The Ryukyu Kajika frog, Buergeria japonica, is a small treefrog native to the Ryukyu Islands of Japan, where it turns up in almost any still or slow-moving fresh water, from rice paddies to roadside ditches. For most of its taxonomic history the same species was thought to range into Taiwan, until 2020, when those Taiwanese and Yaeyama Islands populations were split off as a distinct species, Buergeria choui, narrowing B. japonica to its present Ryukyu range.
Identification
B. japonica is a small frog: males measure about 30 mm snout-vent length, making it one of the more modestly sized members of the family Rhacophoridae. It is also known in English as the Japanese Buerger’s frog or Japanese stream treefrog, names that reflect its ties to running and standing fresh water rather than to trees. The original description, published in 1861 under the name Ixalus japonicus, placed the frog in a now-abandoned genus; it was later moved into Buergeria, the genus it shares with its closest relative, Buergeria choui, split off in 2020. Within that pair, B. japonica is distinguished chiefly by its Ryukyu distribution and its genetic separation from B. choui, since the two were only recently recognised as separate species.
Habitat and range
The species is confined to the Ryukyu Islands of Japan, a chain stretching between the Japanese mainland and Taiwan. Within that range it is a generalist, occupying rivers and intermittent rivers, freshwater marshes, freshwater springs, geothermal wetlands, irrigated land, and canals and ditches. Few amphibians span that many habitat types at once: the same list runs from fully natural settings, like freshwater springs and wetlands warmed by geothermal activity, to thoroughly human-made ones, like irrigation canals and drainage ditches running through farmland. This tolerance for human-altered water bodies, from paddies to irrigation channels and drainage ditches, likely helps explain why the frog remains common across its island range rather than being confined to pristine streams, even as the Ryukyu Islands see continued development pressure on wetlands and waterways.
Behaviour and breeding
Breeding takes place in slow-moving water in ditches and small streams, the same generalist habitats the species uses the rest of the year. Before the 2020 taxonomic split, populations now classified as the separate species B. choui were documented breeding in geothermal hot springs in Taiwan, where the warm water let frogs breed throughout the year instead of being limited to a single season. Tadpoles from that hot-spring breeding strategy showed notable heat tolerance, able to withstand water up to about 42°C (108°F), though few actively entered pool areas warmer than about 37°C (99°F). Since geothermal hot springs are not part of the habitat types recorded for the present-day Ryukyu range, that particular breeding strategy is best read as a trait of the wider Buergeria japonica–choui lineage before the split, rather than a confirmed behaviour of the frogs living in Japan today.
Conservation
The facts box carries the species’ current IUCN Red List category, Least Concern, reflecting its wide use of both natural and artificial freshwater habitats across the Ryukyu Islands and the absence of a single dominant threat to the population as a whole. That said, the frog is locally threatened by habitat loss in parts of its range, even though the species as a whole remains common enough to avoid a higher-risk category. Its reliance on irrigation channels and other artificial waterways helps sustain it where natural wetlands have shrunk, but it also ties the frog’s fortunes to how farmland and drainage systems are managed across the Ryukyu Islands.
Similar species
The closest relative of B. japonica is Buergeria choui, the species split off in 2020 from populations formerly included under B. japonica in northern Taiwan and the Yaeyama Islands. The two are told apart by genetics and geography rather than by appearance, since the split between them was made on genetic grounds only recently.