Marine shore fishes live in the waters close to the shores of Australia. These shores can be divided into the northern tropical shores of warm water and the southern temperate shores of cool water. Our temperate shore waters are separated from other areas of temperate shore waters (in New Zealand and South Africa) by large areas of open ocean. The open ocean forms a barrier, because most shore species cannot swim across them, preventing populations of temperate fishes from mingling. Therefore, these populations remain apart, evolving independently of each other, and over time can become separate, but still related, species. There is a high level of endemism among Australian temperate marine shore fishes because of the above evolutionary scenario.
In contrast, there is a low level of endemism among the marine life of Australia's tropical shores. These shores are close to other tropical shores, such as those of Papua New Guinea, allowing species to move freely between these countries and intermingle. So the vast majority of Australian tropical shore fishes are also found in at least one other tropical area. Of the eight fish families endemic to Australia, three are from freshwater and five are from temperate shore waters. None is from the tropics.
The high level of endemism among Australian marine shore fishes can be traced back to the Ice Ages. During these times, the waters became colder and the fishes had to move north up the east and west coasts, following their preferred temperatures, in order to survive. In the process, the species split into two populations, one east and one west. As the ice caps waxed and waned, water temperatures changed and fish populations moved up and down the coasts accordingly. With the end of the Ice Ages, populations returned south with warmer temperatures. Some eastern and western populations came together and interbred, some came together and couldn't interbreed because they had evolved too many differences and some did not come together, remaining in their separate eastern and western waters.
There are many similar, closely related species or populations of fishes centred on the south-west and south-east coasts of Australia. However, the differences between these fishes vary according to the length of their separation and how quickly they evolved.
Eastern Blue GroperThere are only two species in the wrasse genus Achoerodus. One species, Achoerodus viridis, the Eastern Blue Groper, is found in eastern Australia and the other, Achoerodus gouldii, the Western Blue Groper, is found in Western Australia.
The Upsidedown Pipefish Heraldia nocturna has two distinct populations which differ in colour, number of body rings and size of tail fin. One population is found along the mid New South Wales coast from Seal Rocks to Jervis Bay and the other is found from Victoria to south-west Western Australia.
John Paxton
Ichthyology
Australian Museum
Copyright © Australian Museum, 2003
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