coordinate - Protocol for establishing longitudinal meridians on other heavenly bodies

When we first observe a new heavenly body (it could be a new star, asteroid, etc., but let's say a minor planet in our own solar system), are there any procedures set in place for establishing a system of longitudinal meridians? Being that a prime meridian is an arbitrary concept that you can pick and establish anywhere, is a location decided based on a physical feature, or perhaps from the first points of data we gather when making detailed observations of a heavenly body for the first time? What about heavenly bodies with no easily discernible or non-stationary features (Gas Giants)? Also does the IAU regulate this process? I have always wondered if the USSR and the US shared common reference points for locations such as on the moon.