Nerites (Gastropoda: Neritidae) are prominent members of tropical marine and freshwater gastropod faunas and rich assemblages can be found in many streams of islands in the Indo-Pacific. For example, the streams of Fiji and New Guinea each support at least 23 species of freshwater neritimorphs, with representatives in the genera: Clithon, Neripteron, Neritilia, Neritina, Neritona, Neritodryas, Septaria, and Vittina. The striking diversity of this group in the small coastal streams of Pacific Islands contrast with a paucity of taxa in tropical Australia, despite northern Australia occupying a similar latitude. Just four taxa have been reported from Australia and only two can be considered common. These patterns are in marked contrast to the wide distribution of many marine nerites in the Pacific and conflicts with Island Biogeography Theory. Strikingly, many of these stream taxa have adopted an amphidromous lifestyle; adult gastropods feed and reproduce in freshwater, whereas larvae are swept to the ocean and undergo a marine dispersive phase before settling near the entrance to creeks and re-entering these freshwater systems as crawling juveniles. Rapid transit of larvae to the ocean via short, steep, fast-flowing streams may offer an explanation for this biogeographic conundrum. Larvae that do not reach the ocean within a few days may starve or exhibit poor survival. Hence, the disruption of stream-ocean connectivity may explain the low diversity of these taxa in northern Australia. Sea level rise in northern Australia in the current interglacial has further weakened stream-ocean connectivity with the development of vast flood plains and slow-moving rivers. We contend that: (1) poor stream-ocean connectivity is not conducive to the maintenance of populations of nerites in northern Australia; and (2) new records of freshwater nerites may be revealed by surveys in short, steep coastal streams of northern Australia.
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