Light-emitting Noctilucae planktons are drawing attention in Ine Bay in the Sea of Japan town of Ine, northern Kyoto Prefecture, as they gather and glow blue on the edge of the surf.
Some people are shooting the photogenic scenery, with the blue water adding a fantastical atmosphere to a scene already known for its unique traditional houses incorporating boat moorage.
According to the prefectural Fisheries Technology Department, Noctilucae are planktons 0.15 to 2 millimeters in diameter and emit light when stimulated, such as by waves.
They increase in number in around early May and become red tides when they crowd together, but cause no harm to fishery products, it says.
They are often observed in May but the location varies depending on the wind direction, an official of the local tourism association said, adding, "You would be lucky if you could see them because they are not around every day."