The iwame trout or markless trout, Oncorhynchus masou var. iwame, is a variety of salmonid fish inhabiting some fresh waters of Japan

Japan

Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

. Suspected to be not a real species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 since some time, it was recently confirmed to be a particular intra-specific morph

Polymorphism (biology)

Polymorphism in biology occurs when two or more clearly different phenotypes exist in the same population of a species — in other words, the occurrence of more than one form or morph...

 occurring in some populations of the masu salmon or seema (Oncorhynchus masou). This morph is recessively inherited.

This morph occurs in the amago subspecies ((Oncorhynchus masou ishikawae) and apparently also in the nominate subspecies yamame (Oncorhynchus masou masou), if these are not identical.