Snail shells.

[Stevefm2003]

Help. Is this low calcium or a disease ? I have several Blue Mystery snails. Noticed this on one of them. 

[Shadow_Arbor]

This is a growth deformity. Usually the result of a lack of minerals, including calcium, iodine, magnesium, etc. either in the water, the snails diet, or both. I recommend feeding a food either for shrimp or snails that contains minerals. Also if you have soft water or many snails in a small volume of water, adding wondershells.

Seeing as the snail is living in your aquarium there are likely no predators that will try to eat it. The flesh in contact with water under those cracks will calcify and the snail should be fine as long as you fix whatever issues there are with it's diet or water parameters.

Good Luck!

[Kat_Rigel]

I agree, I think this looks like a calcium deficiency. It looks like there is a little bit of restriction in the shell too, if you look at the top of the picture, the shell kind of goes smaller and then bigger. That as well as the holes is a sign they need more calcium. I have had luck with adding crushed coral to my HOB as well as supplementing feeding with Tums, I just chuck 'em right into the tank and my colony goes right at them. I also crush them and add them to Repashy sometimes.