For a snail/bivalve, just mark it as dead. I’d choose organism for evidence of presence if you like, but it’s not necessary, I’d imagine. For these organisms, I think it’s implied that a shell is part of the evidence in almost all cases.
For a crab shell, it would depend of the shell is a molt or if it’s an actual dead crab. If it’s a molt, then molt for evidence of presence and cannot be determined for alive or dead, unless you are also seeing the organism itself. For a dead crab, just dead and organism should work, I think.