Shell Shield is a hunter pet ability unique to turtles. When this ability is used, the turtle hides in its shell for a short amount of time. This reduces all damage taken by 50%. When this ability is on auto-use, it will activate when the turtle reaches 50% health.
Prior to the Echoes of Doom patch, Kresh in the Wailing Caverns and Ghamoo-ra in Blackfathom Deeps were the only low level (20ish) turtles that knew Shell Shield. It is now a general skill that all turtles know upon taming.