Could you live outside of your species

Life in the underworld

In addition to animals that only live temporarily in the underworld, biologists are constantly discovering new species that spend their entire lives in darkness. These so-called troglobites are not viable outside the caves because they have developed very special adaptations for underground life.

Eyes are superfluous in the dark

It is not uncommon for permanent cave dwellers to have receded eyes or eyes overgrown with skin. In some cave animal species they are even completely absent. While this regression would be life-threatening for living things on the surface, it has proven to be an advantage for cave animals in the course of evolution. Because eyes are of no use to the animal species in the eternal darkness of the caves, but their construction and maintenance costs energy. If they are missing, the cave animals save cell material and metabolic resources.

Another characteristic of many cave animals is that they do not produce pigment, which is why they have pale to white skin. But even that has no disadvantages for the cave dwellers: They are neither exposed to the sun's harmful UV rays, nor do they need to be camouflaged from predators, for example. In this way you save the energy that is required for the production of the pigment melanin. In addition, the skin of the cave animals is also thinner than that of species that live on the surface. This enables them to better absorb moisture and atmospheric oxygen through their body surface.

The biologist Peter J├Ąger from the Senckenberg Research Institute in Frankfurt discovered an example of such an adapted animal: He found the world's first giant, eyeless crab spider in a cave in Laos. Sinopoda scurion is the only one of 1,100 species in this spider group that has neither visual cells nor pigments and is therefore optimally adapted to life without daylight, according to the researcher.

Not just blind, but also deaf

While scientists have proven the lack of vision in cave dwellers a few years ago, a team of researchers led by Matthew Niemiller from Yale University discovered two species of cave fish in which another sense has declined.

In a hearing test, the scientists found that the two fish species Typhlichthys subterraneus and Amblyopsis spelaea from the family of the North American blind fish (Amblyopsidae) reacted sensitively to sounds in the low frequency range of around 100 Hertz. For the higher altitudes from around 800 Hertz, however, the two types of cave were pitch-deaf, as the researchers report. In addition, both species had significantly lower hair cell densities than relatives above ground.

Although it would be obvious that the cave dwellers use their hearing to orient themselves in the dark instead of with their eyes, according to the researchers, this regression also has a biological meaning: Due to their poor hearing, the cave fish block out the strong noise that is in the cave waters caused by water turbulence and the pot water. Niemiller and his colleagues therefore assume that some cave animals will adapt to their habitat, which is typical for some cave animals.

Feel instead of see

But how do the cave dwellers find their way around then? In order to be able to move around in the dark, other sensory organs have strengthened or even newly developed in cave animals to compensate for their lack of vision and the sometimes poor sense of hearing. For example, the well-known cave olm (Proteus anguinus), whose eyes are hidden under the skin, has an even better sense of touch and smell.

Many of the animals that live in caves, such as centipedes, cave isopods, cave crickets or cave beetles also orient themselves with special feelers, whiskers or antennas. Scientists also discovered special vibratory organs and other sense organs of touch in cave animals. Sometimes their extremities are also greatly elongated.

Special physique

For example, fish living in the underworld, such as the loaches of the genus Barbatula, differ from their aboveground relatives: The cave dwellers have elongated tactile processes on their heads, the so-called barbels, and larger nostrils. Researchers also suspect that cave fish like these can be sensitive to currents in the water.

Another example is the spring black species Anurida stereoodorata, which lives underground: During an expedition, researchers led by Rafael Jordana from the Spanish University of Navarra discovered this species of tiny, wingless insects in the so-called Voronya Cave in Abkhazia at a depth of around 100 meters. Instead of normally developed eyes, the scientists found a highly specialized chemical sensory organ in this species. In addition, according to the researchers, these cave springtails have more sensory cells behind their antennae, which are sensors for humidity and temperature.

Changed day-night rhythm

Since the cave dwellers cannot keep the day and sleep rhythm typical for living beings without daylight, they have developed a different internal clock. According to studies by Nicola Cavallari of the University of Ferrara and her colleagues, the timer can be influenced by food instead of light for some blind cave dwellers.