Temporal range: Pleistocene to Present 
|Arctic grayling (T. arcticus)|
Thymallus is a genus of freshwater fish in the salmon family Salmonidae; it is the only genus of subfamily Thymallinae. The type species is Thymallus thymallus, the grayling. The species in the genus are generically called graylings, but without qualification this also refers specifically to T. thymallus.
The fishes of this genus are native to the northern parts of the Palearctic and Nearctic ecozones, ranging from the United Kingdom and northern Europe across Eurasia to Siberia, as well as northern North America. T. thymallus, the grayling, is widespread in Europe, and T. arcticus, the Arctic grayling, is widespread throughout Eurasia east of the Ural Mountains and in the Nearctic. The other species have more localized ranges.
The Thymallus species are distinguished from other members of the salmon family by their larger scales, their small mouths with teeth on the maxillary bone, and most striking of all, their showy, sail-like dorsal fins. This fin is longer in males and highly colourful, with spots of red, orange, purple or green. The body is also colourful; the dorsal surface is a dark purplish to bluish black or gray, grading to dark blue or silver gray on the flanks and gray or white on the belly. The body is further decorated with a smattering of small dark spots; these are much more numerous in juveniles.
The largest of the graylings in terms of length is the Arctic grayling T. arcticus at a maximum length of 76 cm (30 in) and a maximum weight of 3.8 kg (8.4 lb). The grayling, T. thymallus, while somewhat shorter - 60 cm (24 in) - may weigh significantly more, 6.7 kg (15 lb).
The fishes of this genus may live for 18 years or more.
As they are highly sensitive to changes in water quality, Thymallus fishes may be considered indicator species; T. arcticus arcticus has largely disappeared from the Great Lakes Basin. These fishes require cool, well-oxygenated water, preferably with a swift current; they are found in large sandy- or gravelly-bottomed rivers and lakes, but T. thymallus may occasionally be found in brackish conditions. Generally omnivorous, they feed primarily on crustaceans, insects, and zooplankton.
The grayling species, typically for salmonids, spawn in rivers and do not guard their brood, although they do conceal their eggs in silt. The spawning behavior of the Arctic grayling may be typical for the Thymallus genus.
Due to their agreeable taste and attractive form, the grayling species are valued as food and game fishes, and they are occasionally seen in public aquaria. The most economically important of these fishes, for which fisheries and aquaculture operations exist, are the grayling (T. thymallus) and the Arctic grayling (T. arcticus).
- Thymallus arcticus (Pallas, 1776) - Arctic grayling
- Thymallus baicalensis Dybowski, 1874 - Baikal black grayling
- Thymallus brevipinnis Svetovidov (ru), 1931
- Thymallus brevirostris Kessler, 1879 - Mongolian grayling
- Thymallus burejensis Antonov, 2004
- Thymallus grubii Dybowski, 1869
- Thymallus mertensii Valenciennes, 1848
- Thymallus nigrescens Dorogostaisky, 1923 - Kosogol grayling
- Thymallus pallasii Valenciennes, 1848 - East Siberian grayling)
- Thymallus svetovidovi Knizhin & Weiss, 2009 - Upper Yenisei grayling
- Thymallus thymallus (Linnaeus, 1758) - Grayling
- Thymallus tugarinae Knizhin, Antonov, Safronov & Weiss, 2007 - Lower Amur grayling
- Thymallus yaluensis T. Mori, 1928
The Catalog of Fishes also lists the species Thymallus baikalolenensis Matveyev, Samusenok, Pronin & Telpukhovsky, 2005. It reocognizes T. flavomaculatus as a full species, but does not accept T. yaluensis as an independent taxon.
- Sepkoski, Jack (2002). "A compendium of fossil marine animal genera". Bulletins of American Paleontology 364: p.560. Retrieved 2008-01-07.
- Froese, Rainer, and Daniel Pauly, eds. (2012). Species of Thymallus in FishBase. February 2012 version.
- Eschmeyer F. Catalog of Fishes: Thymallus (search) California Academy of Sciences (15.3.2012 version)