Since we have had our crabs (2 months now) our one crab, Spongebob changes his shell at least 2 times a week, somtimes twice in one day. Is this normal? The other 3 crabs we have seem to be happy with the shells they have but for some reason Spongebob just keeps changing. He has one shell he keeps going back to. I have lots of shells in the tat for them, about 10. Has anyone ever had this happen before?