The iwame trout or markless trout is a variety of

salmonid Salmonidae is a family of ray-finned fish that constitutes the only currently extant family in the order Salmoniformes . It includes salmon (both Atlantic and Pacific species), trout (both ocean-going and landlocked), chars, freshwater whitefis ...

fish inhabiting some fresh waters of Japan. It is an intra-specific mutant morph of the masu salmon (''Oncorhynchus masou'') that is characterized by a lack of the lateral spot marks typical of the young fish in most populations. The iwame is a stream-resident morph that does not migrate to the sea. It occurs together with the standard phenotype in some populations. This morph is

recessive In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and ...

ly inherited. This morph occurs in the ''amago'' subspecies ('' Oncorhynchus masou ishikawae'') in Western Japan, and apparently also in the

nominate subspecies In biological classification, subspecies is a rank below species, used for populations that live in different areas and vary in size, shape, or other physical characteristics ( morphology), but that can successfully interbreed. Not all spec ...

''yamame'' (''Oncorhynchus masou masou''), if these are not identical. The morph was scientifically described as a distinct species ''Oncorhynchus iwame'' in 1961 by Kimura & Nakamura. In the 2000s, the iwame morph was shown to occur as a polymorphism in random mating populations of ''O. masou'', and is no more thought to have taxonomical value.

References

* Kano, Y., Shimizu, Y., Kondou, K. 2006. Sympatric, simultaneous, and random mating between markless trout (iwame; ''Oncorhynchus iwame'') and red-spotted masu salmon (amago; ''Oncorhynchus masou ishikawae''). Zoological Science 23:71-77. * Kano, Y., Kondou, K., Shimizu, Y. 2009. Present status and conservation of the markless forms of stream-resident masu salmon ''Oncorhynchus masou'' (the so-called 'iwame') in Japanese mountain streams. Ichthyological Research 57:78-84 * Oncorhynchus Freshwater fish of Japan Taxonomy articles created by Polbot {{Salmoniformes-stub