Hole in starfish?

[Frostyeel]

The starfish in my 20 gallon tank somehow developed a rather large hole in its underside. The other inhabitants of the tank are 3 damslefish, one of them a small clown, and when the hole appeared a pair of banded coral shrimp, now only one. I think the parameters are all fine, but the hole hasn't regenerated at all. I have no clue what caused the hole and what it is really, any help at all would be appreciated.

http://img514.imageshack.us/img514/2971/p6160206large9ow.jpg
Sorry for the picture being a little blurry, I can take another if it will help identify the problem.

[deadly_damsel]

ew.