Simplify the following text for a Grade 8 native English-speaking person.
Original: The highest ranking military officers in the Department of the Navy are the Chief of Naval Operations and the Commandant of the Marine Corps , who are the principal military advisors to the Secretary of the Navy . They supervise their respective military services of the Department of the Navy , and in a separate capacity serves as members of the Joint Chiefs of Staff .
Simplified: While the Navy and the Marine Corps both is under the Department of the Navy , they are two separate branches of the military .