The iwame trout or markless trout, Oncorhynchus masou var. iwame, is a variety of salmonid fish inhabiting some fresh waters of Japan. Suspected to be not a real species for some time, it was recently confirmed to be a particular intra-specific morph occurring in some populations of the masu salmon or seema (Oncorhynchus masou). This morph is recessively inherited.
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. doi:10.2108/zsj.23.71
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 doi:10.1007/s10228-009-0130-1