Hannah M Adkins

Artificial light at night is an emerging environmental stressor with significant implications for aquatic ecosystems, influencing species’ behaviour and physiological processes. We examined the effects of artificial light at night on the routine metabolic rate of two widespread, invasive crayfish species, red swamp, Procambarus clarkii (Girard, 1852) and signal, Pacifastacus leniusculus (Dana, 1852). Crayfishes were exposed to light intensities mimicking urban nighttime lighting (20 lux) and natural daylight (1000 lux) to assess species-specific metabolic responses to artificial light at night. The results revealed distinct patterns: P. clarkii displayed no significant metabolic change (P = 0.331), suggesting a lower sensitivity to artificial light at night, whereas P. leniusculus showed a significant (P < 0.0001) reduction in routine metabolic rate under artificial light at night, likely due to behavioural adaptations related to reduced activity. Both species exhibited higher nocturnal routine metabolic rate compared to diurnal measurements, regardless of treatment, with routine metabolic rate of P. clarkii remaining elevated longer than that of P. leniusculus. These findings underscore the species-specific metabolic responses to artificial light at night, with potential ecological implications for their roles within aquatic ecosystems. The study enhances our understanding of invasion ecology by demonstrating how species-specific responses to environmental stressors like artificial light at night may influence their competitive dynamics and ecological impacts.

The importance of energy budgets in understanding the ecology, conservation and production of crayfishes has long been recognized. Standard metabolic rate (SMR) estimates the minimum metabolic rate required for basic maintenance of an organism while at rest, and is a critical parameter for investigating energy balance and metabolism. Estimating SMR involves quantifying oxygen uptake under specific conditions. Standard methodology for estimating SMR has been described and evaluated for fishes, but not thoroughly investigated for crayfishes. We adapted a recommended protocol developed for fishes in order to determine appropriate methodologies for measuring SMR in crayfishes. Study animals consisted of 18 individuals of Procambarus clarkii (Girard, 1852) collected in Alabama, USA. Respiration rates were measured using an optical respirometry system (Loligo Systems®; Viborg, Denmark) and intermittent respirometry techniques. Crayfish respiration stabilized the morning after initiation of the trial indicating that a 12 h overnight period was sufficient to acclimate crayfish to respirometry chambers. After 12 h of daylight, respiration of acclimated crayfish typically exhibited a short spike when lights were turned off, indicating data collected within ~2 h following a light change should be excluded from the dataset used to calculate SMR. When calculating SMR, a quantile approach was typically more appropriate than the mean of the lowest normal distribution approach. SMR calculated during the day was only marginally higher than SMR calculated during the night, indicating that SMR can be estimated during either period if shelters are provided in the respiration chambers. Due to the wide diversity of crayfish species and ranges, our recommendations may not be appropriate for every crayfish species or subpopulation. The recommendations can serve, however, as a valuable starting point and the described methodology provides a standardized approach for determining appropriate protocols to measure SMR of crayfish species of interest.