The blind olm may breed only once every 12.5 years
Deep in the flooded limestone caves of the Balkans lives a pale salamander locals call the human fish. The olm is nearly blind, keeps its baby gills for life, can survive long starvation, hears vibrations through rock and may sense Earth's magnetic field. A typical female breeds roughly once every 12.5 years.
Olms belong to the genus Proteus, the only group of backboned animals in Europe that lives exclusively in caves. They are found in underground rivers running through the karst of the Dinaric Alps, from the Soča basin near Trieste southwards. Recent studies by two independent teams concluded that what was treated as one species is really nine, each confined to its own cave system. The English name is a German borrowing of unclear origin, perhaps related to the word for newt; the Slovene nickname, human fish, refers to its fleshy skin.
The body is snakelike, usually 20 to 30 centimetres long and occasionally 40, making it one of the largest cave animals anywhere. It swims by twisting like an eel, barely using its tiny legs, which bear three toes in front and two behind. Like the axolotl, it is neotenic, keeping larval features such as external gills into adulthood. Its skin lacks pigment yet can still make melanin, darkening when exposed to light, and one population, the black olm, is always dark.
Darkness has reshaped its senses. Larvae start with normal eyes, but development halts and they waste away after about four months, ending up buried under skin, though still sensitive enough that the animal swims away from light. Smell is sharper than in related amphibians, and the inner ear picks up sound in water across roughly 10 to more than 12,000 hertz, keenest between 1,500 and 2,000. Special organs on the head detect weak electric fields, and in 2002 olms were found to align with natural and artificially altered magnetic fields.
Life runs slowly in water that stays at about 8 to 11 degrees Celsius. Olms swallow small shrimps, snails and insect larvae whole, can gorge when food appears and endure long fasts. Males defend territories but mostly settle disputes by display, since fighting wastes scarce energy. Breeding has only been seen in captivity: a female lays up to 70 eggs, 35 on average, and guards them among rocks.
Source: Olm
