So i often draw stars on snails with sharpies.
Never seen a snail with a Sharpie. 
Snails don’t lose their shells as they age. Maybe your art is stretching out like a tattoo on an older person?
“you can code shells by using colored enamel paints. For instance, a linear arrangement of four dots using five colors will allow you to mark 625 specimens differently. After the paint has dried, cover it with a cyanoacrylate adhesive or acrylic polymer for added protection and durability.”
Don’t know if this helps.
Also, we use calcium carbonate (snail shells, essentially) to mark athletic fields. I don’t know if that’s Alanis morrisette irony or regular irony.
There are different formulations for permanent markers, including Sharpie pens. Some are considered nontoxic and safe for use on skin. Others contain toxic solvents that may cause organ damage from inhalation, ingestion, or skin absorption.
Sharpie Fine Point Markers are the safest pens to use on skin. Even with these pens, it’s a good idea to avoid writing on the lips or near the eyes.
King Size Sharpie, Magnum Sharpie, and Touch-Up Sharpie contain xylene, which is neurotoxic and may damage other organs. Xylene poses a risk via inhalation, ingestion, and absorption across skin and mucous membranes. Writing on skin with these markers isn’t recommended.
There’s also different kinds of sharpies, toluene, alcohol, ethanol or isopropanol, ethylene glycol monobutyl, propanol, butanol, and diacetone. resin or polymer is added as a “binder” that promotes adhesion. Permchrome ink has the alcohol but not glycol ether.
Sharpie snails migrate to the Sharpie Snail Sanctuary (SSS) as shown on the map above. I wondered where all of the star snails came from.
Maybe something to do with snails secreting a layer of protein to protect their shells.
