Cerith shell erosion

[BristleWormHater]

A lot of the ceriths at my favorite lfs have this erosion around their shells, they still clean and seem totally fine and it doesn't seem to be progressing, but what caused this?

 

[BristleWormHater]

Bump

 

[stewy14]

this is just the layer underneath the normal shell layer, its normal, most my astreas have this, I dont know from what, but it is harmless