IN 17th-century Slovenia, people didn’t just believe in dragons, they knew they existed. How? Because every now and then, heavy rains would flush a dragon’s young from its subterranean lair out into the light. The creatures washed from Slovenia’s limestone caves looked like strange snakes with legs, oddly pale and lacking eyes. What could they be but dragon larvae?
Later, when explorers began to probe the region’s underground caverns and passages, they failed to find any fire-breathing monsters. They did, however, discover blind, white salamanders. Confined to caves, the amphibians had evolved into giants some 30 centimetres long that lived for a hundred years. Not dragons, then, but troglobytes.
In the vocabulary of the cave biologist, troglobytes are cave-dwelling creatures that spend their entire lives below ground. Unlike troglophiles, which are happy to live in caves but aren’t obliged to, or trogloxenes – part-timers such as bats, swallows and crickets – troglobytes are so completely adapted to life in caves they cannot survive in the outside world.
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Such confinement means even animals belonging to wildly different groups have many common features. These “troglomorphies” include eyelessness, a well developed sense of smell or touch and lack of pigment. Many invertebrates have gained flexibility by losing most of their hard exoskeleton. The only hard parts you’ll find on Nocticola flabella, the world’s most troglomorphic cockroach, are its mouthparts and genitalia. But even in this bizarre bestiary of cave-life, Slovenia’s salamander stands out. It is one of very few vertebrate troglobytes, and by far the largest.
For intrepid biologists prepared to squeeze into small, dark holes, caves offer excellent prospects of finding new species. Earlier this year, a team of biologists announced that after a three-year survey of 30 caves in California’s Sierra Nevada, they had found at least 27 new species, many of them true troglobytes. These included a fluorescent orange spider, a relative of the woodlouse so translucent they could see its bright yellow liver, and a harvestman with prey-grabbing jaws bigger than its body.
“Confined to caves they evolved into giants that lived a hundred years”
Why such diversity? Caves are a hotbed of speciation, each the equivalent of a test tube with an evolutionary experiment in progress. Whatever drove these animals’ ancestors underground – perhaps deteriorating conditions during an ice age or extended drought – once they were isolated in their rocky bunkers they began to diverge. The result is that after many thousands of years, each cave has its own unique assemblage of species.
With an estimated 90 per cent of the world’s caves yet to be discovered, the stream of new troglobytes is unlikely to dry up any time soon. Unfortunately, there is a flip side to this evolutionary bonanza: confined to one cave, many species consist of a tiny number of individuals. That makes troglobytes not just unique, but uniquely vulnerable.
