Hermit crabs?

[Matt]

I also fell in love with the zebra hermits, but they get bigger than the other hermits I have, and aside from their bullying ways with frags and other hermits (killed a few), they both: a) started to kill snails; and b) taught the other hermits how to kill snails - a trick that they have (thankfully) forgotten without the ongoing leadership of their zebra cousins.

Eventually, the bad boys had to go.