The northern GBR is the region of the Australian continent where the greatest number of molluscan species exist today (about 3000 species). Interestingly however, it is also the region with the lowest endemicity of molluscs, probably reflecting the youthfulness of its geological formation and fact that most species have planktonic larvae and so can mix with populations of the same species elsewhere in the tropical Indo-Pacific Ocean. No molluscan species is endemic to the GBR as such, though two gastropods only occur in the northern section: Peristernia australiensis (Fasciolariidae) and Thecacera boyla (Polyceridae), and Corallastele allenae (Trochidae) seems to be endemic to the Capricorn Group of islands. The diversity of molluscs does attenuate slightly toward the southern extremity of the GBR, but total biodiversity is little changed because of the presence of endemic temperate Australian taxa at the northern limits of their range. Here, the deeper water endemic taxa (Turritellidae, Cypraeidae and Volutidae) have short-lived or direct-developing larvae and there are permanent populations in the cooler inter-reefal areas and channels deeper than 100 metres. By contrast, the eastern Australian shallow-water endemic taxa that do occur on the GBR, like the nudibranch Chromodoris splendida (Chromodorididae), have planktonic larvae and it is not known if their sporadic occurrences on the southern GBR represent breeding populations or adults resulting from chance northward-flowing larvae that have successfully grown to adulthood.

One cannot cover the biogeography of molluscs of the GBR without mentioning the remarkable group of endemic Australian balers popularly known as Heron Island volutes. The genus Cymbiola (Volutidae) contains a complex of about 20 allopatric species, subspecies and forms with very different shaped shells and colour patterns (Fig. 24.2) that live along the eastern Australian coast between 16°S and 32°S and includes by far the majority on the GBR itself (12 taxa alone occur in the section between the Swain Reefs and Fraser Island). Taxa of this assemblage occur from the Ribbon Reefs east of Cape Flattery in the north and extend continuously, in both shallow water on the coral reefs themselves and the deep water channels throughout the GBR, to Lady Elliot Island. South of Lady Elliott Island they are only found in deeper water (i.e. greater than 100 m). Interestingly, the deeper water taxa are ancestral and they have diverged less than the shallow water forms. Like all balers, these taxa of Cymbiola hatch from their egg capsules as crawl-away juveniles, so there is no genetic mixing between them that reinforces their distinctiveness. Although the evolution of these balers must have taken place since the formation of the GBR in the Pleistocene era (2 million years ago) we are a long way from understanding their evolutionary mosaic.

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