Are there species flocks in freshwater diatoms? A review of past reports and a look to the future

We review the literature on species flocks in diatoms. Past descriptions of species flocks in diatoms are of few species and do not demonstrate monophyly of the taxa explicitly. The genus Tetralunata, with 19 taxa described from Lake Toba, Indonesia, may be the best example to date of species flocks in freshwater diatoms. Other examples for this group are evaluated, from ancient lakes, river drainages and islands, to more geologically short-lived areas and habitats. Other examples may reside in the Rift Valley lakes of East Africa, Amazonia, lakes Baikal, and Ohrid, and some islands such as New Caledonia, Sulawesi, and Madagascar. It would appear that the phenomenon is not expressed in Hawaii, which has endemic, but very few, species. It is interesting to note that nearly all of the examples of species flocks of extant diatoms are of those groups that possess a raphe system. The role of molecular studies to study species flocks is discussed. Other features not traditionally examined to recognize species-level distinctions (physiology, ecology) which may be useful in the future to help identify species flocks in freshwater diatoms.