Discovery of a gigantic web of 110,000 spiders

The news has been widely shared around the world: scientists have explored a 100-square-meter underground spider web - the largest known - housing no less than 110,000 spiders of two species which, strangely enough, cohabit without devouring each other. All the more reason to question our knowledge of the social behavior of spiders.

On the border between Albania and Greece, in a Sulfur Cave, a gigantic spider web has been discovered: it extends over 100 square meters. No fewer than 110,000 spiders live in it without eating each other. All the more reason for scientists to take a closer look at this unique colonial web.

Among them, a team from ULB: Jean-François Flot (member of the Department of Organismal Biology, Faculty of Science, as well as of the (IB)² and BLU), Zofia Glabiak and Ninon Lecoquierre - trainee and PhD student respectively in his laboratory - took part in the study of this web.

The scientists observed that this web is built by the domestic begwort (Tegenaria domestica), a common species throughout Europe, including Belgium, where it is frequently found in garages, cellars and attics. But whereas this is normally a solitary predatory species, in Sulfur Cave it builds a gigantic shared web. Some 70,000 Tegenaria domestica spiders live within a few millimetres of each other, without devouring each other. Even more astonishing is the fact that some 40,000 individuals of another spider species (Prinerigone vagans) are also found here.

This peaceful coexistence of over 110,000 spiders of two different species on the same web is unique, and is probably due to the abundance of food. In fact, the web is located in the immediate vicinity of a sulfide stream, on whose surface bacteria multiply to oxidize hydrogen sulfide. These bacteria are consumed by chironomid larvae (small mosquitoes) which, when they become adults, fly off and are devoured by the spiders. Such an abundant energy source is unusual in caves, which are generally energy-poor environments.

Professor Jean-François Flot ’s team’s contribution to this study focused on two points in particular: the sequencing of the DNA of the Sulfur Cave tegenarians (collected on the web but also near the cave entrance), which showed that they are genetically distinct and have probably constituted an isolated population for thousands of years; and the sequencing of their microbiome, which turned out to be considerably depleted compared with that of a spider of the same species sampled a kilometer away.

This study calls into question current knowledge of spider social behavior and highlights the need to protect Sulfur Cave, the only known place in the world to host such a multispecies colonial web.