ɪt wʌz lˈɔŋ θˈɔːt tə kəntˈeɪn ˈoʊnli ɐ sˈɪŋɡəl spˈiːsiːz, ˈɑːɹ. dʒæpˈɑːnɪkə, bˌʌt ɹˈiːsənt stˈʌdiz hæv ɹɪzˈʌltᵻd ɪn sˈɛvɹəl ˈʌðɚ tˈæksə bˌiːɪŋ tɹænsfˈɜːd ˌɪntʊ ðə dʒˈɛnəs.
It was long thought to contain only a single species, R. japonica, but recent studies have resulted in several other taxa being transferred into the genus.