Swabb was secretly in league with Oil Corp, who promised him a proper body. His looked like a human skeleton. The rest of the crew of the Black Gold were the same.
Swabb turned on Kaliko when he thought the Tenth Doctor and Martha Jones were Oil Corp. spies. He ordered the Oil rigs to attack the ship, and it was destroyed. Swabb survived, and threatened to use Martha's body to replace his. The Doctor used his sonic screwdriver to flip Swabb off the deck. Swabb survived the fall and vowed revenge.
According to Kaliko, living skeletons made for better crew men, as they put less of a strain on rations. (TV: The Infinite Quest)