Night can be a scary time for humans, as we generally have trouble distinguishing between a tree and a monster when the light is too low. To make the nighttime world a bit less scary, we've artificially illuminated the night sky and inadvertently brightened the natural world as well. Our brighter nights have wreaked havoc on many nocturnal species, causing them to have trouble finding mates or camouflaging themselves from predators, but sometimes the artificially illuminated sky has longer-lasting effects. During their study of the freshwater shrimp Gammarus pulex, Yuhan He, Anirban Ganguly, Susan Lindgren, Corinne Suvanto and Ulrika Candolin of the University of Helsinki, Finland, along with Laura Quispe of the Université Claude Bernard Lyon 1, France, and Kangshun Zhao of the Chinese Academy of Sciences, China, found that brighter nights can have effects that last throughout the daylight hours as well.
The small crustaceans were collected from a stream near Helsinki, and the scientists then brought them back to the lab and separated the males from the females before beginning the difficult task of raising the shrimp. After a month of lab-rearing, some of the shrimp experienced a brighter nighttime environment for a week. And after the week was over, He and colleagues put a male and female shrimp together in a tank and videoed their mating behaviour during the night and day. Interestingly, a brighter night didn't seem to affect how much the shrimp moved around, but it did make them less likely to form a mating pair once the male had found the female inside the aquarium. If they are both happy with their respective partners, the male will hold the female under his body, preventing other males from mating with the female. This pair formation is incredibly important in this species, as mating can take place over several days with the pair embracing each other until the female moults, shedding her hard exoskeleton so that mating can begin. If the shrimp don't form a pair, or if they become separated too soon, then they won't mate and reproduce.
Because mating takes so long in these shrimp, part of the mating process also takes place during the day. After experiencing artificially brightened nights, the shrimp moved less and spent more time in the shelter of the leaves at the bottom of their aquarium in the daytime. Less time out searching for mates also meant that the shrimp met each other fewer times, leading to less chances for mating. Perhaps surprisingly, the shrimp that experienced a brighter night also separated more frequently, terminating their bond. This suggests that there are carryover effects from the night light that cause the animal stress and make them stop the mating process.
When the team added a second male into the tank, males fought each other more if their nights were artificially illuminated. These fights could also force males to separate from females that they had already paired with. The increased nighttime light pollution also caused more pair separations during the daytime if another male was present. He suggests that ‘if they haven't rested at night, they won't be doing well during daytime’. Whatever the case may be, this is an important example of why it's important to study the effects of artificial light at night during the daylight hours, and also how our artificially brighter nights may be dimming the life of other species.
