We kept a crayfish for a year once. It was a school project with crayfish left over, I brought it a full aquarium kit. We enjoyed it. But after a year it started to grow algae and I mistakenly tried something to get rid of it. Killed the hermit crab as well. But it's worth a try. We had the crayfish when my daughter was 8 and my son 4. Friends were unsuccessful at keeping any alive for any length of time though.
We have also had hermit crabs at one poing but I don't remember them lasting a real long time.
