Shell'n

Shelln (シェルン) is the oyster-like enemy in Mega Man 8 and Mega Man and 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 can guess, it is only vulnerable when the shell is open. A similar enemy called Shellgeek (and others) appears in Megaman Battle Network 2 as a virus. Instead of shooting bubbles, it fires needles down your row.