Artificial light at night enhances the defensive behavior in aquatic shredders under predation risk

Caddisfly larvae (Trichoptera) are mostly nocturnal aquatic invertebrates that contribute to the decomposition of coarse organic matter, such as plant litter in streams. Artificial light at night (ALAN) can interfere with their behavior, particularly in the presence of predation threat. We investigated the effects of various types of ALAN on Lepidostoma hirtum , a common caddisfly species in running waters. We tested its behavior (activity and tendency to shelter) in darkness, as well as under white warm LED light and HPS (sodium lamp) light (2 lux), with and without the presence of predation cues (crushed conspecifics). We found that in darkness caddisflies did not change their behavior in the presence of predation threat. When exposed to ALAN only, caddisflies decreased their activity compared to dark nights, regardless of the type of light. The presence of both predation threat and ALAN enhanced a defensive behavior in caddisflies, especially under LED light. They not only reduced their activity, but also spent more time in shelter, compared to other treatments. This study highlights that impact of ALAN on the behavior of nocturnal aquatic invertebrates may vary depending on the spectral composition of light and may be intensified in the presence of predation threat.