Mating behavior of Gammarus pulex under artificial light at night

Artificial light at night (ALAN) is an emerging environmental problem, originating from e.g. street lightning and infrastructure. It has been found to alter the behavior of many organisms and thereby influence community composition and ecosystem functions. However, most studies on the impact of ALAN on wildlife and the environment have focused on terrestrial ecosystems, while aquatic ecosystems have received less attention, yet are increasingly exposed to artificial light. Gammarus species are amphipods commonly found in freshwater and estuarian ecosystems across Eurasia and North America. They are key components of ecosystems in recycling nutrients and serving as a food source for other species. They are night active and, hence, vulnerable to ALAN. Earlier studies show that ALAN increases their predation risk and induces physiological stress. However, no study has assessed the impact of ALAN on mating behavior, although mating success is a key fitness component that influences population dynamics. Gammarus pulex is a common freshwater detritivore where females are receptive for a short period and males compete for females and then guard them through amplexus, precopulatory guarding. The aim of this master’s thesis was to study if and how ALAN affects the mating behavior of the G. pulex, and if this depends on the presence of male-male competition for females. G. pulex, individuals were collected from a stream in Helsinki and exposed to control (natural light conditions) or ALAN in the absence or presence of male competition, eight treatment combinations in total, for one week under laboratory conditions. In each trial, one female and either one or two males were placed into a tank – in the absence or presence of ALAN – and their behavior recorded for ten minutes, including active time, contact between the sexes, and pair formation and separation. The results show that ALAN influences both night and day mating behaviour, but that the effect depends on the presence of male competition. ALAN had no impact on activity during the night, while male competition decreased it. During the day, ALAN decreased activity, but male competition counteracted and increased activity. Number of contacts between female and male(s) was only impacted by male competition during the night, but by both ALAN and male competition during the day; ALAN reduced contact time during the day in the absence of male competition, while male competition increased contact time. Pair formation was negatively affected by ALAN during the night, while male competition increased formation. Pair separation was higher under ALAN during the day, independent of male competition, but unaffected at night. These results indicate that ALAN causes a carry-over effect, with light conditions during the night influencing mating behaviour during the day. This is most likely through physiological changes incurred during the night because of the artificial light, such as increased stress levels. Further studies should assess how ALAN impacts mating behavior when many individuals are present, as G. pulex is more choosy in mate selection at higher population densities. Follow up studies are also needed to determine if the effect of ALAN depends on the colour of the light, as found for other organisms, as such information is needed in conservation work aiming to reduced negative effects of ALAN on ecosystems through the development of environment friendly lighting systems. Regarding the consequences of the change in mating behaviour when exposed to ALAN, this could alter population dynamics. Given the key ecological role of the species in ecosystems, this could lead to changes in nutrient recycling and the structure of food webs. Considering that the presence and intensity of ALAN is expected to increase in the future, it is of great importance to assess its effects on organisms, not only during the night but also during the day through carry-over effects, as indicated by the present study.