Sea urchin trying to eat hermit crabs?!?

[jayjay5531]

So I just got a Caribbean Shortspine Sea Urchin, and it's trying to eat my 3/4-inch scarlet hermit crabs! I don't know how much success he's having, but right now he's latched onto the shell of one and it's trying to scramble away but he won't let go. He's only about an inch diameter himself. Is this common? Is he going to eat all my snails and crabs? It's a 36 gallon FOWLR with 2 clowns, three firefish, three banggai cardinals, and a small yellow tang (which I will swap out for a smaller one after a year at my LFS). For inverts, I have a brittle star, lots of Nassarius and three turbo snails.

 

[saxman]

Most urchins are opportunistic omnivores, and will eat where they find a meal, esp. if they're starving. Do you feed the urchin? If so, what?
If you have several Nassarius snails and hermits, your urchin is likely getting out-competed for food by them, esp. if there is little in the way of algae in your tank. BTW, Nassarius snails don't eat algae, which isn't helping matter any as far as the urchin is concerned (there is likely little leftover food between them and the hermits).
Try feeding the urchin some dried sheet algae (nori), and offer it some meaty food a couple of times a week in addition to the nori.
The other possibility is that urchins often decorate themselves with shells, bits of substrate, etc. to blend into their surroundings, and it may be trying to use the hermit as decor.