The effect of artificial light at night on settlement patterns of a salt marshes crab

Natural light cycles, particularly lunar phases, regulate settlement patterns of many marine invertebrates. Artificial light at night (ALAN), however, increasingly disrupts these processes, potentially altering lunar-driven settlement and recruitment. Understanding these interactions is critical in salt marshes near urban areas. This study tested the effects of ALAN on the settlement of the granulated crab Neohelice granulata across four lunar phases (new, waxing, full, waning). We deployed four settlement mats per site at four independent sites per treatment (ALAN and unlit control), totaling 32 mats per lunar phase. Early settlers (megalopae to small crabs) and late settlers (juveniles) were counted weekly. Settlement patterns were analyzed with generalized linear mixed-effects models, with treatment and lunar phase as fixed factors and site as a random effect. Model residuals were validated with simulation-based diagnostics, significance tested via Wald χ², and power analyses conducted with the mixedpower package in R. ALAN significantly altered settlement, with effects depending on lunar phase and life stage. For early settlers, ALAN strongly increased settlement during the full moon (χ² = 33.23, p < 0.001), but not in other phases (new: χ² = 0.11, p = 0.86; waxing: χ² = 1.68, p = 0.19; waning: χ² = 0.23, p = 0.77). Late settlers responded mainly during the waning moon (χ² = 10.39, p < 0.01), with no significant effects in new (χ² = 2.97, p = 0.08), full (χ² = 1.91, p = 0.16), or waxing (χ² = 0.57, p = 0.45). Post hoc power analyses indicated adequate sensitivity (all > 0.6). ALAN modifies settlement dynamics of N. granulata, with impacts varying across lunar phases and life stages. Artificial lighting amplified settlement during full moons in early settlers but had inconsistent effects on later stages. These findings show that ALAN can override natural lunar cues, underscoring the need to account for light pollution in ecological assessments and the management of salt marsh ecosystems.