Artificial light at night enhances defensive behaviour in aquatic shredders under predation risk.

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