The U.S. Coast Guard is an independent service, with its own Academy, whose function in peacetime is to ensure the safety and security of traffic in American waters. All of its ships above 65 feet in length, except lightships, are designated as Coast Guard cutters. Ranks are similar to those in the Navy but the insignia are distinctive (featuring shields in place of stars.)
In time of war, the Coast Guard comes under the command of the Navy. The Roosevelt administration took this action on 1 November 1941, a week before the attack on Pearl Habor, with Executive Order 8929. Relations between the two services was not entirely frictionless, as might be anticipated. Coast Guard crews were nonetheless highly professional and performed their wartime duties (mostly antisubmarine patrolling) well.
References
Coast Guard Historian's Office (accessed 2007-5-20)
The Pacific War Online Encyclopedia (c) 2007 by Kent G. Budge. Index