Coregonus is a diverse genus of fish in the salmonfamily. The Coregonus species are known as whitefishes. The genus contains at least 68 described extant taxa, but the true number of species is a matter of debate. The type species of the genus is Coregonus lavaretus. Most Coregonus species inhabit lakes and rivers, and several species, including the Arctic cisco, the Bering cisco, and the least cisco are anadromous, moving between salt water and fresh water. The genus was previously subdivided into two subgenera Coregonus and Leucichthys, Coregonus comprising taxa with sub-terminal mouth and usually a benthic feeding habit, Leucichthys those with terminal or supra-terminal mouth and usually a pelagic plankton-feeding habit. This classification is not natural however: based on molecular data, ciscoes comprise two distinct lineages within the genus. Moreover, the genus Stenodus is not phylogenetically distinct from Coregonus. Many whitefish species or ecotypes, especially from the Great Lakes and the Alpine lakes of Europe, have goneextinct over the past century or are endangered. Among 12 freshwater fish considered extinct in Europe, 6 are Coregonus. All Coregonus species are protected under appendix III of the Bern Convention.
Species diversity
There is much uncertainty and confusion in the classification of the many of species of this genus. Particularly, one extreme view of diversity recognises just two main species in Northern and Central Europe, the common whitefishC. lavaretus and the vendaceC. albula, whereas others would divide these into numerous, often narrowly distributed species. A drastic increase in number of recognized species occurred in 2007, when a review advocated that more than 50 local European populations should be considered as distinct based on morphological differences. It has been estimated that several of them are very young, having separated from each other less than 15,000 years ago. Many of these were primarily defined based on number of gill rakers. Although this largely is hereditary, the number is highly variable, can change relatively fast in response to changes and genetic studies have shown that they often are of limited use in predicting relationships among populations. Genetic differences between several of the recently proposed species, even ones that are relatively distinct morphologically, are very limited and sometimes they are not monophyletic. Various Coregonus, whether regarded as separate species or not, readily interbreed with each other. A review of whitefish in the United Kingdom found that the identification key provided in 2007 did not match most individuals and that solid evidence for more than one species in that region is lacking. Many European lakes have more than one Coregonusmorph differing in ecology and morphology. Such morphs are sometimes partially reproductively isolated from each other, leading to suggestions of recognizing them as separate but clinal species. The morphs or clinal species may rapidly disappear by merging into a single in response to changes in the habitat. A similar pattern can be seen in North America where the ciscoes of the Coregonus artedicomplex in the Great Lakes and elsewhere comprise several, often co-occurring morphs or ecotypes, whose taxonomic status remains controversial. In 2017, FishBase listed 78 species, including the more than 50 proposed for Europe in 2007. Some of these are recently extinct and C. reighardi is likely extinct.
