Light and shade
The olm is renowned for being a pale creature lacking skin pigmentation. However, not all olms are the same. In fact, there is one recognised subspecies, Proteus anguinus parkelj, the black olm, which can be dark brown as well as black. Discovered in 1986, the black olm is restricted to an area smaller than 100sq km near Crnomeljˇ in Slovenia.
As well as having pigmentation, the black olm has a number of other differences to the nominate subspecies ( P. a. anguinus). Its head is slightly shorter, with strong jaw muscles, and it has shorter legs. Though its body length is slightly longer (it has 34–35 vertebrae compared to 29–32 in the nominate subspecies), its tail is shorter, in proportion.
In addition, rather than having completely regressed eyes, the eyes of the black olm develop almost normally, and have principal rods, and both redsensitive and blue-sensitive cones.