From the Document: "The Department of the Navy (DON) is a single military department that includes two military services--the Navy and the Marine Corps. As such, DON has a single civilian leader, the Secretary of the Navy, and two four-star military service chiefs--an admiral whose title is the Chief of Naval Operations (CNO), and a general whose title is the Commandant of the Marine Corps. Although the title 'Secretary of the Navy' includes only the term 'Navy,' the secretary serves as the civilian leader for both the Navy and Marine Corps. The CNO and the Commandant of the Marine Corps are members of the Joint Chiefs of Staff (JCS)."
CRS In Focus, IF10484
Congressional Research Service: https://crsreports.congress.gov/