Comparisons between estuary-associated ichthyofaunas in ecoregions around the Indian Ocean

Abstract

This review examines possible fish colonisation processes that started in the Early Jurassic and gained momentum during the Cretaceous when plate tectonics and continental drift created the Indian Ocean between Africa, Madagascar, Asia and Australia. Initial colonisation of the newly created estuaries by fish is likely to have occurred during the Devonian in eastern Gondwana when these land masses were confined to the temperate waters of the southern hemisphere. A major marine extinction event 372 Ma would probably have eliminated many of these taxa from those estuaries. Tropical marine fish families from the Tethys Sea region would then have used the epicontinental seaways between Africa, Madagascar, India and Australia to colonise the estuaries on these drifting land masses. Speciation by some of these tropical taxa would then have occurred such that the southern temperate waters became occupied by a less species rich cool-water ichthyofauna. A Bray-Curtis similarity matrix was created using the presence/absence of fish families and species from selected ecoregions around the Indian Ocean rim. These analyses showed that fish families present in estuaries on the eastern and western side of the Indian Ocean were over 74 % similar but that the species present were only 40 % similar. In terms of past and present geodispersal of fish taxa, the northern route through tropical coastal waters was always viable but long distance movements through the large central or southern parts of the Indian Ocean basin was not. In addition, dispersal of fish species across arid coastal ecoregions where estuaries are scarce appears to have limited the continuity of ichthyofaunal colonisation processes around the ocean rim.