Isopods - Phreatoicidean

Phreatoicidean isopods are ancient members of Australia's fauna. In fact, fossil evidence shows that these crustaceans have been living in fresh water longer than Australia has been a separate continent.

Phreatoicidean isopods are ancient members of Australia's fauna. In fact, fossil evidence shows that these crustaceans have been living in fresh water longer than Australia has been a separate continent. Phreatoicidean isopods require permanent sources of water, so they are limited to the areas of Australia where water never disappears. Indeed, they are found in places where water has always been present throughout Australia's extremely variable climatic history.

Millions of years ago, when the whole of Australia was as wet as Tasmania is today, phreatoicidean isopods may have been more widespread. Today, Australia is much drier and almost all of these crustaceans live in small, hard-to-find habitats such as sphagnum bogs, caves, springs and groundwater-fed streams and lakes.

Phreatoicidean isopods are an example of endemism over a small area. Some species are found in areas no more than 20 to 50 kilometres across, such as Crenoicus buntiae, found on the Boyd Plateau in New South Wales. Recently, many new species belonging to the genus Eophreatoicus have been found living in small springs within a 20 kilometre circle around Nourlangie Rock in Kakadu National Park (near Jabiluka). Phreatoicidean isopods may be in danger of becoming extinct and we may have already caused the extinction of some species without knowing it. Human changes to the landscape, such as agriculture, dams and bores, deplete their already restricted habitats. For example, free-running, artesian bores have lowered the ground water by more than 100 metres in some parts of Australia, causing springs fed by ground water to dry up.

Furthermore, introductions of non-Australian animals have increased the number of predators that may feed on phreatoicideans. Because these crustaceans cannot swim or fly, they cannot escape if their habitat is infiltrated by new predators. For example, the introduction of trout to the Great Lake in Tasmania has had a serious impact on some species that are now officially listed as endangered. In fact, a large sample of Great Lake phreatoicidean isopods in the Australian Museum's collection came from the stomach of a trout. Some of these Tasmanian species are now officially listed as 'Endangered' by the IUCN (the World Conservation Union).

The advance of the introduced Cane Toad (Bufo marinus) into Kakadu National Park may also endanger species of phreatoicideans that have only recently been discovered.

Words to know:

• ENDEMISM - refers to species which are native to an area or country.
• PHREATOICIDEAN ISOPODS - freshwater crustaceans, survivors of the ancient continent Gondwana.
• GONDWANA - the ancient southern supercontinent that included Australia, New Guinea, South America, Africa, India and Antarctica.