dead snail shells with holes

[marie]

One culprit could be another snail, like a whelk. Look for snails "hitching" a ride on other snails.Once when I ordered a bunch of ceriths, one of them was a carniverous snail (it actually didn't look much like the other snails I don't know why they included it in a cerith order) it ate 3 other snails before I realized it wasn't in the least bit interested in algae
Another culprit could be a mantis shrimp, I don't know if they leave holes in the shells or not though