Full disclosure: I looked into my crabitat tonight and my crab Calypso, who I've had for 16 years, was dead and appeared to have been killed by Frodo. It was very shocking and disturbing, as Calypso was pulled out of the shell and scattered around.
The only reason I can think, other than it was natural causes and Frodo scavenged..ugh, is the substrate being too shallow & maybe Frodo felt threatened? I try my best but the playsand I got from Lowes was orange and weird so I didn't use it, and then the online shop I use was always out-of-stock or temporarily closed...I work six days a week so I did my best, and of course now I am full of regret I didn't try harder to top off their sand sooner. Frodo did not take the shell that Calypso had been in, and they have other shells available, so I don't think it was a shell fight. Otherwise, I keep their tank at 79°F with a Bean Farm heating pad & temperature controller. Their diet is a variety of HCP foods plus worm casings, mineral supplements, and fresh/salt water always available. Regardless of how/why Calypso died, I don't want Frodo to be alone and I don't think I want to get more crabs & start over. I'm very busy and I just want Frodo to have a home with even better care and some friends. I can post some photos of Frodo later...just can't bring myself to do that right now. Frodo loves snacking on dead leaves, and I have some that can go along with him/her.