i think they enjoy snacking on anything new (anything in their diet that is). perhaps if the shrimp were there before the tusk, and the tusk may consider it part of his home. i had a domino damsel with my tusk for 2 years. every time i tryed adding a blue devil damsel, well bye bye. i tried entering it at night one night, and in the morning when i put my face to the tank to see if he survived, the tusk snagged him in front of me, spit it at me dead, then swallowed it again. pretty tramatizing.