A manitis shrimp "BY DESIGN" is an aggressive creature.
It will become aggressive when there is no other food source available to eat.
By giving it a prawn - that's pretty easy pickins - and it took advantage of the easy meal.
If you toss some hunks of beef into a tiger's cage - it will eat and not be near as aggressive. It will take the easy way out to feed.
If you stop feeding the tiger for a period of time - and toss a bunny rabbit in there - I would bet you'd see the aggressive behavior - cause it's hungry and there's no easy food to eat.
Animals conserve energy when they can.
If not other food is available - they will do what they are built for and attack living things.
Mantis shrimp are "built" to attack.
But they'll kick back and grab the easy food too - just as we do.
Ever see 4 hungry guys sitting around a table sharing a large pizza ?
Ever been in one of these situations yourself ?
Ever see what happens when it get's down to the very last slice - and all 4 guys are still hungry ?
Aggressive behavior at it's best LOL
I gotta go now - and attack some eggs and toast