MAttie,
you dont have to wait for the bugger to kick the bucket.
you can still look at the long black spiny , yes you dont want to kiss it or play with it, but they are really a good herivore and will help control algae in the tank ( not all types though)
look at the Diadema setosum dont worry everyone i the tank willknow not to play with it too..also just becasue it is called a long BLACK does not mean you have to get the black one. they do have some suttle color changes in them.
lets talk about the tuxedo Mespilia globulus this guy is really cool look urchin some of the color morphs now a days are neat.they too are herb eaters but will eat some meaty things from time to time. the spines on it are very short and they do "collect " things fromthe tank, they do this on purpose, most likly as a defensive thing.
now look at pencil urchins Eucidaris tribuloides they are an Omivore not really a good thing with fish..also they can get pretty big!