The marines resisted, and got away with it by insisting they didn't have any "special operations" troops (or that "all marines are special operations troops," depending on what day you asked them.) But by late 2001, it was obvious even to the marines that SOCOM was where the action was, and the marines wanted in. After four years of haggling and negotiation, the marines are in, with a combination of traditional commandoes, long range recon, and "ranger" type forces. There are also support troops (dog handlers, interrogators and interpreters, intelligence analysts, supply and transportation) as well as a training unit (to instruct foreign troops, a job the marines have been helping the army Special Forces with already.) The marines have also agreed to provide, as needed, other marine units that are trained to perform jobs SOCOM needs done. The marines have long had their infantry battalions train some of their troops to perform commando type operations (raids, hostage rescue and the like.) This was done so those battalions, when serving on amphibious ships at sea, had some capability to handle a wider range of emergencies (like getting Americans out of some foreign hot spot.)
The marines may contribute more forces to SOCOM in the future, or perhaps the entire Marine Corps will join SOCOM and take it over.