Rise of hybrid giants: invasive hybridization as an urgent conservation challenge to the Japanese Giant Salamander

The Japanese Giant Salamander (JGS; Andrias japonicus ) is an apex aquatic predator endemic to streams in Western Japan. It is threatened with extinction by various factors, and hybridization with introduced Chinese Giant Salamanders (multiple Andrias spp.; CGSs) has become an urgent issue. Historically, CGSs were legally imported into Japan until 1980, when Japan joined CITES. A well-known mass CGS importation occurred in 1972, which is likely to be one of the sources of the invasive hybridization. Hybrid giant salamanders (i.e., hybrid giants) were first identified in Kyoto in 2010. To date, hybrids have been found in nine prefectures, and complete eradication of hybrids from Japan appears difficult. Although hybrids and JGSs are similar in appearance, some differences exist, including those in dorsal patterns and head shape. The larger mouths with potentially stronger bite force likely enable hybrids to outcompete JGSs. However, the ecological impacts of hybrids remain largely unknown. Similarity in reproductive behaviors between the parental species explains successful hybridization. Long-term data from Mie Prefecture showed that hybrid swarms can form rapidly without management interventions. Conservation challenges include illegal releases of hybrids, insufficient initiatives by federal and local governments, and hurdles to conducting field surveys. Practical preservation of a functional ecosystem requires a viable top-predator population, including both JGSs and their ecologically equivalent hybrids. Nonetheless, Japanese citizens are legally obligated to protect the JGS’s genetic integrity as a designated Special Natural Monument. We recommend case-specific plans depending on hybridization progression, river conditions, and habitat connectivity. Finally, we remind that failure to address habitat destruction and climate change as fundamental conservation issues would defeat the ultimate goal of protecting biodiversity, functional ecosystems, and the national treasure.