Invertebrates: Urchins

Urchins can add a lot of personality to an aquarium, but many can be quite destructive, especially as they grow.  In fact, I would avoid urchins entirely if you have an acrylic aquarium or power cords that an urchin could easily climb in the tank.  Urchin bite marks on acrylic will obscure the view and are quite difficult to buff out.  Urchins of various species have also chewed through powercords (I’m sure that nightmare is easy to imagine.)  Do your research before making a purchase that might ruin your aquarium.
Rock-Boring Urchin (Echinometra lacunter / Echinometra mathei): Reef Safe
As far as I’m aware there are no visual markers between the two species – species determination depends on the collection site.  They eat just about all algae, including coralline algae (never fear – the coralline algae spores are released as excrement, so the urchins actually help spread it around).  Their diet should be supplemented with dried algae sheets if the aquarium is lacking algae or if they start to drop their spines.  I had these two guys in my aquarium for a few years and they never bothered any coral.  They did knock over a couple loose frags, so I just had to make sure everything was glued down well.  Also, these guys are nocturnal, so don’t expect much in the way of personality…just more of a nighttime maintenance crew.
Variegated Sea Urchin (Lytechinus variegates) – Reef Safe
I always try to support captive-bred aquarium livestock, and now Oceans Reefs & Aquariums (ORA) has aqua-cultured ones for sale.  These urchins will eat just about all algae (again, including coralline algae), but I never had a problem with them eating any coral.  However, this little bugger sure did love to pick up my zoanthids and carry them as beautiful camouflage.  As long as I made sure all zoanthids were secured to a large base, I didn’t have any issues.  
Black Long-spined Urchin (Diadema sp.) – Reef Safe when Small
This beauty gets a ton of debates on various forums.  It’s midnight black with brilliant blue feet…and an orange anus (no, that’s not an eye…it’s an anus.)  Compare this guy to…oh, say a great dane puppy.  Aquarists think it’s beautiful, that’s it’s so small…what harm could it possibly do?  Yeah, wait til it grows up.  I highly discourage purchasing these urchins for small, immature aquariums.  These urchins quickly grow enormous and are able to mow down just about everything in their path (including corals.)  They can also be quite destructive as their sheer weight can bring rocks tumbling if not secured properly.  In aquariums full of small coral frags, coral loss due to this urchin is noticeable, but a fully stocked mature aquarium may be fine.  If you already have one of these, I would provide a regular supply of algae to help prevent coral loss.
Sea Biscuit (below photo may be Clypeaster rosaceus?) – Expert Only
Sanddollars and seabiscuits are in the class Echinoidea with urchins (so they’re closer to urchins than starfish.)  They are generally filter feeders that use cillia to move food particles to their mouth.  In captivity they slowly starve to death, and they should not be attempted by most aquarists.  A large established reef aquarium dedicated to non-photosynthetic corals and other filter-feeding organisms may be able to support sanddollars and seabiscuits, but I am not aware of any successes.

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