Australia’s freshwater fish fauna, with just over 280 native species, is the smallest for any continent of a similar size. The USA, for example, has almost 1000 freshwater fishes (including non-native species).
The small number of species found in our waters is partly because Australia is the driest continent on earth. Rainfall is sporadic over much of the continent, and fishes cannot live in many of the desert regions of South Australia and Western Australia. Most freshwater species are found in tropical or subtropical regions.
Both primary freshwater species and secondary freshwater species are found in Australia. Primary freshwater fishes evolved entirely in freshwater, whereas secondary freshwater fishes are derived from marine ancestors.
In contrast to other parts of the world, Australia only has three primary freshwater fishes and these are thought to have evolved before the breakup of the supercontinent Gondwana, between 125 and 150 million years ago. These are the Australian Lungfish (Neoceratodus forsteri) and the two species of Saratoga (Scleropages jardinii and S. leichhardti).
Other freshwater fish groups with Gondwanan origins are the families Retropinnidae, Galaxiidae, Aplochitonidae and Percichthyidae. Although the unique Salamanderfish (Lepidogalaxias salamandroides) belongs to an ancient lineage, its origins and relationships continue to be debated.
Many of Australia’s secondary freshwater fishes, especially those found in warmer areas, evolved from marine ancestors that colonised Australia well after the breakup of Gondwana. Most are related to groups found in the tropical Indo-Pacific, and these include the catfishes (Ariidae and Plotosidae), the hardyheads, rainbowfishes and blue-eyes (Atherinidae, Melanotaeniidae and Pseudomugilidae), the grunters (Terapontidae) and the gobies and gudgeons (Gobiidae and Eleotridae).
In Australia (and New Guinea), catfishes, terapontid grunters and eleotrid gudgeons have undergone considerable speciation in freshwater, while they mostly inhabit marine or estuarine environments elsewhere.
While the primary and secondary freshwater species are restricted to freshwater for all or part of their lives, other estuarine or marine fishes may enter the lower reaches of freshwater streams and rivers, and some venture well upstream.
Some freshwater fishes complete their entire life cycle in fresh water, whereas others are diadromous, spending a part of their lives in estuarine or marine environments. A few migrate from the sea as adults to breed in freshwaters, while others migrate from freshwater to breed in estuaries or further seawards. The larvae of other species are carried well out to sea where they develop before migrating back to freshwater as immature juveniles (whitebait).
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Author: Dianne J. Bray