In the dry season if their habitat dries up these crabs construct burrows in clay soils which they sometimes seal with mud. The burrows are usually around 60 cm long and are found in areas that experience yearly flooding. However, they can burrow up to one meter in heavy dense, clay soils in the banks of freshwater rivers and creeks, swamps, farm dams and drainage channels. The humid air trapped inside the burrows gives the crabs enough moisture to survive until wet weather returns. Following heavy rain, the crabs often appear in large numbers in the claypans of river floodplains.
The 'Inland Crab' has adapted to be able to tolerate a loss of almost half of its body water. It can drink water to recover from dehydration, but more importantly for its normal existence, it can absorb moisture which condenses in the burrows as the temperature falls at night. During extensive periods of drought these hardy creatures can plug their burrows with earth and enter a dormant metabolic state, which allows them to live off fat stored in their tissues. Their abundance appears to be positively correlated with the presence of silty substrates, as they are more abundant at instream sites where sediments are finer. This is likely to be related to their increased ability to burrow into these finer sediments.
Around late October to early November, females lay between 100 and 350 large eggs. The eggs are carried under her body and will hatch around December. The eggs do not hatch as larvae but as miniature crabs. Once hatched, the baby crabs also enter an arrested state of development - covered in a thin film of water under the mother's abdomen. In times of drought, she may carry them for several months, until there is enough moisture for them to move on to the next stage of their development.
Keeping & Caring
I maintained a group of Austrothelphusa tigrina for several years, and despite the fact that I provided refuge areas above the water level, I never saw them come out of the water, except for brief periods during waterchanges.
Freshwater crabs are true scavengers and will eat all fish foods, vegetable matter, algae, waterplants, leaves and fruit. Fish and shrimps (alive or dead) are also eagerly devoured, and cannibalism is not uncommon. Cannibalism may occur particularly during moulting when the crabs are soft and vulnerable. Some species are more prone to cannibalism than others. They are particularly fond of mystery snails (Pomacea bridgesii). Therefore, it is wise to keep them in separate aquaria, as they are likely to eat anything they can catch. I have kept them with rainbowfishes however, without any apparent problems.
Freshwater crabs appear to be long-lived (2-5 years) and breeding should present no great difficulties. There is little difference between the sexes except for the width of the abdomen (tail). Males have larger claws and narrow tails, (folded under the body). Females have smaller claws and wider tails. If claws or legs are lost, regrowth will occur with the next moult.
Young crabs hatch from eggs carried for some time under the abdomen of the female. Mating in the few instances witnessed was belly to belly shortly after the female moulted. She then retires to a secluded humid spot out of water, rarely feeds, and apart from an occasional dip into the water does very little until the young disperse.
The male can be left in the aquarium if he does not bother the female otherwise he should be removed. Both parents should be removed after the young are dispersed (the time varying according to temperature) as some will certainly be eaten otherwise. Keep the young under the same conditions as the adults. They grow quite quickly and should be mature in their second year or so.
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Adrian R. Tappin