Aggregation and sheltering behavior of Armadillidium vulgare and Armadillidium nasatum (Isopoda: Armadillidiidae) exposed to fluorescent and ultraviolet light

Abstract

Terrestrial isopods are gregarious invertebrates, aggregating in colonies. Aggregation reduces risk of predation and desiccation. Isopod aggregation and sheltering behavior have been observed under fluorescent and color-filtered light, but not under ultraviolet (UV) light. With UV levels on the rise, it is important to understand how soil ecosystems might be impacted by human-induced ozone depletion. We examined aggregation and sheltering behaviors of the terrestrial isopods Armadillidium vulgare and Armadillidium nasatum under fluorescent light and under fluorescent light with added UV light. Upon testing, each species was randomly assigned into groups of 10 individuals. Each group was released into a circular arena with a black disk in the middle of the lid which functioned to block light and served as a shelter. Sheltering and aggregation behaviors were recorded every 15 minutes for 45 minutes under the presence versus absence of increased UV light. There was no significant difference in the total number of sheltering events, but positive thigmotaxis (movement toward or away from a physical stimulus) may have influenced sheltering. However, there was a significant increase in the average number of aggregated individuals and the total number of aggregates in groups exposed to increased UV conditions, perhaps reflective of their desiccation avoidance behavior and negative phototaxis. Additionally, A. nasatum individuals were significantly more likely to form aggregates than A. vulgare when exposed to increased UV light conditions, perhaps because A. nasatum is less desiccation-resistant than A. vulgare.