leg losing crab

[subopposite]

I have a crab that seems to be losing legs and was wondering why. When I first counted the legs it had 2 on one side and 3 on the other side. I guess it could have had more when I first got it, but when I first counted, that is what it had. Now, it has lost another leg, so it has one leg on one side and 3 on the other and it also lost one of its feeder claws (its a female) Does anyone know what would cause this? Old age...bad water...temperature....salt/no salt. Also, another one of my crabs that had 8 legs lost one this week too. I do have a male in there, so I thought maybe he could have nabbed a leg off of that one, but I actually saw a leg just fall off of the one that now only has one leg on that side. With only one leg on one side, three on the other side and only one feeder, I highly doubt this crab has a chance of survival. I would just like to know if anyone might know why, so I can possibly prevent this in the future. Thanks and help please.

Water parameters are
PH 7.2-7.6
Nitrite 0
Nitrate 20-40