My guess would be that the ending "-je" corresponds to the PIE y-stem. "Morje" obviously comes from *mory and is cognate to Latin "mare" (stem being "mari-"). I don't know much about Slovenian though. In Croatian, "rje" gets monothongised into "re", so the Croatian word for "sea" is "more". This sound change is productive even today. For instance, the genitive singular of "vrijeme" should be "vrjemena", but is actually "vremena". Daniel, what are your thoughts on this?
