Kingdom of the Deep (Shark Wars, Bk. 4)

Peace has finally come to the Big Blue and with it a revelation: Gray is not the last megalodon in the ocean after all. Gray swims deeper into the mystery of his family heritage, uncovering the myth of the megalodons and a legend that goes back to prehistoric times. But when the scheming mako Velenka breaks free from her underwater prison, the peace Gray and Barkley have fought so hard to win is threatened.