Aren't they a menace with regard to eating everything and being clumsy in general?
The same has been said for most species of urchins commonly kept in the trade, including some of my rather good friends in this tank...
Tripneustes gratilla
Lytechinus variegatus
Mespilia globulus
And not to forget some of the residents of my "touch tank" tub...Eucidaris tribuloides, black urchins still unIDed with certainty (either Echinometra lucunter or Arbacia punctulata), and T. gratilla.
Urchins are like many of the Crustaceans I keep - often misunderstood, poorly documented, and not fed properly. Several of the species I have get slammed regularly by hobbyists, but it's almost always down to two things: bad IDs and/or people who don't have a clue about inverts (and unfortunately many don't want a clue either). When advising people about urchins I always have to tack on the clause of "well there are reports of individuals in all commonly kept species eating corals and other beneficial fauna..." but I wouldn't have 13 urchins right now (6 in the 55gal and 7 in the tub) if I didn't find them to be very predictable and manageable.
While there's always room for seeing a surprise with a species I haven't kept before, I hear the same good and bad things about Diadema species as I do about the ones in my 55gal. The main issues that stand out to me with Diadema species in an aquarium is the need for more open space than other "grazers" and more importantly the accidental sting factor, which is what has put me off so far. Echinothrix also gets sold as a "long spine" and might have more surprises; there are some interesting things I've heard about the feeding behavior of E. calamaris in aquaria as it grows (claims of being a voracious carnivore) that seem to directly contradict the result of a gut content study done on wild specimens (algae/kelp).
Well, tossing my hypothesis out the window lol. How far do the polyps retract? And you should try and get a close up of a polyp and Ill see what I can find in my school's database (they have a contract with loads of companies like springer, pubmed, etc. and even have some stuff from USF's DB).
Macro pics of that degree are not really possible; my camera is old enough that even a lot of cell phones these days perform better. There are plenty of pics on the web of exactly what I have in more detail than I can get, but no conclusive ID. Capnella is another genus being bandied around with some of the pics on the web. I'll see if I can find one of those that shows the polyps clearly.
Yale's got pretty endless library access too and it hasn't helped me much in the coral-ID department over time. If you've got a soft coral expert on hand then it might be worth trying to get a sample him/her at some point (or bothering ORA to see how they got theirs IDed if they had an expert), but short of that I don't hold out a lot of hope for certainty in this group of animals. Borneman even says the IDs are a pain and in the past I've read that IDs can be down to things like sclerite morphology, so it has seemed to me to be a rather poorly documented mess.
Speaking of Borneman...not sure why I wasn't noticing this pic before. Page 141 Lithophyton sp. - that could be a match. Too bad the pic isn't bigger.