In my readings of various histories of the USMC, I recall a short passage that a landing party was put together of various ships detachments Marines and sailors and they took a couple of small Mediterranean islands. As mentioned earlier, several Marines were assigned to the OSS and performed operations behind the German lines. Usually as individuals or part of a small mixed group. I think Sterling Hayden, the movie star, was one of those Marines.
The Army had a real hard on for the Marines for upstaging them in the first war and wanted to keep the USMC out of Europe.
I look at it this way, if not for the USMC experimentation with amphibious assualts in the 1930's, the army would never have been able to make their amphibious landings in north Africa, the Med, southern Europe, or Normandy. The war would have gone on for much longer than it did.
ETA, black Marines were primarily assigned to the defense battalions later in the war or they were assigned to supply organizations later in the war. They were present at Iwo Jima and pretty sure they were in the Okinawa campaign also.