IIRC, it's -one- branch of them that sends the shockwave punches, the other branch prefers to slice prey in half with direct blows. something like that, anyway.

and i'm not sure i'd want to take on an adult octopus, even if i were a mantis shrimp. octopuses can lose limbs and keep on ticking, and meanwhile wrap your hard-shelled carapace up like a xmas present with its remaining limbs. every superman has his kryptonite.