Troglobionts or Eutroglobionts are animals that live in caves and are unable to live outside of it. Troglobionts usually have troglomorphic adaptions, which means they have adapted to the special environment. This is generally the reason why they would not survive outside.
A typical adaptation to the cave is the enhancement of the helpful senses: sense of smell, touch and hearing. Many troglobionts have long, sensitive antennae with which they can not only feel well, but are also able to sense vibrations. This enables them to locate moving objects. The sense of smell is particularly interesting for aquatic creatures.
There is also the loss of senses that have become unusable. As there is absolutely no light in caves, vision is superfluous, and the eyes recede and disappear completely over time.
Finally, there are passive adaptations, such as the loss of skin pigmentation. Troglobionts are usually pale, white or even transparent. Sometimes you can see red blood shining through, or it is also transparent. This is not a helpful adaptation, it is simply due to a lack of necessity, but it makes a return to the surface impossible. The troglobiont dies within a very short time from ultraviolet light due to the lack of protective function of the skin pigments. Short-term exposure to sunlight can be fatal. Even a change in temperature of a few degrees is quickly fatal.
A particularly interesting aspect of troglobionts is that they are a local evolutionary branch, producing only endemic species. Many are restricted to individual cave systems or, in the case of aquatic organisms, to a karst water body. On the other hand, the retreat into the cave has protected the respective species from climatic changes on the surface. This is how the descendants of species that have become extinct on the earth's surface, or at least no longer exist in this region, can be found. In general, the food supply is low, so the metabolism is reduced, and at the same time the lifespan is increased.