Crab ID? (came on Diadema Urchen)

[cooperstock]

Hello,
I just got a new diadema urchen yesterday. As I was acclemating him I noticed something funny under him. (it might be a her, my wife named her Diane (the diadema).
So I flipped the urchen over and there was this little crab holding on to the urchen. Maybe for protection? or is it a parasite?
My first thought was 'tick', like the kind you get in the forest and suck blood.
Anyway He came right off and is now in my acclemation bowl. I didn't want to put him into the tank until I knew what he is.
I think I got a pretty good picture.
Size wise the width of the back shell is less than a quarter inch (1/4") so with his legs, he might be 1/2"
If you have any thoughts or suggestions let me know. Also, what would a crab like this eat? Is it worth keeping to put in the sump? or will it eat things in there?
Thanks
David

 

[LeslieH]

Very good picture, which makes identifying it so much easier! The crab is a well known commensal of Echinothrix urchins called Echinoecus pentagonus. Normally they're very host-specific. Either your urchin is mis-identified or the crab somehow lost his normal partner & glommed onto the nearest urchin. Most of these specialized urchin partners won't survive if they're removed. Although they may create a bare patch on the urchin (their home) they're thought to be largely beneficial in keeping the host clean of parasites & helping defend against predators.

 

[cooperstock]

Great! thanks for the info
is there any problem with putting it back on the urchin?

 

[LeslieH]

Nope.