In a group of enlisted captives an E-5 Marine Sergeant is the second most senoir person in the group and disputes an E-9 Navy Corpsman's assertion that he is the senior?

In a military context, rank hierarchy determines seniority, and an E-9 (Master Chief Petty Officer) is indeed senior to an E-5 (Sergeant) regardless of branch. Therefore, the E-9 Navy Corpsman holds the higher position and authority over the E-5 Marine Sergeant. The Sergeant's dispute is unfounded as the rank of E-9 supersedes that of E-5, making the Corpsman the senior member in this scenario.