Metropolitan coastal night lighting aligns with ecological and plastic divergence in closely related Ligia isopods

Artificial light at night (ALAN) is a pervasive urban driver that can reshape coastal communities, yet its links to genetic structure and organismal responses remain unclear. Here, we examined two closely related isopods, Ligia laticarpa and L. furcata, across Tokyo Bay along a gradient from brightly lit inner-bay shorelines to darker outer-bay coasts. Genomic and mitochondrial evidence consistently identified clear genetic segregation between outer- and inner-bay populations corresponding to the two species, with L. laticarpa predominating at inner-bay and L. furcata at outer-bay sites. While individual genetic profiles showed little indication of recent hybridization, population signals in several inner-bay sites pointed to historical inputs or local co-occurrence of distinct lineages, possibly mediated by ships. Bayesian modeling of 28 years of environmental data identified nighttime light intensity, salinity, and vegetation cover as major predictors of species occurrence, highlighting ALAN as a key environmental axis distinguishing their habitats. In laboratory assays, chronic ALAN exposure reduced growth and activity in L. furcata but had limited effects on L. laticarpa. These findings suggest that artificial lighting and maritime activity jointly shape patterns of genetic connectivity, habitat segregation, and physiological plasticity between closely related coastal species, providing insights into how anthropogenic disturbances influence ecological divergence.