ID Sack with hole.

[Gfsrepairs]

I flipped this rock over to expose more of the coral line and I notice this. It looks like a spider egg sack with a hole down the middle. Got any ideas?

 

[nano mania]

Looks like a sponge

 

[aromantis]

Yep, it looks exactly like the sponges in my tank. Actually pretty neat to watch.

 

[SeaSpza]

I think its called an urn sponge.I got a bunch of them