Mercy or messy: distribution and differentiation of native and released Chinese bullfrogs (Hoplobatrachus rugulosus) in Hong Kong using genetic and morphological analyses

The Chinese bullfrog, Hoplobatrachus rugulosus (Wiegmann, 1834), naturally distributed in Thailand, Laos, Vietnam and China, is commonly used in the religious practice of mercy release. The release of H. rugulosus is complicated by the fact that it likely represents a cryptic species complex (multiple species under a single name). In Hong Kong, although there are native and released individuals, the impact of release has not been investigated. In this study, we conduct genetic and morphological analyses of H. rugulosus across Hong Kong to determine the source (native or released) of frogs, the distribution of released populations and identify morphological characteristics that can be used to differentiate native and released individuals. We found that H. rugulosus individuals in Hong Kong belong to the two major genetic clades identified in previous studies (Clade A and Clade B). We consider individuals in Clade A as released frogs based on the genetic result that cluster these individuals with wild H. rugulosus from western, central and eastern Thailand, and individuals from Chinese frog farms. Individuals from the two genetic clades differ based on morphology (dorsal color, coloration of loreal region, presence of wounds and coloration on lateral sides), that can be useful for identification. However, there are four individuals in Clade A that are morphologically similar to native frogs, raising the possibility of hybridization between individuals in the two clades. We strongly recommend that the mercy release of H. rugulosus and other exotic animals should be strictly prohibited by law in Hong Kong and other parts of the world. Eradication guidelines should be developed by experts, as a references for governments, conservation groups and the public.