Fish species diversity in southern African estuarine systems: an evolutionary perspective

Synopsis The ichthyofauna of southern African estuaries consists primarily of juvenile marine species that use these habitats as nursery areas. The abundance and biomass of fishes in estuarine systems are typically high but species diversity is generally low, with only a few taxa dominating the community. This relatively low species diversity is attributed to the fact that estuaries in the region are unpredictable environments which lack any degree of permanence and are dominated by mobile marine eurytopes. Although stenotopes, represented mainly by small resident species from the families Gobiidae, Clinidae and Syngnathidae, are present in southern African estuaries, little speciation appears to have occurred. A possible reason for this lack of speciation, apart from the seasonal and annual variability of the abiotic environment, is that the lifetime of individual systems is usually limited to a few thousand years. In addition, fishes utilising southern African estuaries need to remain flexible (eurytopic) in their responses to an external environment which is unlikely to become more stable in the future. Thus the lack of permanence and fluctuating nature of southern African estuaries on both a spatial and temporal scale, together with the dominance of eurytopes in these systems, does not favour the evolution of new species. A preliminary examination of the available literature indicates that a detailed review of estuarine ichthyofaunal communities on a global basis will probably mirror the trends outlined above, and reveal a domination of these dynamic ecosystems by eurytopic taxa with low speciation potential.