Caves are very dark places. If you have never experienced the complete and utter blackness of a cave, it is somewhat difficult to appreciate just how black black can be. Sitting in your wardrobe with your hands over your eyes on the darkest night of the year doesn’t even come close. This total absence of light has had a profound effect on the animals that live their entire lives within caves. Known as troglobites – not to be confused with troglodytes, which are people that live in caves – these animals are only matched by the inhabitants of the very deep-sea in their bizarre and alien appearances.
But, of course, if you live in an environment where not even a single photon of natural light can reach, looks just don’t matter. You can afford to be as ugly as you want.
Eyes need light to see. If there is no light, eyes become useless and that means they are just expending extra (precious) energy. It is much better to reroute the energy needed to maintain a visual system to other, more useful senses, such as touch. And, indeed, most cave specialists have done away with eyes altogether. Swimming in the cold, still, underground lakes found in many large caverns throughout the world are various cave fish, almost all of them small, blind and devoid of pigment – three classic troglobite characteristics.
In the picture above, there seems to be two types of fish: one blind pink fish and two decidedly normal-looking ones. In actual fact, they belong to the same species, the Mexican tetra. These are the two forms of the fish – one that lives in caves and one that lives on the surface (and, since they are the same species, ‘hybrids’ between the forms can occasionally be found). Interestingly, this species is born with eyes, but those belonging to the cave form become covered with an opaque layer of skin as it grows older. The coding for eyes is obviously there but it gets overridden as the fish matures and the need for vision becomes virtually nil.
Another exclusive cave-dwelling fish, this time from Thailand, shares many similarities with the blind form of the Mexican tetra, except it has four flattened wing-like fins, which has given it the name of cave angel. These fins have microscopic hooks on their underside, which allow the fish not only to climb up cave waterfalls to find new and higher pools but also to crawl out of the water and navigate damp, vertical walls. Its behaviour, other than it feeds on bacteria in oxygenated water, is largely unknown and it is almost never seen in the wild, showing just how unexplored and unstudied this subterranean world really is.
In the 1600s, the people of southern Europe occasionally found strange pink, blind lizard-like creatures around cave entrances following sudden downpours. People back then still believed in dragons and they assumed that these rarely-seen and bizarre animals must be the underdeveloped offspring of these mythological monsters.
In reality, these ‘dragon babies’ were cave-dwelling salamanders called olm. Heavy rainfall very occasionally washes them up out of their caves, but every other time they are only found inhabiting deep underground lakes. Their bodies reflect this lifestyle. The olm’s limbs are so tiny and so far apart that their use in locomotion has become practically non-existent. It instead uses its long, snake-like body to swim through the water with graceful undulations. Its skin has lost all of its pigment and has become pink and translucent, its blood vessels and organs just about visible underneath.
Food is exceptionally scarce in these water-filled caves and, since it has no eyes, the olm must rely on its keen sense of smell to locate small crustaceans and other invertebrates. Even so, edible matter is so rarely encountered that the olm has developed an extremely low metabolic rate and scientific experiments have shown that olms can survive, at a stretch, for up to ten years without food, although it starts to digest its own internal organs after a while. Though they may live up to seventy years or more, olms only become sexually mature when they are fourteen and this factor, coupled with its very restricted natural range (it only lives in a small number of cave systems), means that is not only one of the strangest amphibians, but also one of the most endangered.
Not every cave in the world is completely devoid of light. In a few cave systems in New Zealand, strange and mystical lights illuminate the passageways from above. If you were a first time visitor to these caves, you would be forgiven for thinking that the cave roof had suddenly disappeared, giving a clear view of the star-studded sky outside. But the ceiling of the cave is indeed still there, and the lights are produced by a type of glowworm, the bioluminescent larvae of a species of fungus gnat called Arachnocampa. Many species of fungus gnat, as their name implies, feed on the fruiting bodies of fungi. Arachnocampa, however, is different from the rest of its family – it is highly carnivorous.
When a grub hatches, it produces a line of silk, up to 40cm long, which it dangles down from the cave ceiling where it lives. At intervals along the thread, there are small droplets of sticky mucus, each applied by the grub as it constructs the strand. Streams often flow into these cave systems, and they bring with them immature insects such as mayflies, which are ready to begin their short adult lives above water. Little do they know, their lives are about to be shortened even further.
Emerging from the water, the mayflies are fooled by the bioluminescent gnat grubs into believing they are outside, and they instinctively fly feebly upwards, blundering into sticky threads as they do so and becoming stuck. The gnat larva feels the struggling insect and reels in the strand, eating both the silk and the mucus – recycling them both – and, finally, the captured victim.
Of course, there comes a time when the Arachnocampa gnats must leave behind their larval lives and become adults. Males lose their bioluminescence when they mature, but the females retain them. It is thought that the females intermittently light up whilst in their pupal form to attract nearby males. The males wait until she emerges from her pupa and then tussle for the right to breed with her. Like many species of gnat, though, adult life is a short affair. Neither males nor females have mouthparts and so cannot feed. The adults do not survive for very long after their eggs are laid and many of them probably end up as food for the next generation.