Invertebrates

Of the estimated 200,000 animal species in Australia, about 96% are invertebrates. While the full extent of invertebrate diversity is uncertain, 90% of insects and molluscs are considered to be endemic. Invertebrates occupy all ecological niches and are important in all ecosystems as decomposers, pollinators, and food sources. The largest group of invertebrates are the insects, comprising 75% of Australia's known species of animals. The most diverse insect orders are the Coleoptera, with 28,200 species of beetles and weevils out of an estimated 300,000 worldwide. There are 20,816 species of Lepidoptera, including butterflies and moths, out of an estimated 100,000 and 12,781 species of Hymenoptera the order that included the ants, bees and wasps from 100,000 worldwide. Order Diptera, which includes the flies and mosquitoes, comprises 7,786 species out of 150,000, and of the estimated 60,000 species of Hemiptera (including bugs, aphids and hoppers), 5,650 are found in Australia. There are 2,827 species of order Orthoptera, including grasshoppers, crickets and katydids, out of an estimated global total of 20,000. Introduced species that have and continute to cause significant environmental degradation include the European wasp, the red fire ant, the yellow crazy ant and feral honeybees which competes with native bees.

Australia has a wide variety of arachnids, including 135 species of spider that are familiar enough to have common names. There are a number of highly venomous species, including the notorious Sydney funnel-web and Red-back spiders, whose bite can be deadly. There are thousands of species of mites and ticks from order Acarina. Australia also has eight species of pseudoscorpion and nine scorpion species.

There are two families of native terrestrial worms: the Enchytraeidae, and the Megascolecidae that includes the world's largest earthworm, the Giant Gippsland Earthworm. The Giant Earthworm is found only in Gippsland, Victoria and can reach up to 3.7 m in length. There are many more families of aquatic oligochaetes than there are terrestrial families.

Freshwater crustaceans include the large family Parastacidae, which incorporates 124 species of freshwater crayfish. Australian species include the world's smallest crayfish, Swamp Crayfish which does not exceed 30 mm in length, and the world's largest crayfish, the Tasmanian Giant Freshwater Crayfish, measuring up to 76 cm long and weighing 4.5 kg. The genus Cherax includes the Common Yabby—the most widely distributed species—in addition to the farmed species Marron and Queensland Red Claw. Species from the genus Engaeus, commonly known as the land crayfish, are also found in Australia. Engaeus species are not entirely aquatic, because they spend most of their lives living in burrows. Australia has seven species of freshwater crab from the genus Austrothelphusa. The crabs live burrowed into the banks of waterways and can plug their burrows, surviving several years through drought. The freshwater mountain shrimp occur only in Tasmania; they are a unique group because they are extremely primitive, resembling species found in the fossil record from 200 MYA.

Other problematic marine invertebrates include the native species Purple Sea-urchin and the White Urchin which have been able to take over marine habitats and form urchin barrens due to the over harvesting of their natural predators. Introduced pests include the Asian Mussel, New Zealand Green-lipped Mussel, Black-striped Mussel and the Northern Pacific Seastar, all of which displace native shellfish.

There are many unique marine crustaceans in Australian waters among the country's seven represented classes. The best known class, to which all the edible species of crustacean belong, is Malacostraca. The warm waters of northern Australia are home to many species of Decapod crustaceans, including crabs, false crabs, hermit crabs, lobsters, shrimps, and prawns. The Peracarids including the amphipods and isopods are more diverse in the colder waters of southern Australia. Less well known marine groups include the classes Remipedia, Cephalocarida, Branchiopoda, Maxillopoda (which includes the barnacles, copepods and fish lice), and the Ostracoda. Notable species include the Tasmanian Giant Crab, the second largest crab species in the world, found in deep water, and weighing up to 13 kg. Species called lobsters in Australia including the Western Rock Lobster; these are distinct from other lobster species, since they do not have claws.