I think you misunderstood. Different Marine units have different missions. You may have talked with Marines who had missions similar to those the Army and Navy would normally assign to their "special forces", but there are no Marines who have that kind of designation. There are only Marines who have trained for those kinds of missions.
I recently visited Camp Geiger (where I graduated infantry training school a few decades ago!) and there is a sign over the entry to a walkway among the old white buildings which has the following quote:
"Only in the Marine Corps can you take an artillery man, or tanker, or a cook and tell him that he is an infantryman and he'll know what to do." - General CE Mundy
I took a picture of that sign. Again, the point is that ALL Marines are "special forces".
FRegards,
LH
You need to update your information in light of what Marsoc is doing. Marsoc operators are being trained to the same standards as SEALS and SF.
My son was started out in 3/6 moved to Recon then to Force Recon and is now part of Marsoc (Marine Special Ops Command).