I have a longspined urchin (named spike!)
I got him for my nano cube. He was very small then.
I am still pretty new at this. Anyway, he was doing pretty well, running around eating all my coralline algae. I raised the salt level apparently to fast, when He started shedding his spines everywhere. The same thing happened when I was trying to control my PH and alk. Well, apparently I raised it to much. He started shedding his spines again. I didn't realize how sensitive he was to those changes.
He survived though. He was nearly bald. I almost killed him twice.
He's now in my 110g tank (He's twice the size now).
I've thought about changing his name to "lucky", but the name is already taken by my coral banded shrimp that I had in my sump. When I came home from a trip, he was gone. I thought he had died (sucked up by the return pump). I discovered him a month later in the same chamber with the skimmer pump. I don't know how he survived or what he ate. But he "earned" that name.