The colonial spider web in Sulfur Cave, is home to a mixed colony of Tegenaria domestica and Prinerigone vagans. (Photo : Marek Audy).
Deep beneath the Balkan mountains, scientists have discovered a phenomenon never before seen in nature: tens of thousands of spiders of two normally solitary species living together in a vast, communal web inside a toxic sulfur cave.
The finding, published in the peer-reviewed journal Subterranean Biology (Vol. 53, 2025), describes a thriving colony of more than 110,000 spiders—roughly 69,000 individuals of Tegenaria domestica and over 42,000 of Prinerigone vagans—all occupying the same cave system known as Sulfur Cave, which straddles the border between Albania and Greece.
“This is the first time we have observed such a large-scale communal structure among species that are typically solitary,” said lead researcher István Urák of the Sapientia Hungarian University of Transylvania. “It completely changes what we know about spider social behavior.”
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Spider Colony Cave
The Sulfur Cave is no ordinary habitat. Its air is heavy with hydrogen sulfide (H₂S), a gas toxic to most animals, and its walls are coated in pale, sticky microbial mats that feed on chemical reactions instead of sunlight.
Amid this environment, the research team discovered a massive tangle of silk, covering more than 100 square meters of rock surface—an unprecedented communal web complex.
Normally, T. domestica and P. vagans live alone, building separate webs and showing territorial aggression. Yet in this cave, they coexist peacefully, their webs merging into one continuous structure that teems with life.
What makes this discovery even more remarkable is how the entire food web functions. Instead of relying on plants or surface detritus, the Sulfur Cave ecosystem is sustained by chemoautotrophic bacteria—microbes that generate organic energy by oxidizing sulfur compounds.
Stable-isotope analysis of carbon and nitrogen conducted by the researchers revealed a clear energy pathway: sulfur-oxidizing microbes form the base of the food chain, which supports fly larvae (Chironomidae), and in turn, the spiders.
“This cave is a self-contained biosphere driven by chemical energy, not sunlight,” Urák explained. “It’s one of the few known terrestrial ecosystems where predators like spiders depend entirely on underground microbial production.”
Signs of Rapid Adaptation
Genetic tests confirmed that the spiders belong to the same species as their surface counterparts, yet they show distinct haplotypes—minor genetic variations suggesting recent adaptation rather than ancient isolation.
The researchers believe the colony formed when individual spiders from nearby surface habitats wandered into the cave, found an abundance of food and stable conditions, and stayed. Over time, their populations exploded, creating a dense super-colony.
Why would solitary spiders tolerate such proximity? The scientists propose that the constant humidity, stable temperatures, and reliable food supply inside the cave removed the pressures that normally drive competition.
“In this environment, fighting for territory may be pointless,” Urák said. “Instead, cooperation—intentional or not—offers survival advantages.”
This phenomenon is described as facultative coloniality: the ability of species to switch from solitary to communal living when conditions favor it. It is rarely seen among spiders and had never been documented at such a scale.
Microbial Symbiosis and Simplification
The study also uncovered microbial differences between the cave spiders and their surface relatives.
Cave-dwelling T. domestica hosted a far less diverse microbiome, dominated by symbiotic bacteria such as Mycoplasma and Wolbachia. The simplified microbiome could reflect the cave’s chemical isolation and lack of environmental microbes typically encountered outdoors.
While the discovery has fascinated biologists, it also raises concerns about conservation. The Sulfur Cave’s delicate balance depends on its unique chemical composition and microbial communities. Any disturbance—from tourism, mining, or pollution—could disrupt the chemoautotrophic base that sustains the entire system.
“Such environments are incredibly fragile,” Urák warned. “If the microbial mats die, the insects disappear, and so will the spiders. It’s a reminder that even the most resilient forms of life can depend on delicate chemistry.”
The discovery redefines scientists’ understanding of how life can adapt to extreme environments—thriving not through sunlight and plants, but through the invisible chemistry of the Earth itself. (Sulung Prasetyo)
