I've always liked looking at sea urchins so it was natural that I would want one for my tank. However, most urchins are not reef safe. Those that are can often get large and do a number on rock work. That's where a tuxedo urchin comes in.
This video is of the urchin I keep in my 29 gallon BioCube which is currently the smallest tank I keep.
A few years ago I read about a great little urchin in one of my favorite invertebrates books that is commonly called a blue tuxedo urchin (Mespilla globulus). This urchin only gets about two inches in size and is considered reef safe. Like many reef safe crabs and shrimp there are specimens that will develop a liking for polyps, but most stick with algae. I've never been afraid of algae so my tanks will usually have a good supply for snails and other grazers. I knew there was enough for it to eat. Also, their spines are not poisonous like other urchins (though still sharp and caution should be used).
Some things to look for are when buying a blue tuxedo urchin are
Spines in tact. A stressed urchin or one that has not been acclimated properly, will often lose its spines.
Bits of debris stuck to the top of it. the urchin puts this there and a lack of such cover is often a sign of an unhealthy urchin.
No damage to tube feet. These urchins have tube feet all over their bodies.
Acclimation should be over a 2 - 2.5 hour period using a slow drip method. Failure to properly acclimate the urchin will most likely result in its death over the first couple of weeks after introduction to the tank. The first signs of this will be spines dropping off of the urchin. Soon it will be dead and nothing will be left but the test (the name for the urchin's skeleton).
For your safety do not pick up the urchin. The spines are very sharp and can easily puncture the skin. While they are not poisonous like other urchins they can contain bacteria which will infect the puncture wound.
For the urchin's safety never introduce anything containing copper to the tank. This acts as a poison to them and will kill them.
Reef Tank Concerns
Many urchins (maybe most) eat corals and sometimes other invertebrates if they can catch them. The tuxedo urchin is considered "reef-safe with caution" which means it normally won't cause a problem. Where the caution comes in is the above mentioned camouflage. Bits of shell or algae are no problem, but when the urchin chooses your prize zoanthus colony to wear as a hat, this can be frustrating. This is something to consider if thinking about buying this urchin.
They do eat coralline algae so if you do not have a healthy growth of it you are likely to loose what you do have to the urchin's appetite. However, in a mature tank the urchin will not be able to out eat the continual growth of coralline experienced in a mature tank.
Because the urchin grazes for algae the tank must contain enough of it to feed the urchin. It will not generally eat algae added to the tank as its method of eating is by scraping with extremely sharp teeth (sometimes known as jaws). Because of this we only suggest these urchins for mature tanks (1 year old or older).
Information contributed by: Chad
Arizona Fishkeeping PO Box 6302 Mesa, Arizona 85216-6302