Shell Blade

Shell Blades appear in both and. They are clam-like enemies, which attack by charging at Link and defend themselves by retreating into their shells. In both games, Link must defeat them by stabbing their innards just as the shell is opened.

Ocarina of Time
Shell Blades were found in the Water Temple, and only ever underwater. On spotting Link, they would open their shells wide (leaving themselves open to attack by the Hookshot, which can kill them in one hit) before clamping shut, spinning round, and dashing at Link, repeating slicing their back spikes through the air before sinking back to the ground.

Twilight Princess
Shell Blades in this game were much larger, sizeable enough to swallow Link whole. This time they would attack by rushing at Link front-first to try and trap him in their shells, which would snap shut with distinctive bangs. If they miss they cower in their shells until they have the strength to launch another attack. This type of Shell Blade is vulnerable even when attacking, and the additional fact that Link can use weapons other that his hookshot underwater (again, Shell Blades are always found submerged in TP) make them much easier to kill.