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I have two of these that have mysteriously appeared in my reef tank. I am guessing they were hitchhikers on my live rock. Are they spiny sea urchins or something like that and are they reef safe? Thanks for the help
They're relatively harmless and tend to feed on coraline algae it finds on rocks and elsewhere in the tank.
Like many other urchins, when they get larger they be capable of moving or knocking over corals that aren't firmly affixed. I had hitch hiker rock boring sea urchin which was about the size of a plain m&m candy and grew to about the size of a golf ball before I removed it (because it was knocking over stuff).