Shrimp are a lot faster and harder to catch. Also, octopuses seem to come with something of an instinct for catching crabs, but have to learn to catch shrimp.
When she was little, my bimac used to chase them head first, then try to turn around and catch them with her arms. Didn't work the shrimp were gone.
She perfected her techniques throughout her life and eventually hunted them at night, upside down, at the water's surface. She seemed to really love hunting live shrimp!
I've come to like the little shrimp and they are useful in a tank. Right now I have one left from my last shipment and he eats from my hand.
Nancy