Key to Australian Freshwater and Terrestrial Invertebrates

An Introduction to Terrestrial and Freshwater Invertebrates
Debatably, the most well-known animals are vertebrates (of the subphylum Vertebrata); mammals, reptiles, fish and birds. However, the vast majority of the animal kingdom actually consists of invertebrates � animals without a backbone. The word �invertebrate� does not refer to a true taxonomical subdivision, but rather contains all the animals apart from those in the subphylum Vertebrata. In fact, it is estimated that 90% of animals are invertebrates, but less than 15% of them are formally described!

Invertebrates are incredibly diverse and can be found in every environment imaginable. From the freezing ice of Antarctica to humid rainforests, from high up in the atmosphere to deep beneath the ground, a species of invertebrate lives just about everywhere. Including animals such as crabs, jellyfish, sponges, millipedes, spiders, ants, snails, and worms, invertebrates are all around us � you�ve almost certainly seen one today!

Arthropoda - Crustacea -
Malacostraca - Decapoda... a hermit crab!
Image credit: Vanessa Pike-Russell
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Terrestrial invertebrates live on land. They include the insects, the arachnids, the myriapods, and some of the annelids, molluscs and crustaceans. They may live in burrows under the ground like earthworms, build webs like spiders, live in nests like wasps, hide under rocks or in leaf litter, fly through the air or burrow under the bark of a tree. Freshwater invertebrates are those that live in rivers, lakes, creeks and springs, and include crustaceans, molluscs, jellyfish, annelids and sponges. Many invertebrates such as crustaceans, jellyfish and annelids are also found in oceans, but marine species are not included in this key.

Biodiversity of Australian Invertebrates
Australia�s invertebrate fauna is highly diverse due to the wide variety of environments found on the continent. The number of freshwater and terrestrial invertebrate species in Australia is difficult to estimate, but it is thought that up to 300,000 species of invertebrate are found on land, and many thousands of additional species live in freshwater habitats. A large proportion of Australia�s invertebrate fauna is endemic � this means that they are not found anywhere else in the world. Australia has so many endemic species because of the long geological isolation of the continent from other land masses and the presence of many endemic flora species in a wide range of habitats.

Latrodectus hasselti
Redback spider guarding her eggsac
Image credit: Natalie Tapson
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Much of Australia�s invertebrate fauna is understudied, and species new to science are constantly being discovered and described by researchers. Documenting this biodiversity is extremely important for developing our knowledge of Australia�s ecosystems and understanding the conservation requirements of different environments. When species are described they are given a unique scientific name and placed within a hierarchical classification system. For example, the scientific name of the redback spider is Latrodectus hasselti; Latrodectus is the name of genus, which is shared by a group of related spiders, whilst hasselti is the species name � the combination of the two is unique to the redback spider. All animals are found within the kingdom Animalia, and Australia�s terrestrial and freshwater invertebrates can be found across 15 phyla: Acanthocephala, Annelida, Arthropoda, Bryozoa, Cnidaria, Gastrotricha, Mollusca, Nematoda, Nematomorpha, Nemertea, Onychophora, Platyhelminthes, Porifera, Rotifera and Tardigrada.

The Role of Invertebrates in the Environment
Although they often go unnoticed because of their small size, invertebrates play a variety of important roles in our environment.

Many vertebrates depend on invertebrates as a primary food source. Species of bird, fish, reptiles and mammals all consume invertebrates, and if you�ve eaten lobster, shellfish or squid before, you�ve eaten invertebrates too! Food webs are always a delicate balance, and a decline in an invertebrate prey species can have disastrous effects on the vertebrate population relying on that species for survival.

Invertebrates are extremely important for maintaining the health of soils. Termites, ants and earthworms are among the invertebrates that recycle nutrients by decomposing organic matter in the soil, allowing the nutrients to become available to plants again.

Insecta - Hymenoptera - native bee, Western Australia
Image credit: Wi Bing Tan
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Many plant species are pollinated by invertebrates, most often by insect species such as bees, butterflies and beetles. These insects are necessary for the reproduction of their associated plant species, and the decline in some endangered plants has been linked with the decline of their insect pollinator.

Some freshwater invertebrate species are used as environmental indicator species. This means that the population of the invertebrate species can be correlated with the overall health of the water system because of the sensitivity of the species to pollution or disturbance.

Invertebrates can be herbivores (plant eaters), detritivores (feeding off of decaying organic matter) or predatory (they eat other invertebrates). Only rarely will an invertebrate prey on a vertebrate. There are also several groups of invertebrates which are parasitic. This means they depend on another organism for survival and may live attached to the outside or the inside of their host.

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