A Novel Light-Induced Collective Circular Movement in Armadillo sordidus Isopods

Collective movement in terrestrial isopods has rarely been documented and almost never discussed. Here, we report a novel behavioral phenomenon in isopods from the species Armadillo sordidus: large nocturnal aggregations forming coordinated circular movements involving thousands of individuals. The behavior was observed naturally in several locations in northern Israel and could be experimentally induced using artificial white light. Ultraviolet light and magnetic fields did not induce this behavior. Image analysis revealed approximately 5500 individuals within a single aggregation, and manual tracking confirmed a clear circular movement pattern. The observed sex ratio (1:4 males to females) and presence of many gravid females suggest that the behavior is not reproductive. Instead, the circular motion appears to represent a density-dependent, light-induced aggregation response. To our knowledge, this is the first formal documentation of collective circular movement in isopods, potentially arising from anthropogenic light pollution.