Chromeshell Crab
Morph (You may cast this card face down as a 2/2 creature for 
. Turn it face up any time for its morph cost.)
When this creature is turned face up, you may exchange control of target creature you control and target creature an opponent controls.
Illustrated by Joseph Meehan
This printing
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The creatures can’t attack until you start a turn with them under your control. So you can’t swap and then attack.
You choose the two target creatures when the triggered ability is put onto the stack. You do not have to choose Chromeshell Crab. You choose whether or not to do the exchange on resolution.
The exchange fails if either you no longer control your creature or the other creature is now controlled by you.
The exchange fails if either target is not still on the battlefield at that time.
The trigger occurs when you use the Morph ability to turn the card face up, or when an effect turns it face up. It will not trigger on being revealed or on leaving the battlefield.
