Shell Blade

Shell Blades are aquatic clam-like enemies that appear in, Majora's Mask and Twilight Princess. They will will lunge themselves at Link if he gets too close. When they do this, Link must block them with his shield or move out of the way. They will stop and eventually open up, exposing their muscle ligaments which Link can attack with the Hookshot in, as Zora Link in Majora's Mask, and with the Clawshot or sword in Twilight Princess. In Majora's Mask, Shell Blades guard some of the stolen Zora Eggs in Pirates' Fortress.

It should be noted that when encountered out of water in Ocarina of Time, these enemies can be defeated with a larger spectrum of weapons. The Megaton Hammer is useful as it may shatter a Shell Blade in a single blow, as will the bombs if timed correctly.

Trivia
In Twilight Princess, it is the only normal enemy that will bleed when defeated.