Tasmania is the world's hotspot of freshwater crayfish diversity. Many of our species are found nowhere else on Earth, and some have very restricted distributions within Tasmania. Consequently, and due to other factors such as loss of habitat and environmental degradation of water catchments, some are listed as threatened species.
The freshwater burrowing crayfish is one of four groups of freshwater crayfish native to Tasmania. They are small with two prominent claws and a general body length under ten centimetres. They vary in colour from orange to reddish brown, grey- blue and purple. During the breeding season (late spring to summer) females carry large orange eggs and recently hatched young under their tail.
The other groups of freshwater crayfish in Tasmania are Astacopsis species (also known as the giant freshwater crayfish or lobster) which is entirely endemic to Tasmania; the genera Ombrastacoides and Spinasticoides which comprise 15 species, all of which are endemic to Tasmania; and Geocharax species which are also found on the mainland.
Freshwater crayfish in Tasmania can be distinguished by the orientation and shape of their claw, the size of the grooves along their body and the location and number of spines on their body. The Engaeus species hold their claws more or less vertically (an adaptation to their burrowing habitat), whereas the three other groups of crayfish hold their claws more or less horizontally, as they do not burrow.
Occasionally they can lose a claw and can be seen with one claw notably smaller than the other. This is quite natural and they are able to regrow their missing appendage.
Fifteen species of Engaeus occur in Tasmania. Thirteen are endemic to Tasmania, meaning they do not occur anywhere else in the world. Five species have been listed as threatened at both a State and Commonwealth level and are the focus of a recovery plan.
The small size of the burrowing crayfish and their unique burrowing habitat distinguish them from freshwater lobsters, which are significantly larger and live in river and creek beds, not burrows.
Are yabbies and burrowing freshwater crayfish the same?
The mainland yabbies of the Cherax group are not native to Tasmania. However, yabbies of the species Cherax destructor have been introduced to Tasmania in the past and have infested some farm dams in the State. Yabbies can occur in a wide variety of natural and artificial habitats including creeks, rivers, lakes, swamps, farm dams and irrigation channels. Cherax destructor has the potential to carry disease and parasites harmful to our native crayfish and can degrade the natural habitats of native species by destroying aquatic vegetation and increasing water turbidity. Their burrowing activity can also damage farm dam walls and irrigation channels. The introduced yabby has been declared a ‘controlled fish’ Inland Fisheries Act (1995).
What do burrowing freshwater crayfish live in?
The name burrowing crayfish comes from the fact that they dig and live in muddy burrows. The burrows can be simple and shallow or complex, deep and extensive, and can often be the product of several generations of crayfish families, digging activity.
Burrows may be directly connected to streams or lakes, may connect to the water table, or may simply rely on run-off to stay wet. From species to species the reliance on water availability and other environmental conditions does vary. Those burrowing crayfish relying solely on run-off are found only in Australia and are specific to Engaeus species. Due to their reliance on infrequent water, they are the most terrestrial of the world’s freshwater crayfish. As a consequence, the dispersal of the Engaeus species through waterways is limited to when the water is flowing and has led to restricted ranges and large physical differences between local species.
The crayfish build their burrows by rolling little balls of soil to the burrow entrance, building distinctive chimneys that, in sheltered areas, may reach up to 40 cm above ground level.
Where are burrowing crayfish found?
Tasmanian Engaeus species are mostly found in the north and west of the state, where they have distinct groups and sub-groups. The genus displays remarkable diversity given the relatively small geographic area over which they occur. The state is characterised by a jig-saw pattern of distinct and interlocking ranges for individual species of burrowing crayfish. Overlap between species does occasionally occur, but is not common.
Many species live in buttongrass moorlands, where they play a major role in soil turnover, drainage and aeration.
Freshwater burrowing crayfish are believed to eat rotting wood, detritus, root material and occasionally animal material.
Threats to freshwater burrowing crayfish
Four species of burrowing crayfish are of high conservation concern due to their severely restricted habitat ranges and the presence of actively threatening processes within these areas.
Threatening processes include those that affect water quality and quantity, soil and food (wood/plant) availability.
All crayfish are very susceptible to any alteration of their environment during mating, moulting, nurturing their young and at times when they venture to the surface.
Changes to crayfish habitat can be brought about through the following actions:
- agricultural processes including stock grazing (which churns and compacts soil), dam construction, clearance of riparian vegetation and ploughing;
- forestry activities (e.g. clearing, burning, conversion to plantation) which impose significant physical disturbance on stream headwaters and seepage channels (to which crayfish can show varying degrees of tolerance);
- general road and drainage activities (urban and non- urban) impacting on seepage/wetland/stream bank habitat quality, and any activities (e.g. alluvial mining) that degrade river bank integrity and enhance erosion.
How can we help them?
Everyone can assist in protecting these unique and vulnerable creatures. Most importantly people working on the land can play a vital role in assisting the survival of the crayfish populations. The improvement of water quality, through the reduction of pollutants, pesticides and removal of pest species will assist in rejuvenating the burrowing crayfish populations. Maintaining and restoring riparian vegetation around streams and rivers and more simply, excluding stock access to stream sides, for at least part of the stream length, will create natural habitat for these creatures and allow the crayfish to regain a stronghold throughout Tasmania.
The National Parks & Wildlife Act 1973, protects all invertebrates found within national parks, state reserves, nature reserves and historic areas.
Threatened freshwater burrowing crayfish
Not all burrowing crayfish species in Tasmania are threatened. However, some of those in the genus Engaeus are threatened due to habitat disturbance, and can only be found in very small and specific sites in northern Tasmania.
Some of the threatened species are outlined below.
Mt Arthur burrowing crayfish Engaeus orramakunna
Distributed around Mt Arthur, Lilydale, Nabowla and possibly to Launceston. Vulnerable status.
Scottsdale burrowing crayfish Engaeus spinicaudatus
Found north of Scottsdale, in an area containing only some four square kilometres of suitable habitat. Endangered status.
Burnie burrowing crayfish Engaeus yabbimunna
hey can be found extending from Burnie to the west covering about 130 km2. The species has only been known since 1992, when discovered in the Burnie Park, and has since been sited in Seabrook Creek and other catchments to the west of Burnie. Vulnerable status.
Furneaux burrowing crayfish Engaeus martigener
Previously known only from isolated locations at higher altitude on Mt Strzlecki and the Darling Ranges on Flinders Island and Mt Munro on Cape Barren Island. Vulnerable status.
Central North Burrowing Crayfish Engaeus granulatus