111,000 Spiders In The Same Cave!

In a cavity on the border between Greece and Albania, researchers have uncovered a spectacle that would have scared away more than one person. More than 111,000 spiders living in a single gigantic webwoven on the walls of a cave called Sulfur Cave. A surreal setting worthy of a Ridley Scott film or a dungeon Diablo IVbut completely real, which the researchers reported in this study published in the journal Subterranean Biology.

A nightmare for arachnophobes, but this immense organic city turns out to be one of the most extreme examples of chemotropic adaptation and animal cooperation never observed in the world.

The wall, which appears mineral, is in reality entirely biological: a matrix of silk and organic debris, in which each alcove houses a spider. © Marek Audy

A spidery empire

Nestled several dozen meters underground, this suffocating cavity, loaded with sulfur gases (hence its name), contains a living carpet of intertwined webs. The census was carried out by the team from the University of Tirana and the Natural History Museum of Crete, and the figures are truly mind-blowing: in total 111,513 spiders are grouped together on the same web, extending over more than 106 m².

Never before has such a colony been discovered anywhere else in the world, and even more surprisingly, two species of spiders coexist peacefully there, with a normally solitary lifestyle. The domestic tegenaria (Tegenaria domestica), which we find quite commonly in our homes, once summer arrives (69,000 individuals). The second, less close to us, the Prinerigone vagansa small weaving spider that usually frequents humid areas, mosses and temperate undergrowth in Europe (42,000 individuals).

These two spiders are logically reluctant to share their territories and can even be aggressive when confronted. Here, none of that, their webs form a common network, on which they live in community: such a case of interspecies cohabitation on this scale had never been documented before.

Why did these creatures, usually fierce and territorial, choose to live side by side in the same cave? Because this ecosystem functions according to different laws of the surface. The Sulfur Cave is plunged into permanent darkness, so sunlight never penetrates it, unlike other caves which benefit from external organic contributions.

At the base of this food chain, bacteria called chimioautotrophes use a gas, hydrogen sulfide (H₂S) to produce their own organic matter. These bacterial biofilms cover rocks, attract and feed a population of tiny midgesTHE Tanytarsus albisutuswhich swarm above the sulfurous stream flowing into the cave. According to the team’s estimates, there would be 45,000/m², which explains the small white streaks you see in the photo above.

These midges, in turn, are the main food resource for these two spider species. They therefore did not choose to cohabit for pleasure or sociability, but because they benefit from this almost inexhaustible food abundance.

To explain this phenomenon, researchers speak of a “ optional coloniality »: a collective way of life adopted only when environmental conditions are exceptionally favorable. Here the main condition being the hundreds of thousands of midges constantly flying in the cave. Spiders have even developed there cooperative behavior, once again an extremely rare fact : sharing of webs for hunting, mutual protection of eggs, tolerance of females during reproduction and synchronization of reproductive cycles. A form of collective intelligenceexplained by a fundamental principle of Charles Darwin’s theory: rivalry loses its evolutionary interest when the density of available resources goes beyond the need to defend a territory. Good news for arachnophobes: these tens of thousands of spiders have absolutely no no reason to come out of their little underground cocoon !