The Cottoidei and Hexagrammoidei center their distribution in the eastern North Pacific Ocean, but antitropical (both hemispheres) distributions occur for cottoids in some instances. The Psychrolutidae include North Pacific and North Atlantic species, plus several species off New Zealand, Australia, and South Africa. Cottids similarly occur in the Northern Hemisphere (but in both marine and fresh waters) and off New Zealand, eastern Australia, and Argentina. Li-paridids occur in both warm and cold marine waters of all the world's oceans, although rarely in the Indian Ocean. The Cy-clopteridae occur only in cold, marine waters of the Northern Hemisphere. Agonids are found in the North Pacific, North Atlantic, and off southern South America.
The unique families of Lake Baikal cottoid fishes have been mentioned. Molecular genetic studies of Baikal cottoids (Kir-ilchik and Slobodyanyuk, 1997) reveal close relatedness with freshwater cottids such as Cottus bairdii from Lake Michigan and Cottus cognatus from Lake Michigan and Siberia. The genus Cottus is very widely distributed across North America and Eurasia, and the different species are all a very generalized cottid body type. The Baikal cottoids, although closely related to other freshwater cottids, have radiated into 22 endemic Baikal species (another four cottocomephorid species also occurring in other drainages of Siberia).
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