Urchin id please
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IMHO, I believe it's a pencil urchin. I have a couple of rock boring urchins and they don't have the white band at the base of the spines.
The white rings at the base of the spines are super distinctive - to my current knowledge, your specimen could only be either an Echinometra species, Parasalenia gratiosa, or (sometimes) Heterocentrotus mamillatus.
For the colors your urchin has, H. mamillatus (the Red Slate Pencil Urchin) is extremely unlikely, so that leaves either an Echinometra species or P. gratiosa. An easy way to distinguish adult P. gratiosa from adults of Echinometra species like E. mathaei or E. oblonga is size: if the urchin is bigger than ~3.5cm, it's an Echinometra. The other easy way to distinguish is by looking at the periproct plates (the "teeth" around the urchin's anus); P. gratiosa will only have 3-5 periproct plates while an Echinometra species will have a bunch (for examples, see the links below; the second pic of the Lizard Island Guide shows P. gratiosa's periproct plates, and figure 8 from the Semantic Scholar link shows the periproct of E. mathaei - it's a lot harder to see the periproct of E. mathaei clearly on live specimens, but they also have a pic of the area in figure 1). The French Wikipedia page lists some more subtle differences, but the methods above would be my suggestion for differentiating them.
Edit: Forgot to add the French Wikipedia link:
fr.wikipedia.org
All that said, Echinometra species are reportedly much more common, so I'd assume that's what you have.
Regardless, P. gratiosa is reportedly a primarily herbivorous detritivore (though it may - like pretty much all urchins - occasionally eat sedentary inverts), while Echinometra species are reportedly herbivores, so both would be good for a tank. Echinometra species are rock-boring, while P. gratiosa seems to basically go wherever it wants.
For the colors your urchin has, H. mamillatus (the Red Slate Pencil Urchin) is extremely unlikely, so that leaves either an Echinometra species or P. gratiosa. An easy way to distinguish adult P. gratiosa from adults of Echinometra species like E. mathaei or E. oblonga is size: if the urchin is bigger than ~3.5cm, it's an Echinometra. The other easy way to distinguish is by looking at the periproct plates (the "teeth" around the urchin's anus); P. gratiosa will only have 3-5 periproct plates while an Echinometra species will have a bunch (for examples, see the links below; the second pic of the Lizard Island Guide shows P. gratiosa's periproct plates, and figure 8 from the Semantic Scholar link shows the periproct of E. mathaei - it's a lot harder to see the periproct of E. mathaei clearly on live specimens, but they also have a pic of the area in figure 1). The French Wikipedia page lists some more subtle differences, but the methods above would be my suggestion for differentiating them.
species: Parasalenia gratiosa in taxonomy (Lizard Island Field Guide)
lifg.australian.museum
Parasalenia gratiosa — Wikipédia
Regardless, P. gratiosa is reportedly a primarily herbivorous detritivore (though it may - like pretty much all urchins - occasionally eat sedentary inverts), while Echinometra species are reportedly herbivores, so both would be good for a tank. Echinometra species are rock-boring, while P. gratiosa seems to basically go wherever it wants.
