Sea Urchin?

[Gary Ellis]

I bought a purple sea urchin to clean up some algae. I put him in and he went into a cave and has not moved for 4 days. Is he dead or is this normal for an Urchin?

 

[New&no clue]

Does he still have his spines? If so then he is fine, if the spines have fallen off he is dead.

 

[Hydrored]

Not normal in my experience, does it have all its spines?