Behavioural rhythms of two amphipod species Marinogammarus marinus and Gammarus pulex under increasing levels of light at night

Abstract

Artificial light at night (ALAN) is proliferating at an alarming rate across the globe, particularly around aquatic habitats. Natural and predictable light cycles dictate much of an individual organism's life by acting as a major signal for their circadian clock, driving rhythmic behaviours and physiological changes throughout the body. Light cycles also help populations coordinate group behaviour and greatly impact the interspecies dynamics of a community. Research into the ecological impacts of ALAN has highlighted numerous effects on these biological processes, including higher predation rates, impaired growth and development, and diminished reproductive success. Invertebrates play an undeniable role in ecosystem functioning and show robust daily rhythms. As such, it is vital to understand how ALAN may disrupt their behavioural patterns. The aim of this study was to monitor the impacts of increasing levels of light at night (0 lux - 80 lux), as well as constant light and constant darkness, on the behavioural rhythms of the intertidal amphipod, Marinogammarus marinus, and the freshwater species, Gammarus pulex. Gammarus pulex activity was not strongly synchronised to any of the light at night treatments. Marinogammarus marinus, however, exhibited strong behavioural rhythmicity in diurnal cycles with dark night periods. All the ALAN treatments resulted in a significant decrease in M. marinus rhythmicity and overall activity. Moreover, ALAN between 1-50 lux disrupted nocturnality in this species. These results indicate that while some amphipods show some adaptive plasticity when it comes to light pollution, others may experience strong direct impacts on their activity. This may be relevant to individual and population level fitness of vulnerable species in more heavily urbanised areas.

Fig 1. Average activity profiles (left column) and double-plotted actograms (right column) of Marinogammarus marinus (A-B) and Gammarus pulex (C-D) under the LD light regime.

Activity profiles display average activity levels (black line) with standard deviation (grey areas), along with the light levels (dotted lines) over 24 hours. Actograms display the average, normalised behaviour across the 7-day assays, with each row showing two 24-hour cycles. Bars above actograms denote light levels; on/off transitions are not shown.

Fig 2. Locomotor activity of Marinogammarus marinus (left column) and Gammarus pulex (right column).

(A-B) Barplots with standard error bars of total amount of activity counts logged during the seven-day assay period. (C-D) Boxplots of relative rhythmic power (RRP), which shows how strongly the activity repeated over a set period (e.g., 24 hours). An RRP ≥ 1 denotes rhythmic behaviour (dotted line); an RRP ≥ 1.5 denotes strongly rhythmic behaviour (dashed line). (E-F) Boxplots of period length, which shows the length of time across which an activity pattern occurs in individuals with rhythmic behaviour (RRP ≥ 1). (G-H) Boxplots of nocturnality, which shows the proportion of activity that occurred during the subjective night phase (ZT12 – ZT23:59). Different lower-case letters indicate significant differences (p < 0.05) between treatments. Light conditions outlined in Table 1.