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Olm
The only vertebrate in Europe that lives its whole life in caves, the olm is a pale salamander of the underground waters beneath the karst of Slovenia and the western Balkans. Its eyes are atrophied and covered with skin, yet it still senses light through the skin of its whole body and relies on other senses to find food.
The slowness is the striking part: an olm can go years without eating and is thought to live for around a century. Postojna Cave keeps a few on view, and in January 2016 one laid eggs in a large aquarium inside the cave. Because olms live only in underground water, whatever drains from the surface into the karst ends up in theirs.
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Leptodirus
Leptodirus hochenwartii, the narrow-necked blind cave beetle
The first cave insect ever described. An assistant lamplighter, Luka Čeč, found it in Postojna Cave in 1831, and Ferdinand Schmidt named it the following year, which helped start the scientific study of cave life. It has no eyes and no pigment, a thin, stretched-out front body and a rounded abdomen where it stores food reserves, a sensible design where meals are rare. At 8 to 11 mm it is easy to miss. It depends on the steady cool temperature and high humidity of large caves and lives only in the Slovenian Karst and its surroundings, so it cannot simply move elsewhere if a cave dries out or warms.
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Cryptops speleorex
Movile Cave in Romania was cut off from the outside world millions of years ago, and workers came upon it only in 1986. Nothing there runs on sunlight: its food chain starts with bacteria living on carbon dioxide and methane, in air that may hold about half the usual oxygen.
This venomous centipede, 46 to 52 mm long, is the largest animal known from the cave and sits at the top of that chain. Its name, given when it was described in 2020, means King of the Cave. It evolved from a surface-dwelling relative over millions of years of isolation, one of many species found nowhere else but Movile.
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Plutomurus ortobalaganensis
The record for depth. This eyeless springtail was collected about 1,980 meters below the surface in Krubera-Voronja Cave in the western Caucasus, and when it was described in 2012 it was the deepest land animal known. It lives on fungi, far beyond the reach of any plant.
Unusually for a deep cave animal it still carries pigment, which may mean it reached those depths fairly recently. The same expedition turned up three other new springtail species, a reminder of how little of the deep underground has ever been sampled.
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Mexican tetra
Mexican blind cavefish, the cave form of the Mexican tetra
One species, two ways of living: a silvery, sighted river fish and pale, eyeless fish in 29 deep caves in Mexico. That makes it one of the clearest natural experiments in cave evolution. Cave fish are born with eyes that degenerate as they grow, and populations in separate, unconnected caves such as Pachón, Tinaja and Molino lost both sight and sleep independently, through different genetic changes.
The difference in sleep is stark: surface fish sleep around 13 hours a day, the cave fish just 2 to 4.
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Waterfall climbing cave fish
A tiny loach from the Tham Maelana and Tham Susa karst of northern Thailand that walks. It climbs steep rock in fast-flowing waterfalls using the same diagonal gait as a salamander, braced by a robust pelvic girdle with features usually linked to land vertebrates.
It is depigmented, has no visible eyes and grows to only about 2.8 cm. A 2016 study of how it moves drew wide attention because that pelvis echoes the anatomy of early four-legged animals. The fish is rare and protected, which has kept research on it limited.
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Xibalbanus tulumensis
Remipedes are blind, cave-dwelling crustaceans of flooded caves along the coasts of Mexico and Central America, places reached only by cave divers. This species, from the Yucatán, gave the group its claim to fame: a 2013 study showed it was the first crustacean known to use venom to capture and kill prey.
A pair of glands in the front of its body makes the venom, and one of its toxins closely resembles a type of neurotoxin first described in spider venom. In water where prey is scarce and hard to find, disabling whatever it catches is a sound strategy.
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Texas blind salamander
Found only in the water-filled caverns of the Edwards Aquifer around San Marcos, Texas, this salamander is smooth and unpigmented, with a large, broad head, reduced eyes and long, slender limbs. It keeps its external gills throughout life and averages about 12 cm long.
It was listed as endangered on March 11, 1967, and its fate is tied to the aquifer's water: groundwater overconsumption and contamination are the threats. For anyone who knows only the olm, it is a useful comparison, a salamander on another continent arriving at much the same pale, gilled, nearly blind shape.
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Kentucky cave shrimp
A shrimp about 1.25 inches long that lives only in the immediate vicinity of Mammoth Cave National Park, in the lowest water-filled passages. It has no eyes and no pigment in its shell, which makes it transparent and very hard to spot. It finds food with its antennules, grazing the microbial films that coat underwater cave surfaces.
The cave's streams are fed from the surface, and during floods water flows backwards into the springs, carrying sewage and runoff from farms, parking lots and highways. Pollution is the leading suspect in the shrimp's decline, and it is federally endangered.
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Alabama cavefish
Known only from Key Cave, on Tennessee Valley Authority land in Lauderdale County, northwest Alabama, and widely counted among the rarest cave fish in North America. Searches of more than a hundred other caves in the area have found no confirmed second population.
It has no eyes and practically no pigment and reaches only 30 to 58 mm. Its food arrives from outside: adults move toward water disturbed by falling droplets or bat guano, so the bats roosting in the cave are part of its life support. Falling and degraded groundwater threaten it, and much of the land that feeds water into the cave remains unprotected.
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Kauaʻi cave wolf spider
Its most distinctive feature is a complete lack of eyes, unique among wolf spiders. Discovered in 1971, it has only been found in the Koloa Basin of Kauaʻi, Hawaii, in the dark zone of lava caves where the humidity may approach 100 percent. It finds prey by smell, probably feeding mainly on the Kauaʻi cave amphipod that shares its caves.
Family life is scaled down too: females produce fewer than 30 spiderlings per clutch, against 100 to 300 for most wolf spiders, and carry the unusually large young on their backs for a few days. It is endangered, threatened mainly by development and the loss of native plants above the caves.
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