Slavic name for Ruegen

Epistola Wibaldi ad Bernardum Hildesheimensem episcopum (mid-12th c.)
goes:
_a Teutonicis Rugiana a Sclavis autem Rana dicitur_.
While it's not quite clear for me what is meant by "Teutonicus"
_Rugiana_ phonetically ([rujan-]?), I can't think of a Germanic
prototype to account both for _Rugiana_ and _Rana_ (and the etymology
of the toponym itself, < *Rug- 'Rug(i)'). [ruja:n-] > [rUjan-] > [Ran-
] ~ [r'an-] ([R] - r-affricate or something of the kind)?

Sergei