Minecraft wiki entry for Shulker, associated text: When its shell is closed, a shulker has 20 ( × 10) armor points and deflects arrows. When the shell opens, it loses this natural armor and takes damage as normal. When at less than half health, a shulker has a 25% chance of teleporting (as described above) after taking damage. When a shulker is attacked, other shulkers in the area target the attacker as well.