I have kept cichlids for a long time and I’ve kept several species of shell dwellers. One of my favorites is Lamprologus ocellatus, a wonderful dwarf cichlid from Lake Tanganyika. If you keep African species and you get the opportunity, invest in a small colony of these little beauties. They are great fun. If you want to learn more about them, Tropical Fish Hobbyist magazine published a great article about this fish in 2014.
Anyway, a long time ago I had several ocellatus and bred them. I never replaced the adults when they passed away. A few months ago after deciding to breed them again, I bought several juveniles from a breeder in New York. Assuming that they were all probably siblings and not wanting to interbreed, about a month ago I picked up another juvenile pair from my LFS, who I know doesn’t get fish from the same breeder. I put this new pair into a 20g long to raise them until I can sex them. The tank has aragonite substrate and several shells.
I was doing a water change on that tank this evening when one of ocellatus nose dived into the sand rather than seek shelter in one of the open shells. It burrowed until the only thing I could see was its eyes. In all the years I kept ocellatus, that is one behavior I never witnessed. I must say I was pretty shocked. See the photo above with the little guy circled in orange.