Just as evolution via natural selection can give, it can also taketh away. If an animal lives in an environment in which a particular trait becomes unnecessary, then that trait may become reduced – or even lost altogether.
This might be the wings of large flightless birds, or the diminished pelvic bone still seen in snakes. It can also be a sense, like vision, and the eyes that grant it. Animals that live in the dark – whether that be underground or in the deep sea – are often found to have greatly reduced vision, eyes that are obscured by skin or hair so as to render them non-functional, or even lack eyes entirely. But what is the point of getting rid of an eye, even if it's not used? Indeed, natural selection only works on traits that influence survival and reproductive success in some way.
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What is genetic drift?
There's also an evolutionary process that goes some way in explaining the loss of certain useless features: genetic drift. Genetic drift is a random process by which the frequency of a trait in a population (and particularly a neutral one in terms of survival and fitness) can change. This process may work on eyes if they don’t provide any meaningful advantage.
However, it doesn't fully explain why blindness can evolve as rapidly as it does in some dark-dwellers. The general idea is that there must be a cost to having eyes – both in the making of them and in the upkeep. Eyes, though of course very useful in the light, may become a disadvantage. It takes energy to form them and they are vulnerable and easily damaged. In the dark, the blind may become the fittest.
Blind animals
Texas blind salamander
Found only in underground streams and deep pools in one region of Texas, this amphibian lacks pigment and is translucent-white as it has no need for protection from the sun. Functioning eyes are also obsolete in the complete darkness of its home, so they are reduced to small black dots covered by skin. Instead, it senses its prey through changes in water pressure.
Olm
Another salamander, this time from Europe, the olm looks similar to the Texas blind salamander, with the same underdeveloped eyes covered by skin. However, it has a more serpentine body and can reach 40cm in length in comparison to the more diminutive 13cm of its Texan cousin.
Mexican tetra
Found in Mexico and southern USA, this species of freshwater fish exists in two forms: one that lives in the light and is fairly unremarkable-looking, and the famed ‘blind cave fish’ that is a strange pinkish-white. Some populations have no eyes at all, with just empty sockets in their place. It detects prey using a ‘lateral line’ – a motion-sensing organ found in fish and amphibians.
Kaua‘i cave wolf spider
A handful of caves on the Hawaiian island of Kaua‘i host this species of wolf spider, which is notable for being the only member of the family to lack eyes. Rather than constructing a web, it catches its prey – the Kaua‘i cave amphipod – by sensing its movements through vibrations and chemical signals.
Pacific lamprey
This parasitic Pacific Ocean dweller has a round sucker mouth, lined with sharp teeth used to latch onto other fish and even sperm whales. While it has well-developed eyes as an adult, the lamprey spends its first five years as a filter-feeder on the seafloor with undeveloped eyes covered in skin. The adult stage only lasts a couple of years, before it spawns and then, days later, dies.
Star-nosed mole
Though not completely blind – it has small eyes that can detect light and dark – the star-nosed mole uses its unique, star-shaped, fleshy nose, which is packed full of nerve endings, to identify and consume food items in a record-breaking 120 milliseconds!
We named it one of the weirdest animals in the world.
Eyeless shrimp
This fascinating species of crustacean was only discovered in 2012 around one of the world’s deepest undersea hydrothermal vents, which spews scalding water up to 400°C. It hosts bacteria inside its mouth and gills that use the hydrogen sulphide from the vents to produce organic matter that the shrimp then eats. Despite its common name, it does have non-functional eyes – not a problem for an animal that lives 3km below the surface where light can’t reach.
Giant blind mole-rat
Endemic to the North Caucasus region of Russia, this large rodent can be found living in underground burrows that it digs with its sizeable teeth. Like the star-nosed mole, it may not be completely blind, with some related species able to distinguish light from dark, and even possibly different colours. However, its eyes are very small and covered by a layer of skin and thick fur.
Sinopoda scurion
Discovered living in a cave in Laos in 2012, Sinopoda scurion is the only huntsman spider that lacks eyes. Like many on this list, its blindness is a consequence of life in an environment in which vision would be an unnecessary expense. It also has no need for pigment, and as such is ghostly pale.
Thaumastochelidae
Members of this family of rare deep-sea lobsters are set apart from other lobsters by their blindness, as well as their bizarre, elongated, spiny pincers that look a little like a row of sharp teeth. Distributed widely across the globe, they are found within the Twilight Zone, which lies from 200-1,000m below the water’s surface.
Brahminy blind snake
One of the world's weirdest snakes, the Brahminy blind snake is a tiny mysterious reptile that moves unseen beneath the soil’s surface. Although blind it can still find and devour a plentiful supply of insects especially ants and termites. However this isn't the snake's weirdest attribute as did you know there are no males? To date, every single brahminy blind snake is female. It is the world’s only unisexual snake and can reproduce rapidly.
Meet more weird and wonderful creatures:
- Weirdest sea creatures - meet 13 strange sea creatures of the deep
- 10 flightless birds: Meet some curious - and often strange - land-based avian wonders
- Transparent animals: Discover 10 weird invisible creatures you can see right though
- Weirdest birds: meet 14 strange and weird wonders of the avian world
