Shell'n

Shell'n (シェルン) is an oyster-like enemy in Mega Man 8 and Mega Man & Bass. Appropriately, it dwells in the aquatic levels of Aqua Man and Pirate Man. It attacks by opening its shell to spit out dangerous bubbles. As one could guess, it is only vulnerable when the shell is open.

Trivia
A similar enemy called ShellGeek (and its variations) appear in Mega Man Battle Network 2 as a virus. Instead of shooting bubbles, it fires needles down the player's row.