I had always used waterproof and fadeproof ink to mark the shells of live land snails. However, one problem with ink is that it isn't absorbed into the periostracum but stays on it and, sooner or later, even the most "permanent" ink flakes off. Nevertheless, I have had some success with ink-marking of live snails. For example, shells of live Vertigo pygmaea marked with a Pigma pen remained marked over about 4 winter months. But in that case, the snails were mostly dormant and didn’t do much crawling around.
Last winter I started ink-marking the shells of some live Triodopsis with Staedtler Permanent Lumocolor pens. The last marking was on 17 April when I marked 9 adults with a green pen. On 7 August I found 2 of those snails but their marks had become so fragmented that I didn’t realize they were marked until after I put them under the microscope.
So this time I decided to try something different and filed small notches into the lips of the shells of about a dozen Triodopsis and released them back where I had collected them.
The snails will probably repair their lips, but I am hoping that the scars will still be visible. I remember reading about this in a paper, but I haven’t had a chance to dip up the citation. Obviously, filing marks on shells wouldn’t be practical in the case of tiny species like Vertigo.
If I ever recover any of those Triodopsis, I will post an update here.