Artificial Nighttime Light Draws a Species Boundary Across Tokyo Bay's Coastline
Tokyo Bay's coastline is one of the most illuminated urban shorelines on Earth. Sodium-vapor street lamps, shipping-terminal floodlights, and the perpetual glow of one of the world's largest metropolitan areas mean that true darkness never falls on the bay's inner shores. A study published in PNAS Nexus finds that this light gradient is doing something ecologists had not previously quantified: it is drawing a genetic and ecological boundary between two closely related species of coastal crustacean, separating them as effectively as a physical barrier.
The research, led by Daiki Sato of Chiba University's Graduate School of Science, combined genome-wide population analysis, 28 years of environmental monitoring data, and controlled laboratory exposure experiments to trace the influence of artificial light at night - known as ALAN - on the distribution and physiology of two Ligia isopod species across the bay.
Why isopods and why Tokyo Bay
Isopods of the genus Ligia are flat, fast-moving crustaceans found on coastal rock faces and seawalls throughout the world's temperate coastlines. They occupy a narrow intertidal strip - often just a meter or two of vertical space between the waterline and dry land - which means they experience whatever conditions exist at the land-sea interface. In a heavily urbanized coast like Tokyo Bay, that interface is dominated by concrete seawalls and breakwaters, many of which are directly illuminated at night.
Tokyo Bay provided an unusually clear natural experiment: a continuous coastline with a steep gradient from the intensely lit inner bay to the comparatively darker outer bay, allowing sampling across the full light range within a single geographic region.
What the genomes revealed
Genetic analysis showed a clear ecological partition between the two species that matched the bay's illumination pattern with striking precision. Ligia laticarpa was most abundant along the brightly lit inner-bay shorelines, while Ligia furcata dominated the darker outer-bay areas. Individual genetic profiles showed no evidence of recent hybridization, confirming that the two species remain reproductively isolated despite occupying the same coastal habitat type.
At the population level, the genetic signal was more complex. Some inner-bay sites showed signatures of admixture consistent with an additional Ligia species. That admixture signal correlated with ship-traffic density, suggesting maritime commerce may occasionally transport individuals between ports. Statistical analysis of the 28-year environmental dataset identified nighttime light intensity, salinity, and vegetation cover as the three primary variables driving the species partition.
Laboratory tests confirm physiological asymmetry
To test whether the field pattern reflected genuine physiological differences, Sato conducted laboratory exposure experiments. Long-term exposure to artificial nighttime light reduced both growth rates and activity levels in L. furcata - the outer-bay, darkness-adapted species - but produced limited effects on L. laticarpa, the inner-bay species that coexists with chronic illumination.
This asymmetry provides a mechanistic link between the environmental gradient and the species distribution. L. furcata appears physiologically suited to darkness and pays a measurable cost under chronic artificial light. L. laticarpa tolerates or has adapted to illuminated conditions. The ecological boundary in the field corresponds to where those physiological tolerances diverge.
Urban planning implications
The study focused on a single highly urbanized bay and two closely related species. Whether similar light-driven partitioning occurs at other illuminated coastlines, and whether it extends to other taxonomic groups, remains untested. The mechanisms driving any light-tolerance differences between species - whether behavioral, physiological, or genetic - also await investigation.
"Rather than viewing artificial light and other anthropogenic stressors solely as degradative forces, this work shows that some species can persist, diverge, and potentially adapt within human-altered systems," Sato noted. "Recognizing human-mediated dynamics in coastal systems can help inform more ecologically sensitive urban planning, in which factors such as lighting can be adjusted to support, rather than undermine, biodiversity."