111,000 Spiders Coexist in a Sulfur Cave, Redefining Two Species Once Deemed Solitary

In the Vromoner Gorges, straddling the border between Greece and Albania, a sulfur-rich cave houses the largest spider web ever recorded. More than 111,000 individuals from two species that are normally solitary coexist on 106 square meters, overturning several scientific certainties about their behavior.

Sulfuric-acid-carved cave harbors 106 m² of web, the largest on record across all species

The Sulfur Cave lives up to its name. Carved by sulfuric acid produced from the oxidation of hydrogen sulfide, it stands apart from most European caves. A sulfur-laden stream continually traverses it. It saturates the air with toxic compounds and maintains a constant temperature of 26°C.

This cave is located in the Vromoner Gorges, about 450 kilometers from Athens. The entrance lies on the Greek side, but the deepest sections extend into Albanian territory. Czech speleologists first spotted the extraordinary textile structure in 2022 during an expedition in this sparsely traveled canyon.

In 2024, an international team carried out a thorough analysis of the cavern. In a narrow passage near the entrance, researchers discovered a web spanning 106 m². Thousands of funnel-shaped structures weave together, forming what is likely the largest arachnid network ever recorded.

Two species normally hostile to each other live in peace in total darkness, defying predatory logic

The census reveals roughly 69,000 Tegenaria domestica and more than 42,000 Prinerigone vagans. These two species, common near human dwellings, are typically solitary. Under ordinary conditions, Tegenaria actively preys on P. vagans, which are much smaller and more vulnerable.

Yet, within the Sulfur Cave, these natural enemies share the same web in peaceful coexistence. The researchers hypothesize that the perpetual darkness sufficiently impairs the Tegenaria’s vision to dampen their predatory instinct. This relative blindness turns formidable predators into unlikely roommates.

Sulfur-oxidizing bacteria at the base of the chain, the spiders adapt within a few generations

The entire food chain of the Sulfur Cave rests on sulfur-oxidizing bacteria. These micro-organisms secrete whitish biofilms in the sulfurous waters. Clouds of chironomids, small harmless midges, feed on these films and become the direct prey of the spiders.

This diet, saturated with sulfur compounds, leaves a lasting imprint on the spiders. Analyses of their gut contents reveal a microbiome less diverse than that of individuals living on the surface. The extreme diet has measurably shaped their internal ecology.

Molecular data bring an additional surprise. Within the Sulfur Cave, the spiders show genetic differences measurable compared with external populations. In just a few generations, this hostile environment has selected genetic variants adapted to these subterranean conditions.

An unexpected behavioral plasticity in ordinary species, a major lesson on the adaptability of life

István Urák of the Sapientia University of Transylvania emphasizes a reality that is often overlooked. Some species possess remarkable genetic plasticity that only manifests under sufficiently intense environmental pressures. The Sulfur Cave triggers behaviors completely absent from these spiders’ usual repertoire.

This unique colony also poses conservation challenges. Its border location between Greece and Albania complicates coordinated protection. Teams are continuing studies to better understand the adaptation mechanisms at work in this exceptional environment.